Parrot Intermediate Representation

The Parrot intermediate representation (PIR) is the primary way to program Parrot directly. It used to be an overlay on top of the far more primitive Parrot assembly language (PASM). However, PIR and PASM have since diverged semantically in a number of places and no longer are directly related to one another. PIR has many high-level features that will be familiar to programmers, such as basic operator syntax. However, it's still very low-level, and is closely tied to the underlying virtual machine. In fact, the Parrot developers specifically want to keep in that way for a number of reasons. PASM, the Parrot assembly language, is discussed in more detail in Chapter 5.

As a convention, files containing pure PIR code generally have a .pir extension. PASM files typically end with .pasm. Compiled Parrot Bytecode (PBC) files have a .pbc extension. We'll talk more about PBC and PASM in later chapters.

PIR is well documented, both in traditional documentation and in instructional code examples. The project documentation in docs/ are good sources for information about the current syntax, semantics, and implementation. The test suite in t/compilers/imcc/ shows examples of proper working code. In fact, the test suite is the definitive PIR resource, because it shows how PIR actually works, even when the documentation may be out of date.

Statements

The syntax of statements in PIR is much more flexible then is commonly found in assembly languages, but is more rigid and "close to the machine" then some higher-level languages like C are. PIR has a very close relationship with the Parrot assembly language, PASM. PASM instructions, with some small changes and caveats, are valid PIR instructions. PIR does add some extra syntactic options to help improve readability and programmability, however. The statement delimiter for both PIR and PASM is a newline \n. Each statement has to be on its own line This isn't entirely true when you consider things like macros and heredocs, but we'll tackle those issues when we come to them., but empty whitespace lines between statements are okay. Statements may also start with a label, for use with jumps and branches. Comments are marked by a hash sign (#), and continue until the end of the line. POD blocks may be used for multi-line documentation. We'll talk about all these issues in more detail as we go.

To help with readability, PIR has some high-level constructs, including symbol operators:

  $I1 = 5                       # set $I1, 5

named variables:

  .param int count
  count = 5

and complex statements built from multiple keywords and symbol operators:

  if $I1 <= 5 goto LABEL        # le $I1, 5, LABEL

We will get into all of these in more detail as we go. Notice that PIR does not, and will not, have high-level looping structures like while or for loops. PIR has some support for basic if branching constructs, but will not support more complicated if/then/else branch structures. Because of these omissions PIR can become a little bit messy and unwieldy for large programs. Luckily, there are a large group of high-level languages (HLL) that can be used to program Parrot instead. PIR is used primarily to write the compilers and libraries for these languages, while those languages can be used for writing larger and more complicated programs.

Directives

PIR has a number of directives, instructions which are handle specially by the parser to perform operations. Some directives specify actions that should be taken at compile-time. Some directives represent complex operations that require the generation of multiple PIR or PASM instructions. PIR also has a macro facility to create user-defined directives that are replaced at compile-time with the specified PIR code.

Directives all start with a . period. They take a variety of different forms, depending on what they are, what they do, and what arguments they take. We'll talk more about the various directives and about PIR macros in this and in later chapters.

Variables and Constants

Parrot Registers

PIR code has a variety of ways to store values while you work with them. Actually, the only place to store values is in a Parrot register, but there are multiple ways to work with these registers. Register names in PIR always start with a dollar sign, followed by a single character that shows whether it is an integer (I), numeric (N), string (S), or PMC (P) register, and then the number of the register:

  $S0 = "Hello, Polly.\n"
  print $S0

Integer (I) and Number (N) registers use platform-dependent sizes and limitations There are a few exceptions to this, we use platform-dependent behavior when the platforms behave sanely. Parrot will smooth out some of the bumps and inconsistencies so that behavior of PIR code will be the same on all platforms that Parrot supports. Both I and N registers are treated as signed quantities internally for the purposes of arithmetic. Parrot's floating point values and operations are all IEEE 754 compliant.

Strings (S) are buffers of data with a consistent formatting but a variable size. By far the most common use of S registers and variables is for storing text data. S registers may also be used in some circumstances as buffers for binary or other non-text data. However, this is an uncommon usage of them, and for most such data there will likely be a PMC type that is better suited to store and manipulate it. Parrot strings are designed to be very flexible and powerful, to account for all the complexity of human-readable (and computer-representable) text data.

The final data type is the PMC, a complex and flexible data type. PMCs are, in the world of Parrot, similar to what classes and objects are in object-oriented languages. PMCs are the basis for complex data structures and object-oriented behavior in Parrot. We'll discuss them in more detail in this and in later chapters.

Constants

As we've just seen, Parrot has four primary data types: integers, floating-point numbers, strings, and PMCs. Integers and floating-point numbers can be specified in the code with numeric constants in a variety of formats:

  $I0 = 42       # Integers are regular numeric constants
  $I1 = -1       # They can be negative or positive
  $I2 = 0xA5     # They can also be hexadecimal
  $I3 = 0b01010  # ...or binary

  $N0 = 3.14     # Numbers can have a decimal point
  $N1 = 4        # ...or they don't
  $N2 = -1.2e+4  # Numbers can also use scientific notation.

String literals are enclosed in single or double-quotes:

  $S0 = "This is a valid literal string"
  $S1 = 'This is also a valid literal string'

Strings in double-quotes accept all sorts of escape sequences using backslashes. Strings in single-quotes only allow escapes for nested quotes:

  $S0 = "This string is \n on two lines"
  $S0 = 'This is a \n one-line string with a slash in it'

Here's a quick listing of the escape sequences supported by double-quoted strings:

  \xhh        1..2 hex digits
  \ooo        1..3 oct digits
  \cX         Control char X
  \x{h..h}    1..8 hex digits
  \uhhhh      4 hex digits
  \Uhhhhhhhh  8 hex digits
  \a          An ASCII alarm character
  \b          An ASCII backspace character
  \t          A tab
  \n          A newline
  \v          A vertical tab
  \f
  \r
  \e
  \\          A backslash
  \"          A quote

Or, if you need more flexibility, you can use a heredoc string literal:

  $S2 = << "End_Token"

  This is a multi-line string literal. Notice that
  it doesn't use quotation marks. The string continues
  until the ending token (the thing in quotes next to
  the << above) is found. The terminator must appear on
  its own line, must appear at the beginning of the
  line, and may not have any trailing whitespace.

  End_Token

Strings: Encodings and Charsets

Strings are complicated. We showed three different ways to specify string literals in PIR code, but that wasn't the entire story. It used to be that all a computer system needed was to support the ASCII charset, a mapping of 128 bit patterns to symbols and English-language characters. This was sufficient so long as all computer users could read and write English, and were satisfied with a small handful of punctuation symbols that were commonly used in English-speaking countries. However, this is a woefully insufficient system to use when we are talking about software portability between countries and continents and languages. Now we need to worry about several character encodings and charsets in order to make sense out of all the string data in the world.

Parrot has a very flexible system for handling and manipulating strings. Every string is associated with an encoding and a character set (charset). The default for Parrot is 8-bit ASCII, which is simple to use and is almost universally supported. However, support is built in to have other formats as well.

Double-quoted string constants, like the ones we've seen above, can have an optional prefix specifying the charset or both the encoding and charset of the string. Parrot will maintain these values internally, and will automatically convert strings when necessary to preserve the information. String prefixes are specified as encoding:charset: at the front of the string. Here are some examples:

  $S0 = utf8:unicode:"Hello UTF8 Unicode World!"
  $S1 = utf16:unicode:"Hello UTF16 Unicode World!"
  $S2 = ascii:"This is 8-bit ASCII"
  $S3 = binary:"This is treated as raw unformatted binary"

The binary: charset treats the string as a buffer of raw unformatted binary data. It isn't really a "string" per se because binary data isn't treated as if it contains any readable characters. These kinds of strings are useful for library routines that return large amounts of binary data that doesn't easily fit into any other primitive data type.

Notice that only double-quoted strings can have encoding and charset prefixes like this. Single-quoted strings do not support them.

When two types of strings are combined together in some way, such as through concatenation, they must both use the same character set an encoding. Parrot will automatically upgrade one or both of the strings to use the next highest compatible format, if they aren't equal. ASCII strings will automatically upgrade to UTF-8 strings if needed, and UTF-8 will upgrade to UTF-16. Handling and maintaining these data and conversions all happens automatically inside Parrot, and you the programmer don't need to worry about the details.

Named Variables

Calling a value "$S0" isn't very descriptive, and usually it's a lot nicer to be able to refer to values using a helpful name. For this reason Parrot allows registers to be given temporary variable names to use instead. These named variables can be used anywhere a register would be used normally ...because they actually are registers, but with fancier names. They're declared with the .local statement which requires a variable type and a name:

  .local string hello
  set hello, "Hello, Polly.\n"
  print hello

This snippet defines a string variable named hello, assigns it the value "Hello, Polly.\n", and then prints the value. Under the hood these named variables are just normal registers of course, so any operation that a register can be used for a named variable can be used for as well.

The valid types are int, num, string, and pmc Also, you can use any predefined PMC class name like BigNum or LexPad. We'll talk about classes and PMC object types in a little bit.. It should come as no surprise that these are the same as Parrot's four built-in register types. Named variables are valid from the point of their definition to the end of the current subroutine.

The name of a variable must be a valid PIR identifier. It can contain letters, digits and underscores but the first character has to be a letter or an underscore. There is no limit to the length of an identifier, especially since the automatic code generators in use with the various high-level languages on parrot tend to generate very long identifier names in some situations. Of course, huge identifier names could cause all sorts of memory allocation problems or inefficiencies during lexical analysis and parsing. You should push the limits at your own risk.

Register Allocator

Now's a decent time to talk about Parrot's register allocator it's also sometimes humorously referred to as the "register alligator", due to an oft-repeated typo and the fact that the algorithm will bite you if you get too close to it. When you use a register like $P5, you aren't necessarily talking about the fifth register in memory. This is important since you can use a register named $P10000000 without forcing Parrot to allocate an array of ten million registers. Instead Parrot's compiler front-end uses an allocation algorithm which turns each individual register referenced in the PIR source code into a reference to an actual memory storage location. Here is a short example of how registers might be mapped:

  $I20 = 5       # First register, I0
  $I10000 = 6    # Second register, I1
  $I13 = 7       # Third register, I2

The allocator can also serve as a type of optimization. It performs a lifetime analysis on the registers to determine when they are being used and when they are not. When a register stops being used for one thing, it can be reused later for a different purpose. Register reuse helps to keep Parrot's memory requirements lower, because fewer unique registers need to be allocated. However, the downside of the register allocator is that it takes more time to execute during the compilation phase. Here's an example of where a register could be reused:

  .sub main
    $S0 = "hello "
    print $S0
    $S1 = "world!"
    print $S1
  .end

We'll talk about subroutines in more detail in the next chapter. For now, we can dissect this little bit of code to see what is happening. The .sub and .end directives demarcate the beginning and end of a subroutine called main. This convention should be familiar to C and C++ programmers, although it's not required that the first subroutine or any subroutine for that matter be named "main". In this code sequence, we assign the string "hello " to the register $S0 and use the print opcode to display it to the terminal. Then, we assign a second string "world!" to a second register $S1, and then print that to the terminal as well. The resulting output of this small program is, of course, the well-worn salutation hello world!.

Parrot's compiler and register allocator are smart enough to realize that the two registers in the example above, $S0 and $S1 are used exclusively of one another. $S0 is assigned a value in line 2, and is read in line 3, but is never accessed after that. So, Parrot determines that its lifespan ends at line 3. The register $S1 is used first on line 4, and is accessed again on line 5. Since these two do not overlap, Parrot's compiler can determine that it can use only one register for both operations. This saves the second allocation. Notice that this code with only one register performs identically to the previous example:

  .sub main
    $S0 = "hello "
    print $S0
    $S0 = "world!"
    print $S0
  .end

In some situations it can be helpful to turn the allocator off and avoid expensive optimizations. Such situations are subroutines where there are a small fixed number of registers used, when variables are used throughout the subroutine and should never be reused, or when some kind of pointer reference needs to be made to the register this happens in some NCI calls that take pointers and return values. To turn off the register allocator for certain variables, you can use the :unique_reg modifier:

  .local pmc MyUniquePMC :unique_reg

Notice that :unique_reg shouldn't affect the behavior of Parrot, but instead only changes the way registers are allocated. It's a trade off between using more memory in exchange for less time spent optimizing the subroutine.

PMC variables

PMC registers and variables act much like any integer, floating-point number, or string register or variable, but you have to instantiate a new PMC object into a type before you use it. The new instruction creates a new PMC of the specified type:

  $P0 = new 'PerlString'     # This is how the Perl people do it
  $P0 = "Hello, Polly.\n"
  print $P0

This example creates a PerlString object, stores it in the PMC register $P0, assigns the value "Hello, Polly.\n" to it, and prints it. With named variables the type passed to the .local directive is either the generic pmc or a type compatible with the type passed to new:

  .local PerlString hello    # or .local pmc hello
  hello = new 'PerlString'
  hello = "Hello, Polly.\n"
  print hello

PIR is a dynamic language, and that dynamicism is readily displayed in the way PMC values are handled. Primitive registers like strings, numbers, and integers perform a special action called autoboxing when they are assigned to a PMC. Autoboxing is when a primitive scalar type is automatically converted to a PMC object type. There are PMC classes for String, Number, and Integer which can be quickly converted to and from primitive int, number, and string types. Notice that the primitive types are in lower-case, while the PMC classes are capitalized. If you want to box a value explicitly, you can use the box opcode:

  $P0 = new 'Integer'       # The boxed form of int
  $P0 = box 42
  $P1 = new 'Number'        # The boxed form of num
  $P1 = box 3.14
  $P2 = new 'String'        # The boxed form of string
  $P2 = "This is a string!"

The PMC classes Integer, Number, and String are thin overlays on the primitive types they represent. However, these PMC types have the benefit of the VTABLE interface. VTABLEs are a standard API that all PMCs conform to for performing standard operations. These PMC types also have special custom methods available for performing various operations, they may be passed as PMCs to subroutines that only expect PMC arguments, and they can be subclassed by a user-defined type. We'll discuss all these complicated topics later in this chapter and in the next chapter. We will discuss PMC and all the details of their implementation and interactions in Chapter 11.

Named Constants

The .const directive declares a named constant. It's very similar to .local, and requires a type and a name. The value of a constant must be assigned in the declaration statement. As with named variables, named constants are visible only within the compilation unit where they're declared. This example declares a named string constant hello and prints the value:

  .const string hello = "Hello, Polly.\n"
  print hello

Named constants may be used in all the same places as literal constants, but have to be declared beforehand:

  .const int the_answer = 42        # integer constant
  .const string mouse = "Mouse"     # string constant
  .const num pi = 3.14159           # floating point constant

In addition to normal local constants, you can also specify a global constant which is accessible from everywhere in the current code file:

  .globalconst int days = 365

Currently there is no way to specify a PMC constant in PIR source code, although a way to do so may be added in later versions of Parrot.

Symbol Operators

PIR has many other symbol operators: arithmetic, concatenation, comparison, bitwise, and logical. All PIR operators are translated into one or more Parrot opcodes internally, but the details of this translation stay safely hidden from the programmer. Consider this example snippet:

  .local int sum
  sum = $I42 + 5
  print sum
  print "\n"

The statement sum = $I42 + 5 translates to the equivalent statement add sum, $I42, 5. This in turn will be translated to an equivalent PASM instruction which will be similar to add I0, I1, 5. Notice that in the PASM instruction the register names do not have the $ symbol in front of them, and they've already been optimized into smaller numbers by the register allocator. The exact translation from PIR statement to PASM instruction isn't too important Unless you're hacking on the Parrot compiler!, so we don't have to worry about it for now. We will talk more about PASM, its syntax and its instruction set in Chapter 5. Here are examples of some PIR symbolic operations:

  $I0 = $I1 + 5      # Addition
  $N0 = $N1 - 7      # Subtraction
  $I3 = 4 * 6        # Multiplication
  $N7 = 3.14 / $N2   # Division
  $S0 = $S1 . $S2    # String concatenation

PIR also provides automatic assignment operators such as +=, -=, and >>=. These operators help programmers to perform common manipulations on a data value in place, and save a few keystrokes while doing them.

A complete list of PIR operators is available in Chapter 13.

= and Type Conversion

We've mostly glossed over the behavior of the = operator, although it's a very powerful and important operator in PIR. In it's most simple form, = stores a value into one of the Parrot registers. We've seen cases where it can be used to assign a string value to a string register, or an integer value to an int register, or a floating point value into a number register, etc. However, the = operator can be used to assign any type of value into any type of register, and Parrot will handle the conversion for you automatically:

  $I0 = 5     # Integer. 5
  $S0 = $I0   # Stringify. "5"
  $N0 = $S0   # Numify. 5.0
  $I0 = $N0   # Intify. 5

Notice that conversions between the numeric formats and strings only makes sense when the value to convert is a number.

  $S0 = "parrot"
  $I0 = $S0        # 0

We've also seen an example earlier where a string literal was set into a PMC register that had a type String. This works for all the primitive types and their autoboxed PMC equivalents:

  $P0 = new 'Integer'
  $P0 = 5
  $S0 = $P0      # Stringify. "5"
  $N0 = $P0      # Numify. 5.0
  $I0 = $P0      # De-box. $I0 = 5

  $P1 = new 'String'
  $P1 = "5 birds"
  $S1 = $P1      # De-box. $S1 = "5 birds"
  $I1 = $P1      # Intify. 5
  $N1 = $P1      # Numify. 5.0

  $P2 = new 'Number'
  $P2 = 3.14
  $S2 = $P2      # Stringify. "3.14"
  $I2 = $P2      # Intify. 3
  $N2 = $P2      # De-box. $N2 = 3.14

Labels

Any line in PIR can start with a label definition like LABEL:, but label definitions can also stand alone on their own line. Labels are like flags or markers that the program can jump to or return to at different times. Labels and jump operations (which we will discuss a little bit later) are one of the primary methods to change control flow in PIR, so it is well worth understanding.

Labels are most often used in branching instructions, which are used to implement high level control structures by our high-level language compilers.

Compilation Units

Compilation units in PIR are roughly equivalent to the subroutines or methods of a high-level language. Though they will be explained in more detail later, we introduce them here because all code in a PIR source file must be defined in a compilation unit. We've already seen an example for the simplest syntax for a PIR compilation unit. It starts with the .sub directive and ends with the .end directive:

  .sub main
      print "Hello, Polly.\n"
  .end

Again, we don't need to name the subroutine main, it's just a common convention. This example defines a compilation unit named main that prints a string "Hello, Polly.". The first compilation unit in a file is normally executed first but you can flag any compilation unit as the first one to execute with the :main marker.

  .sub first
      print "Polly want a cracker?\n"
  .end

  .sub second :main
      print "Hello, Polly.\n"
  .end

This code prints out "Hello, Polly." but not "Polly want a cracker?". This is because the function second has the :main flag, so it is executed first. The function first, which doesn't have this flag is never executed. However, if we change around this example a little:

  .sub first :main
      print "Polly want a cracker?\n"
  .end

  .sub second
      print "Hello, Polly.\n"
  .end

The output now is "Polly want a cracker?". Execution in PIR starts at the :main function and continues until the end of that function only. If you want to do more stuff if your program, you will need to call other functions explicitly.

Chapter 4 goes into much more detail about compilation units and their uses.

Flow Control

Flow control in PIR is done entirely with conditional and unconditional branches to labels. This may seem simplistic and primitive, but remember that PIR is a thin overlay on the assembly language of a virtual processor, and is intended to be a simple target for the compilers of various. high-level languages. High level control structures are invariably linked to the language in which they are used, so any attempt by Parrot to provide these structures would work well for some languages but would require all sorts of messy translation in others. The only way to make sure all languages and their control structures can be equally accommodated is to simply give them the most simple and fundamental building blocks to work with. Language agnosticism is an important design goal in Parrot, and creates a very flexible and powerful development environment for our language developers.

The most basic branching instruction is the unconditional branch: goto.

  .sub _main
      goto L1
      print "never printed"
  L1:
      print "after branch\n"
      end
  .end

The first print statement never runs because the goto always skips over it to the label L1.

The conditional branches combine if or unless with goto.

  .sub _main
      $I0 = 42
      if $I0 goto L1
      print "never printed"
  L1: print "after branch\n"
      end
  .end

In this example, the goto branches to the label L1 only if the value stored in $I0 is true. The unless statement is quite similar, but branches when the tested value is false. An undefined value, 0, or an empty string are all false values. Any other values are considered to be true values.

The comparison operators (<, <=, ==, !=, >, >=) can combine with if ... goto. These branch when the comparison is true:

  .sub _main
      $I0 = 42
      $I1 = 43
      if $I0 < $I1 goto L1
      print "never printed"
  L1:
      print "after branch\n"
      end
  .end

This example compares $I0 to $I1 and branches to the label L1 if $I0 is less than $I1. The if $I0 < $I1 goto L1 statement translates directly to the PASM lt branch operation.

The rest of the comparison operators are summarized in "PIR Instructions" in Chapter 11.

PIR has no special loop constructs. A combination of conditional and unconditional branches handle iteration:

  .sub _main
      $I0 = 1               # product
      $I1 = 5               # counter

  REDO:                     # start of loop
      $I0 = $I0 * $I1
      dec $I1
      if $I1 > 0 goto REDO  # end of loop

      print $I0
      print "\n"
      end
  .end

This example calculates the factorial 5!. Each time through the loop it multiplies $I0 by the current value of the counter $I1, decrements the counter, and then branches to the start of the loop. The loop ends when $I1 counts down to 0 so that the if doesn't branch to REDO. This is a do while-style loop with the condition test at the end, so the code always runs the first time through.

For a while-style loop with the condition test at the start, use a conditional branch together with an unconditional branch:

  .sub _main
      $I0 = 1               # product
      $I1 = 5               # counter

  REDO:                     # start of loop
      if $I1 <= 0 goto LAST
      $I0 = $I0 * $I1
      dec $I1
      goto REDO
  LAST:                     # end of loop

      print $I0
      print "\n"
      end
  .end

This example tests the counter $I1 at the start of the loop. At the end of the loop, it unconditionally branches back to the start of the loop and tests the condition again. The loop ends when the counter $I1 reaches 0 and the if branches to the LAST label. If the counter isn't a positive number before the loop, the loop never executes.

Any high-level flow control construct can be built from conditional and unconditional branches, because this is the way almost all computer hardware operates at the lowest-level, so all modern programming languages use branching constructs to implement their most complex flow control devices.

Fortunately, libraries of macros have been developed that can implement more familiar syntax for many of these control structures. We will discuss these libraries in more detail in "PIR Standard Library".

Subroutines

Code reuse has become a cornerstone of modern software engineering. Common tasks are routinely packaged as libraries for later reuse by other developers. The most basic building block of code reuse is the "function" or "subroutine". A calculation like "the factorial of a number", for example, may be used several times in a large program. Subroutines allow this kind of functionality to be abstracted into a single stand-alone unit for reuse. PIR is a subroutine-based language in that all code in PIR must exist in a subroutine. Execution starts, as we have seen, in the :main subroutine, and others can be called to perform the tasks of a program. From subroutines we can construct more elaborate chunks of code reusability methods and objects. In this chapter we will talk about how subroutines work in PIR, and how they can be used by developers to create programs for Parrot.

Parrot supports multiple high-level languages, and each language uses a different syntax for defining and calling subroutines. The goal of PIR is not to be a high-level language in itself, but to provide the basic tools that other languages can use to implement them. PIR's syntax for subroutines may seem very primitive for this reason.

Parrot Calling Conventions

The way that Parrot calls a subroutine--by passing arguments, altering control flow, and returning results--is called the "Parrot Calling Conventions", or PCC. The details of PCC are generally hidden from the programmer, being partially implemented in C and being partially implemented in PASM. PIR has several constructs to gloss over these details, and the average programmer will not need to worry about them. PCC uses the Continuation Passing Style (CPS) to pass control to subroutines and back again. Again, the details of this can be largely ignored for developers who don't need it, but the power of this approach can be harnessed by those who do. We'll talk more about PCC and CPS in this and in later chapters as well.

Subroutine Calls

PIR's simplest subroutine call syntax looks much like a subroutine call from a high-level language. This example calls the subroutine fact with two arguments and assigns the result to $I0:

  $I0 = 'fact'(count, product)

This simple statement hides a great deal of complexity. It generates a subroutine PMC object, creates a continuation PMC object to return control flow after the subroutine, passes the arguments, looks up the subroutine by name (and by signature if it's been overloaded), it calls the subroutine, and finally it assigns the results of the call to the given register variables. This is quite a lot of work for a single statement, and that's ignoring the computational logic that the subroutine itself implements.

Expanded Subroutine Syntax

The single line subroutine call is incredibly convenient, but it isn't always flexible enough. So PIR also has a more verbose call syntax that is still more convenient than manual calls. This example pulls the subroutine fact out of the global symbol table into a PMC register and calls it:

  find_global $P1, "fact"

  .begin_call
    .arg count
    .arg product
    .call $P1
    .result $I0
  .end_call

The whole chunk of code from .begin_call to .end_call acts as a single unit. The .arg directive sets up and passes arguments to the call. The .call directive calls the subroutine, returns control flow after the subroutine has completed. The .result directive retrieves returned values from the call.

Subroutine Declarations

In addition to syntax for subroutine calls, PIR provides syntax for subroutine definitions. Subroutines are defined with the .sub directive, and end with the .end directive. We've already seen this syntax in our earlier examples. The .param defines input parameters and creates local named variables for them:

  .param int c

The .return directive allows the subroutine to return control flow to the calling subroutine, and optionally returns result output values.

Here's a complete code example that implements the factorial algorithm. The subroutine fact is a separate compilation unit, assembled and processed after the main function. Parrot resolves global symbols like the fact label between different units.

  # factorial.pir
  .sub main
     .local int count
     .local int product
     count = 5
     product = 1

     $I0 = 'fact'(count, product)

     print $I0
     print "\n"
     end
  .end

  .sub fact
     .param int c
     .param int p

  loop:
     if c <= 1 goto fin
     p = c * p
     dec c
     branch loop
  fin:
     .return (p)
  .end

This example defines two local named variables, count and product, and assigns them the values 1 and 5. It calls the fact subroutine passing the two variables as arguments. In the call, the two arguments are assigned to consecutive integer registers, because they're stored in typed integer variables. The fact subroutine uses .param and the .return directives for retrieving parameters and returning results. The final printed result is 120.

Execution of the program starts at the :main subroutine or, if no subroutines are declared with :main at the first subroutine in the file. If multiple subroutines are declared with :main, the last of them is treated as the starting point. Eventually, declaring multiple subroutines with :main might cause a syntax error or some other bad behavior, so it's not a good idea to rely on it now.

Named Parameters

Parameters that are passed in a strict order like we've seen above are called positional arguments. Positional arguments are differentiated from one another by their position in the function call. Putting positional arguments in a different order will produce different effects, or may cause errors. Parrot supports a second type of parameter, a named parameter. Instead of passing parameters by their position in the string, parameters are passed by name and can be in any order. Here's an example:

 .sub 'MySub'
    .param string yrs :named("age")
    .param string call :named("name")
    $S0 = "Hello " . call
    $S1 = "You are " . yrs
    $S1 = $S1 . " years old"
    print $S0
    print $S1
 .end

 .sub main :main
    'MySub'("age" => 42, "name" => "Bob")
 .end

In the example above, we could have easily reversed the order too:

 .sub main :main
    'MySub'("name" => "Bob", "age" => 42)    # Same!
 .end

Named arguments can be a big help because you don't have to worry about the exact order of variables, especially as argument lists get very long.

Optional Parameters

Sometimes there are parameters to a function that don't always need to be passed, or values for a parameter which should be given a default value if a different value hasn't been explicitly provided. Parrot provides a mechanism for allowing optional parameters to be specified, so an error won't be raised if the parameter isn't provided. Parrot also provides a flag value that can be tested to determine if an optional parameter has been provided or not, so a default value can be supplied.

Optional parameters are actually treated like two parameters: The value that may or may not be passed, and the flag value to determine if it has been or not. Here's an example declaration of an optional parameter:

  .param string name :optional
  .param int has_name :opt_flag

The :optional flag specifies that the given parameter is optional and does not necessarily need to be provided. The :opt_flag specifies that an integer parameter contains a boolean flag. This flag is true if the value was passed, and false otherwise. This means we can use logic like this to provide a default value:

  .param string name :optional
  .param int has_name :opt_flag
  if has_name goto we_have_a_name
      name = "Default value"
  we_have_a_name:

Optional parameters can be positional or named parameters. When using them with positional parameters, they must appear at the end of the list of positional parameters. Also, the :opt_flag parameter must always appear directly after the :optional parameter.

  .sub 'Foo'
    .param int optvalue :optional
    .param int hasvalue :opt_flag
    .param pmc notoptional          # WRONG!
    ...

  .sub 'Bar'
     .param int hasvalue :opt_flag
     .param int optvalue :optional  # WRONG!
     ...

  .sub 'Baz'
    .param int optvalue :optional
    .param pmc notoptional
    .param int hasvalue :opt_flag   # WRONG!
    ...

Optional parameters can also be mixed with named parameters:

  .sub 'MySub'
    .param int value :named("answer") :optional
    .param int has_value :opt_flag
    ...

This could be called in two ways:

  'MySub'("answer" => 42)  # with a value
  'MySub'()                # without

Sub PMCs

Subroutines are a PMC type in Parrot, and references to them can be stored in PMC registers and manipulated like other PMC types. You can get a subroutine in the current namespace with the get_global opcode:

  $P0 = get_global "MySubName"

Or, if you want to find a subroutine from a different namespace, you need to first select the namespace PMC and then pass that to get_global:

  $P0 = get_namespace "MyNamespace"
  $P1 = get_global $P0, "MySubName"

With a Sub PMC, there are lots of things you can do. You can obviously invoke it:

  $P0(1, 2, 3)

You can get its name or change its name:

  $S0 = $P0               # Get the current name
  $P0 = "MyNewSubName"    # Set a new name

You can get a hash of the complete metadata for the subroutine:

  $P1 = inspect $P0

The metadata fields in this hash are

• pos_required

The number of required positional parameters to the Sub

• pos_optional

The number of optional positional parameters to the Sub

• named_required

The number of required named parameters to the Sub

• named_optional

The number of optional named parameters to the Sub

• pos_slurpy

Returns 1 if the sub has a slurpy parameter to eat up extra positional args

• named_slurpy

Returns 1 if the sub has a slurpy parameter to eat up extra named args

Instead of getting the whole inspection hash, you can look for individual data items that you want:

  $I0 = inspect $P0, "pos_required"

If you want to get the total number of defined parameters to the Sub, you can call the arity method:

  $I0 = $P0.'arity'()

To get the namespace PMC that the Sub was defined into, you can call the get_namespace method:

  $P1 = $P0.'get_namespace'()

Subroutine PMCs are very useful things, and we will show more of their uses throughout this chapter.

The Commandline

Programs written in Parrot have access to arguments passed on the command line like any other program would.

  .sub MyMain :main
    .param pmc all_args :slurpy
    ...
  .end

Continuation Passing Style

Continuations are snapshots, a frozen image of the current execution state of the VM. Once we have a continuation we can invoke it to return to the point where the continuation was first created. It's like a magical timewarp that allows the developer to arbitrarily move control flow back to any previous point in the program there's actually no magic involved, just a lot of interesting ideas and involved code.

Continuations are not a new concept, they've been boggling the minds of Lisp and Scheme programmers for many years. However, despite all their power and flexibility they haven't been well-utilized in most modern programming languages or in their underlying libraries and virtual machines. Parrot aims to change that: In Parrot, almost every control flow manipulation including all subroutine, method, and coroutine calls, are performed using continuations. This mechanism is mostly hidden from developers who build applications on top of Parrot. The power and flexibility is available if people want to use it, but it's hidden behind more familiar constructs if not.

Doing all sorts of flow control using continuations is called Continuation Passing Style (CPS). CPS allows parrot to offer all sorts of neat features, such as tail-call optimizations and lexical subroutines.

Tailcalls

In many cases, a subroutine will set up and call another subroutine, and then return the result of the second call directly. This is called a tailcall, and is an important opportunity for optimization. Here's a contrived example in pseudocode:

  call add_two(5)

  subroutine add_two(value)
    value = add_one(value)
    return add_one(value)

In this example, the subroutine add_two makes two calls to c<add_one>. The second call to add_one is used as the return value. add_one is called and its result is immediately returned to the caller of add_two, it is never stored in a local register or variable in add_two, it's immediately returned. We can optimize this situation if we realize that the second call to add_one is returning to the same place that add_two is, and therefore can utilize the same return continuation as add_two uses. The two subroutine calls can share a return continuation, instead of having to create a new continuation for each call.

In PIR code, we use the .tailcall directive to make a tailcall like this, instead of the .return directive. .tailcall performs this optimization by reusing the return continuation of the parent function to make the tailcall. In PIR, we can write this example:

  .sub main :main
      .local int value
      value = add_two(5)
      say value
  .end

  .sub add_two
      .param int value
      .local int val2
      val2 = add_one(value)
      .tailcall add_one(val2)
  .end

  .sub add_one
      .param int a
      .local int b
      b = a + 1
      .return (b)
  .end

This example above will print out the correct value "7".

Creating and Using Continuations

Most often continuations are used implicitly by the other control-flow operations in Parrot. However, they can also be created and used explicitly when required. Continuations are like any other PMC, and can be created using the new keyword:

  $P0 = new 'Continuation'

The new continuation starts off in an undefined state. Attempting to invoke a new continuation after it's first been created will raise an exception. To prepare the continuation for use, a destination label must be assigned to it with the set_addr opcode:

    $P0 = new 'Continuation'
    set_addr $P0, my_label

  my_label:
    ...

To jump to the continuation's stored label and return the context to the state it was in when the continuation was created, use the invoke opcode or the () notation:

  invoke $P0  # Explicit using "invoke" opcode
  $P0()       # Same, but nicer syntax

Notice that even though you can use the subroutine notation $P0() to invoke the continuation, it doesn't make any sense to try and pass arguments to it or to try and return values from it:

  $P0 = new 'Continuation'
  set_addr $P0, my_label

  $P0(1, 2)      # WRONG!

  $P1 = $P0()    # WRONG!

Lexical Subroutines

As we've mentioned above, Parrot offers support for lexical subroutines. What this means is that we can define a subroutine by name inside a larger subroutine, and our "inner" subroutine is only visible and callable from the "outer" outer. The "inner" subroutine inherits all the lexical variables from the outer subroutine, but is able to define its own lexical variables that cannot be seen or modified by the outer subroutine. This is important because PIR doesn't have anything corresponding to blocks or nested scopes like some other languages have. Lexical subroutines play the role of nested scopes when they are needed.

If the subroutine is lexical, you can get its :outer with the get_outer method on the Sub PMC:

  $P1 = $P0.'get_outer'()

If there is no :outer PMC, this returns a NULL PMC. Conversely, you can set the outer sub:

  $P0.'set_outer'($P1)

Scope and HLLs

Let us diverge for a minute and start looking forward at the idea of High Level Languages (HLLs) such as Perl, Python, and Ruby. All of these languages allow nested scopes, or blocks within blocks that can have their own lexical variables. Let's look back at the C programming language, where this kind of construct is not uncommon:

  {
      int x = 0;
      int y = 1;
      {
          int z = 2;
          // x, y, and z are all visible here
      }
      // only x and y are visible here
  }

The code above illustrates this idea perfectly without having to get into a detailed and convoluted example: In the inner block, we define the variable z which is only visible inside that block. The outer block has no knowledge of z at all. However, the inner block does have access to the variables x and y. This is an example of nested scopes where the visibility of different data items can change in a single subroutine. As we've discussed above, Parrot doesn't have any direct analog for this situation: If we tried to write the code above directly, we would end up with this PIR code:

  .param int x
  .param int y
  .param int z
  x = 0
  y = 1
  z = 2
  ...

This PIR code is similar, but the handling of the variable z is different: z is visible throughout the entire current subroutine, where it is not visible throughout the entire C function. To help approximate this effect, PIR supplies lexical subroutines to create nested lexical scopes.

PIR Scoping

In PIR, there is only one structure that supports scoping like this: the subroutine and objects that inherit from subroutines, such as methods, coroutines, and multisubs, which we will discuss later. There are no blocks in PIR that have their own scope besides subroutines. Fortunately, we can use these lexical subroutines to simulate this behavior that HLLs require:

  .sub 'MyOuter'
      .local int x,y
      .lex 'x', x
      .lex 'y', y
      'MyInner'()
      # only x and y are visible here
  .end

  .sub 'MyInner' :outer('MyOuter')
      .local int z
      .lex 'z', z
      #x, y, and z are all "visible" here
  .end

In the example above we put the word "visible" in quotes. This is because lexically-defined variables need to be accessed with the get_lex and set_lex opcodes. These two opcodes don't just access the value of a register, where the value is stored while it's being used, but they also make sure to interact with the LexPad PMC that's storing the data. If the value isn't properly stored in the LexPad, then they won't be available in nested inner subroutines, or available from :outer subroutines either.

Lexical Variables

As we have seen above, we can declare a new subroutine to be a nested inner subroutine of an existing outer subroutine using the :outer flag. The outer flag is used to specify the name of the outer subroutine. Where there may be multiple subroutines with the same name such is the case with multisubs, which we will discuss soon, we can use the :subid flag on the outer subroutine to give it a different--and unique--name that the lexical subroutines can reference in their :outer declarations. Within lexical subroutines, the .lex command defines a local variable that follows these scoping rules.

LexPad and LexInfo PMCs

Information about lexical variables in a subroutine is stored in two different types of PMCs: The LexPad PMC that we already mentioned briefly, and the LexInfo PMCs which we haven't. Neither of these PMC types are really usable from PIR code, but are instead used by Parrot internally to store information about lexical variables.

LexInfo PMCs are used to store information about lexical variables at compile time. This is read-only information that is generated during compilation to represent what is known about lexical variables. Not all subroutines get a LexInfo PMC by default, you need to indicate to Parrot somehow that you require a LexInfo PMC to be created. One way to do this is with the .lex directive that we looked at above. Of course, the .lex directive only works for languages where the names of lexical variables are all known at compile time. For languages where this information isn't known, the subroutine can be flagged with :lex instead.

LexPad PMCs are used to store run-time information about lexical variables. This includes their current values and their type information. LexPad PMCs are created at runtime for subs that have a LexInfo PMC already. These are created each time the subroutine is invoked, which allows for recursive subroutine calls without overwriting variable names.

With a Subroutine PMC, you can get access to the associated LexInfo PMC by calling the 'get_lexinfo' method:

  $P0 = find_global "MySubroutine"
  $P1 = $P0.'get_lexinfo'()

Once you have the LexInfo PMC, there are a limited number of operations that you can call with it:

  $I0 = elements $P1    # Get the number of lexical variables from it
  $P0 = $P1["name"]     # Get the entry for lexical variable "name"

There really isn't much else useful to do with LexInfo PMCs, they're mostly used by Parrot internally and aren't helpful to the PIR programmer.

There is no easy way to get a reference to the current LexPad PMC in a given subroutine, but like LexInfo PMCs that doesn't matter because they aren't useful from PIR anyway. Remember that subroutines themselves can be lexical and that therefore the lexical environment of a given variable can extend to multiple subroutines and therefore multiple LexPads. The opcodes find_lex and store_lex automatically search through nested LexPads recursively to find the proper environment information about the given variables.

Compilation Units Revisited

The term "compilation unit" is one that's been bandied about throughout the chapter and it's worth some amount of explanation here. A compilation unit is a section of code that forms a single unit. In some instances the term can be used to describe an entire file. In most other cases, it's used to describe a single subroutine. Our earlier example which created a 'fact' subroutine for calculating factorials could be considered to have used two separate compilation units: The main subroutine and the fact subroutine. Here is a way to rewrite that algorithm using only a single subroutine instead:

  .sub main
      $I1 = 5           # counter
      bsr fact
      say $I0
      $I1 = 6           # counter
      bsr fact
      say $I0
      end

  fact:
      $I0 = 1           # product
  L1:
      $I0 = $I0 * $I1
      dec $I1
      if $I1 > 0 goto L1
      ret
  .end

The unit of code from the fact label definition to ret is a reusable routine, but is only usable from within the main subroutine. There are several problems with this simple approach. In terms of the interface, the caller has to know to pass the argument to fact in $I1 and to get the result from $I0. This is different from how subroutines are normally invoked in PIR.

Another disadvantage of this approach is that main and fact share the same compilation unit, so they're parsed and processed as one piece of code. They share registers. They would also share LexInfo and LexPad PMCs, if any were needed by main. The fact routine is also not easily usable from outside the c<main> subroutine, so other parts of your code won't have access to it. This is a problem when trying to follow normal encapsulation guidelines.

Namespaces, Methods, and VTABLES

PIR provides syntax to simplify writing methods and method calls for object-oriented programming. We've seen some method calls in the examples above, especially when we were talking about the interfaces to certain PMC types. We've also seen a little bit of information about classes and objects in the previous chapter. PIR allows you to define your own classes, and with those classes you can define method interfaces to them. Method calls follow the same Parrot calling conventions that we have seen above, including all the various parameter configurations, lexical scoping, and other aspects we have already talked about.

Classes can be defined in two ways: in C and compiled to machine code, and in PIR. The former is how the built-in PMC types are defined, like ResizablePMCArray, or Integer. These PMC types are either built with Parrot at compile time, or are compiled into a shared library called a dynpmc and loaded into Parrot at runtime. We will talk about writing PMCs in C, and dealing with dynpmcs in chapter 11.

The second type of class can be defined in PIR at runtime. We saw some examples of this in the last chapter using the newclass and subclass opcodes. We also talked about class attribute values. Now, we're going to talk about associating subroutines with these classes, and they're called methods. Methods are just like other normal subroutines with two major changes: they are marked with the :method flag, and they exist in a namespace. Before we can talk about methods, we need to discuss namespaces first.

Namespaces

Namespaces provide a mechanism where names can be reused. This may not sound like much, but in large complicated systems, or systems with many included libraries, it can be very handy. Each namespace gets its own area for function names and global variables. This way you can have multiple functions named create or new or convert, for instance, without having to use Multi-Method Dispatch (MMD) which we will describe later. Namespaces are also vital for defining classes and their methods, which we already mentioned. We'll talk about all those uses here.

Namespaces are specified with the .namespace [] directive. The brackets are not optional, but the keys inside them are. Here are some examples:

  .namespace [ ]               # The root namespace
  .namespace [ "Foo" ]         # The namespace "Foo"
  .namespace [ "Foo" ; "Bar" ] # Namespace Foo::Bar
  .namespace                   # WRONG! The [] are needed

Using semicolons, namespaces can be nested to any arbitrary depth. Namespaces are special types of PMC, so we can access them and manipulate them just like other data objects. We can get the PMC for the root namespace using the get_root_namespace opcode:

  $P0 = get_root_namespace

The current namespace, which might be different from the root namespace can be retrieved with the get_namespace opcode:

  $P0 = get_namespace             # get current namespace PMC
  $P0 = get_namespace ["Foo"]     # get PMC for namespace "Foo"

Namespaces are arranged into a large n-ary tree. There is the root namespace at the top of the tree, and in the root namespace are various special HLL namespaces. Each HLL compiler gets its own HLL namespace where it can store its data during compilation and runtime. Each HLL namespace may have a large hierarchy of other namespaces. We'll talk more about HLL namespaces and their significance in chapter 10.

The root namespace is a busy place. Everybody could be lazy and use it to store all their subroutines and global variables, and then we would run into all sorts of collisions. One library would define a function "Foo", and then another library could try to create another subroutine with the same name. This is called namespace pollution, because everybody is trying to put things into the root namespace, and those things are all unrelated to each other. Best practices requires that namespaces be used to hold private information away from public information, and to keep like things together.

As an example, the namespace Integers could be used to store subroutines that deal with integers. The namespace images could be used to store subroutines that deal with creating and manipulating images. That way, when we have a subroutine that adds two numbers together, and a subroutine that performs additive image composition, we can name them both add without any conflict or confusion. And within the image namespace we could have sub namespaces for jpeg and MRI and schematics, and each of these could have a add method without getting into each other's way.

The short version is this: use namespaces. There aren't any penalties to them, and they do a lot of work to keep things organized and separated.

Namespace PMC

The .namespace directive that we've seen sets the current namespace. In PIR code, we have multiple ways to address a namespace:

  # Get namespace "a/b/c" starting at the root namespace
  $P0 = get_root_namespace ["a" ; "b" ; "c"]

  # Get namespace "a/b/c" starting in the current HLL namespace.
  $P0 = get_hll_namespace ["a" ; "b" ; "c"]
  # Same
  $P0 = get_root_namespace ["hll" ; "a" ; "b" ; "c"]

  # Get namespace "a/b/c" starting in the current namespace
  $P0 = get_namespace ["a" ; "b" ; "c"]

Once we have a namespace PMC we can retrieve global variables and subroutine PMCs from it using the following functions:

  $P1 = get_global $S0            # Get global in current namespace
  $P1 = get_global ["Foo"], $S0   # Get global in namespace "Foo"
  $P1 = get_global $P0, $S0       # Get global in $P0 namespace PMC

Operations on the Namespace PMC

We've seen above how to find a Namespace PMC. Once you have it, there are a few things you can do with it. You can find methods and variables that are stored in the namespace, or you can add new ones:

  $P0 = get_namespace
  $P0.'add_namespace'($P1)      # Add Namespace $P1 to $P0
  $P1 = $P0.'find_namespace'("MyOtherNamespace")

  # Find namespace "MyNamespace" in $P0, create it if it
  #    doesn't exist
  $P1 = $P0.'make_namespace'("MyNamespace")

  $P0.'add_sub'("MySub", $P2)   # Add Sub PMC $P2 to the namespace
  $P1 = $P0.'find_sub'("MySub") # Find it

  $P0.'add_var'("MyVar", $P3)   # Add variable "MyVar" in $P3
  $P1 = $P0.'find_var'("MyVar") # Find it

  # Return the name of Namespace $P0 as a ResizableStringArray
  $P3 = $P0.'get_name'()

  # Find the parent namespace that contains this one:
  $P5 = $P0.'get_parent'()

  # Get the Class PMC associated with this namespace:
  $P6 = $P0.'get_class'()

There are a few other operations that can be done on Namespaces, but none as interesting as these. We'll talk about Namespaces throughout the rest of this chapter.

Calling Methods

Now that we've discussed namespaces, we can start to discuss all the interesting things that namespaces enable, like object-oriented programming and method calls. Methods are just like subroutines, except they are invoked on a object PMC, and that PMC is passed as the c<self> parameter.

The basic syntax for a method call is similar to the single line subroutine call above. It takes a variable for the invocant PMC and a string with the name of the method:

  object."methodname"(arguments)

Notice that the name of the method must be contained in quotes. If the name of the method is not contained in quotes, it's treated as a named variable that does. Here's an example:

  .local string methname = "Foo"
  object.methname()               # Same as object."Foo"()
  object."Foo"()                  # Same

The invocant can be a variable or register, and the method name can be a literal string, string variable, or method object PMC.

Defining Methods

Methods are defined like any other subroutine except with two major differences: They must be inside a namespace named after the class they are a part of, and they must use the :method flag.

  .namespace [ "MyClass"]

  .sub "MyMethod" :method
    ...

Inside the method, the invocant object can be accessed using the self keyword. self isn't the only name you can call this value, however. You can also use the :invocant flag to define a new name for the invocant object:

(See TT #483)

  .sub "MyMethod" :method
    $S0 = self                    # Already defined as "self"
    say $S0
  .end

  .sub "MyMethod2" :method
    .param pmc item :invocant     # "self" is now called "item"
    $S0 = item
    say $S0
  .end

This example defines two methods in the Foo class. It calls one from the main body of the subroutine and the other from within the first method:

  .sub main
    .local pmc class
    .local pmc obj
    newclass class, "Foo"       # create a new Foo class
    new obj, "Foo"              # instantiate a Foo object
    obj."meth"()                # call obj."meth" which is actually
    print "done\n"              # in the "Foo" namespace
    end
  .end

  .namespace [ "Foo" ]          # start namespace "Foo"

  .sub meth :method             # define Foo::meth global
     print "in meth\n"
     $S0 = "other_meth"         # method names can be in a register too
     self.$S0()                 # self is the invocant
  .end

  .sub other_meth :method       # define another method
     print "in other_meth\n"    # as above Parrot provides a return
  .end                          # statement

Each method call looks up the method name in the object's class namespace. The .sub directive automatically makes a symbol table entry for the subroutine in the current namespace.

When a .sub is declared as a :method, it automatically creates a local variable named self and assigns it the object passed in P2. You don't need to write .param pmc self to get it, it comes free with the method.

You can pass multiple arguments to a method and retrieve multiple return values just like a single line subroutine call:

  (res1, res2) = obj."method"(arg1, arg2)

VTABLEs

PMCs all subscribe to a common interface of functions called VTABLEs. Every PMC implements the same set of these interfaces, which perform very specific low-level tasks on the PMC. The term VTABLE was originally a shortened form of the name "virtual function table", although that name isn't used any more by the developers, or in any of the documentation In fact, if you say "virtual function table" to one of the developers, they probably won't know what you are talking about. The virtual functions in the VTABLE, called VTABLE interfaces, are similar to ordinary functions and methods in many respects. VTABLE interfaces are occasionally called "VTABLE functions", or "VTABLE methods" or even "VTABLE entries" in casual conversation. A quick comparison shows that VTABLE interfaces are not really subroutines or methods in the way that those terms have been used throughout the rest of Parrot. Like methods on an object, VTABLE interfaces are defined for a specific class of PMC, and can be invoked on any member of that class. Likewise, in a VTABLE interface declaration, the self keyword is used to describe the object that it is invoked upon. That's where the similarities end, however. Unlike ordinary subroutines or methods, VTABLE methods cannot be invoked directly, they are also not inherited through class hierarchies like how methods are. With all this terminology discussion out of the way, we can start talking about what VTABLES are and how they are used in Parrot.

VTABLE interfaces are the primary way that data in the PMC is accessed and modified. VTABLES also provide a way to invoke the PMC if it's a subroutine or subroutine-like PMC. VTABLE interfaces are not called directly from PIR code, but are instead called internally by Parrot to implement specific opcodes and behaviors. For instance, the invoke opcode calls the invoke VTABLE interface of the subroutine PMC, while the inc opcode on a PMC calls the increment VTABLE interface on that PMC. What VTABLE interface overrides do, in essence, is to allow the programmer to change the very way that Parrot accesses PMC data in the most fundamental way, and changes the very way that the opcodes act on that data.

PMCs, as we will look at more closely in later chapters, are typically implemented using PMC Script, a layer of syntax and macros over ordinary C code. A PMC compiler program converts the PMC files into C code for compilation as part of the ordinary build process. However, VTABLE interfaces can be written and overwritten in PIR using the :vtable flag on a subroutine declaration. This technique is used most commonly when subclassing an existing PMC class in PIR code to create a new data type with custom access methods.

VTABLE interfaces are declared with the :vtable flag:

  .sub 'set_integer' :vtable
      #set the integer value of the PMC here
  .end

in which case the subroutine must have the same name as the VTABLE interface it is intended to implement. VTABLE interfaces all have very specific names, and you can't override one with just any arbitrary name. However, if you would like to name the function something different but still use it as a VTABLE interface, you could add an additional name parameter to the flag:

  .sub 'MySetInteger' :vtable('set_integer')
      #set the integer value of the PMC here
  .end

VTABLE interfaces are often given the :method flag also, so that they can be used directly in PIR code as methods, in addition to being used by Parrot as VTABLE interfaces. This means we can have the following:

  .namespace [ "MyClass" ]

  .sub 'ToString' :vtable('get_string') :method
      $S0 = "hello!"
      .return($S0)
  .end

  .namespace [ "OtherClass" ]

  .local pmc myclass = new "MyClass"
  say myclass                 # say converts to string internally
  $S0 = myclass               # Convert to a string, store in $S0
  $S0 = myclass.'ToString'()  # The same

Inside a VTABLE interface definition, the self local variable contains the PMC on which the VTABLE interface is invoked, just like in a method declaration.

Roles

As we've seen above and in the previous chapter, Class PMCs and NameSpace PMCs work to keep classes and methods together in a logical way. There is another factor to add to this mix: The Role PMC.

Roles are like classes, but don't stand on their own. They represent collections of methods and VTABLES that can be added into an existing class. Adding a role to a class is called composing that role, and any class that has been composed with a role does that role.

Roles are created as PMC and can be manipulated through opcodes and methods like other PMCs:

  $P0 = new 'Role'
  $P1 = get_global "MyRoleSub"
  $P0.'add_method'("MyRoleSub", $P1)

Once we've created a role and added methods to it, we can add that role to a class, or even to another role:

  $P1 = new 'Role'
  $P2 = new 'Class'
  $P1.'add_role'($P0)
  $P2.'add_role'($P0)
  add_role $P2, $P0    # Same!

Now that we have added the role, we can check whether we implement it:

  $I0 = does $P2, $P0  # Yes

We can get a list of roles from our Class PMC:

  $P3 = $P2.'roles'()

Roles are very useful for ensuring that related classes all implement a common interface.

Coroutines

We've mentioned coroutines several times before, and we're finally going to explain what they are. Coroutines are similar to subroutines except that they have an internal notion of state And the cool new name!. Coroutines, in addition to performing a normal .return to return control flow back to the caller and destroy the lexical environment of the subroutine, may also perform a .yield operation. .yield returns a value to the caller like .return can, but it does not destroy the lexical state of the coroutine. The next time the coroutine is called, it continues execution from the point of the last .yield, not at the beginning of the coroutine.

In a Coroutine, when we continue from a .yield, the entire lexical environment is the same as it was when .yield was called. This means that the parameter values don't change, even if we call the coroutine with different arguments later.

Defining Coroutines

Coroutines are defined like any ordinary subroutine. They do not require any special flag or any special syntax to mark them as being a coroutine. However, what sets them apart is the use of the .yield directive. .yield plays several roles:

• Identifies coroutines

When Parrot sees a yield, it knows to create a Coroutine PMC object instead of a Sub PMC.

• Creates a continuation

Continuations, as we have already seen, allow us to continue execution at the point of the continuation later. It's like a snapshot of the current execution environment. .yield creates a continuation in the coroutine and stores the continuation object in the coroutine object or later resuming from the point of the .yield.

• Returns a value

.yield can return a value or many values, or no values to the caller. It is basically the same as a .return in this regard.

Here is a quick example of a simple coroutine:

  .sub MyCoro
    .yield(1)
    .yield(2)
    .yield(3)
    .return(4)
  .end

  .sub main :main
    $I0 = MyCoro()    # 1
    $I0 = MyCoro()    # 2
    $I0 = MyCoro()    # 3
    $I0 = MyCoro()    # 4
    $I0 = MyCoro()    # 1
    $I0 = MyCoro()    # 2
    $I0 = MyCoro()    # 3
    $I0 = MyCoro()    # 4
    $I0 = MyCoro()    # 1
    $I0 = MyCoro()    # 2
    $I0 = MyCoro()    # 3
    $I0 = MyCoro()    # 4
  .end

This is obviously a contrived example, but it demonstrates how the coroutine stores it's state. The coroutine stores it's state when we reach a .yield directive, and when the coroutine is called again it picks up where it last left off. Coroutines also handle parameters in a way that might not be intuitive. Here's an example of this:

  .sub StoredConstant
    .param int x
    .yield(x)
    .yield(x)
    .yield(x)
  .end

  .sub main :main
    $I0 = StoredConstant(5)       # $I0 = 5
    $I0 = StoredConstant(6)       # $I0 = 5
    $I0 = StoredConstant(7)       # $I0 = 5
    $I0 = StoredConstant(8)       # $I0 = 8
  .end

Notice how even though we are calling the StoredConstant coroutine with different arguments each time, the value of parameter x doesn't change until the coroutine's state resets after the last .yield. Remember that a continuation takes a snapshot of the current state, and the .yield directive takes a continuation. The next time we call the coroutine, it invokes the continuation internally, and returns us to the exact same place in the exact same condition as we were when we called the .yield. In order to reset the coroutine and enable it to take a new parameter, we must either execute a .return directive or reach the end of the coroutine.

Multiple Dispatch

Multiple dispatch is when there are multiple subroutines in a single namespace with the same name. These functions must differ, however, in their parameter list, or "signature". All subs with the same name get put into a single PMC called a MultiSub. The MultiSub is like a list of subroutines. When the multisub is invoked, the MultiSub PMC object searches through the list of subroutines and searches for the one with the closest matching signature. The best match is the sub that gets invoked.

Defining MultiSubs

MultiSubs are subroutines with the :multi flag applied to them. MultiSubs (also called "Multis") must all differ from one another in the number and/or type of arguments passed to the function. Having two multisubs with the same function signature could result in a parsing error, or the later function could overwrite the former one in the multi.

Multisubs are defined like this:

  .sub 'MyMulti' :multi
      # does whatever a MyMulti does
  .end

Multis belong to a specific namespace. Functions in different namespaces with the same name do not conflict with each other this is one of the reasons for having multisubs in the first place!. It's only when multiple functions in a single namespace need to have the same name that a multi is used.

Multisubs take a special designator called a multi signature. The multi signature tells Parrot what particular combination of input parameters the multi accepts. Each multi will have a different signature, and Parrot will be able to dispatch to each one depending on the arguments passed. The multi signature is specified in the :multi directive:

  .sub 'Add' :multi(I, I)
    .param int x
    .param int y
    .return(x + y)
  .end

  .sub 'Add' :multi(N, N)
    .param num x
    .param num y
    .return(x + y)
  .end

  .sub Start :main
    $I0 = Add(1, 2)      # 3
    $N0 = Add(3.14, 2.0) # 5.14
    $S0 = Add("a", "b")  # ERROR! No (S, S) variant!
  .end

Multis can take I, N, S, and P types, but they can also use _ (underscore) to denote a wildcard, and a string that can be the name of a particular PMC type:

  .sub 'Add' :multi(I, I)  # Two integers
    ...

  .sub 'Add' :multi(I, 'Float')  # An integer and Float PMC
    ...

                           # Two Integer PMCs
  .sub 'Add' :multi('Integer', _)
    ...

When we call a multi PMC, Parrot will try to take the most specific best-match variant, and will fall back to more general variants if a perfect best-match cannot be found. So if we call 'Add'(1, 2), Parrot will dispatch to the (I, I) variant. If we call 'Add'(1, "hi"), Parrot will match the (I, _) variant, since the string in the second argument doesn't match I or 'Float'. Parrot can also choose to automatically promote one of the I, N, or S values to an Integer, Float, or String PMC.

To make the decision about which multi variant to call, Parrot takes a Manhattan Distance between the two. Parrot calculates the distance between the multi signatures and the argument signature. Every difference counts as one step. A difference can be an autobox from a primitive type to a PMC, or the conversion from one primitive type to another, or the matching of an argument to a _ wildcard. After Parrot calculates the distance to each variant, it calls the function with the lowest distance. Notice that it's possible to define a variant that is impossible to call: for every potential combination of arguments there is a better match. This isn't necessarily a common occurrence, but it's something to watch out for in systems with a lot of multis and a limited number of data types in use.

Classes and Objects

It may seem more appropriate for a discussion of PIR's support for classes and objects to reside in its own chapter, instead of appearing in a generic chapter about PIR programming "basics". However, part of PIR's core functionality is its support for object-oriented programming. PIR doesn't use all the fancy syntax as other OO languages, and it doesn't even support all the features that most modern OO languages have. What PIR does have is support for some of the basic structures and abilities, the necessary subset to construct richer and higher-level object systems.

PMCs as Classes

PMCs aren't exactly "classes" in the way that this term is normally used in object-oriented programming languages. They are polymorphic data items that can be one of a large variety of predefined types. As we have seen briefly, and as we will see in more depth later, PMCs have a standard interface called the VTABLE interface. VTABLEs are a standard list of functions that all PMCs implement or, PMCs can choose not to implement each interface explicitly and instead let Parrot call the default implementations.

VTABLEs are very strict: There are a fixed number with fixed names and fixed argument lists. You can't just create any random VTABLE interface that you want to create, you can only make use of the ones that Parrot supplies and expects. To circumvent this limitation, PMCs may have METHODS in addition to VTABLEs. METHODs are arbitrary code functions that can be written in C, may have any name, and may implement any behavior.

VTABLE Interfaces

Internally, all operations on PMCs are performed by calling various VTABLE interfaces.

Class and Object PMCs

The details about various PMC classes are managed by the Class PMC. Class PMCs contain information about the class, available methods, the inheritance hierarchy of the class, and various other details. Classes can be created with the newclass opcode:

  $P0 = newclass "MyClass"

Once we have created the class PMC, we can instantiate objects of that class using the new opcode. The new opcode takes either the class name or the Class PMC as an argument:

  $P1 = new $P0        # $P0 is the Class PMC
  $P2 = new "MyClass"  # Same

The new opcode can create two different types of PMC. The first type are the built-in core PMC classes. The built-in PMCs are written in C and cannot be extended from PIR without subclassing. However, you can also create user-defined PMC types in PIR. User-defined PMCs use the Object PMC type for instantiation. Object PMCs are used for all user-defined type and keep track of the methods and VTABLE override definitions. We're going to talk about methods and VTABLE overrides in the next chapter.

Subclassing PMCs

Existing built-in PMC types can be subclassed to associate additional data and methods with that PMC type. Subclassed PMC types act like their PMC base types, by sharing the same VTABLE methods and underlying data types. However, the subclass can define additional methods and attribute data storage. If necessary new VTABLE interfaces can be defined in PIR and old VTABLE methods can be overridden using PIR. We'll talk about defining methods and VTABLE interface overrides in the next chapter.

Creating a new subclass of an existing PMC class is done using the subclass keyword:

  # create an anonymous subclass
  $P0 = subclass 'ResizablePMCArray'

  # create a subclass named "MyArray"
  $P0 = subclass 'ResizablePMCArray', 'MyArray'

This returns a Class PMC which can be used to create and modify the class by adding attributes or creating objects of that class. You can also use the new class PMC to create additional subclasses:

  $P0 = subclass 'ResizablePMCArray', 'MyArray'
  $P1 = subclass $P0, 'MyOtherArray'

Once you have created these classes, you can create them like normal with the new keyword:

  $P0 = new 'MyArray'
  $P1 = new 'MyOtherArray'

Attributes

Classes and subclasses can be given attributes in addition to methods, which we will talk about in the next chapter which are named data fields. Attributes are created with the addattribute opcode, and can be set and retrieved with the setattribute and getattribute opcodes respectively:

  # Create the new class with two attributes
  $P0 = newclass 'MyClass'
  addattribute $P0, 'First'
  addattribute $P0, 'Second'

  # Create a new item of type MyClass
  $P1 = new 'MyClass'

  # Set values to the attributes
  setattribute $P1, 'First', 'First Value'
  setattribute $P1, 'Second', 'Second Value'

  # Get the attribute values
  $S0 = getattribute $P1, 'First'
  $S1 = getattribute $P1, 'Second'

Those values added as attributes don't need to be strings, even though both of the ones in the example are. They can be integers, numbers or PMCs too.

Input and Output

Like almost everything else in Parrot, input and output are handled by PMCs. Using the print opcode or the say opcode like we've already seen in some examples does this internally without your knowledge. However, we can do it explicitly too. First we'll talk about basic I/O, and then we will talk about using PMC-based filehandles for more advanced operations.

Basic I/O Opcodes

We've seen print and say. These are carry-over artifacts from Perl, when Parrot was simply the VM backend to the Perl 6 language. print prints the given string argument, or the stringified form of the argument, if it's not a string, to standard output. say does the same thing but also appends a trailing newline to it. Another opcode worth mentioning is the printerr opcode, which prints an argument to the standard error output instead.

We can read values from the standard input using the read and readline ops. read takes an integer value and returns a string with that many characters. readline reads an entire line of input from the standard input, and returns the string without the trailing newline. Here is a simple echo program that reads in characters from the user and echos them to standard output:

  .sub main
    loop_top:
      $S0 = read 10
      print $S0
      goto loop_top
  .end

Filehandles

The ops we have seen so far are useful if all your I/O operations are limited to the standard streams. However, there are plenty of other places where you might want to get data from and send data to. Things like files, sockets, and databases all might need to have data sent to them. These things can be done by using a file handle.

Filehandles are PMCs that describe a file and keep track of an I/O operations internal state. We can get Filehandles for the standard streams using dedicated opcodes:

  $P0 = getstdin    # Standard input handle
  $P1 = getstdout   # Standard output handle
  $P2 = getstderr   # Standard error handle

If we have a file, we can create a handle to it using the open op:

  $P0 = open "my/file/name.txt"

We can also specify the exact mode that the file handle will be in:

  $P0 = open "my/file/name.txt", "wa"

The mode string at the end should be familiar to C programmers, because they are mostly the same values:

  r  : read
  w  : write
  wa : append
  p  : pipe

So if we want a handle that we can read and write to, we write the mode string "rw". If we want to be able to read and write to it, but we don't want write operations to overwrite the existing contents, we use "rwa" instead.

When we are done with a filehandle that we've created, we can shut it down with the close op. Notice that we don't want to be closing any of the standard streams.

  close $P0

With a filehandle, we can perform all the same operations as we could earlier, but we pass the filehandle as an additional argument to tell the op where to write or read the data from.

  print "hello"       # Write "hello!" to STDOUT

  $P0 = getstdout
  print $P0, "hello"  # Same, but more explicit

  say $P0, " world!"  # say to STDOUT

  $P1 = open "myfile.txt", "wa"
  print $P1, "foo"    # Write "foo" to myfile.txt

Filehandle PMCs

Let's see a little example of a program that reads in data from a file, and prints it to STDOUT.

  .sub main
    $P0 = getstdout
    $P1 = open "myfile.txt", "r"
    loop_top:
      $S0 = readline $P1
      print $P0, $S0
      if $P1 goto loop_top
    close $P1
  .end

This example shows that treating a filehandle PMC like a boolean value returns whether or not we have reached the end of the file. A true return value means there is more file to read. A false return value means we are at the end. In addition to this behavior, Filehandle PMCs have a number of methods that can be used to perform various operations.

$P0.'open'(STRING filename, STRING mode)

Opens the filehandle. Takes two optional strings: the name of the file to open and the open mode. If no filename is given, the previous filename associated with the filehandle is opened. If no mode is given, the previously-used mode is used.

  $P0 = new 'Filehandle'
  $P0.'open'("myfile.txt", "r")

  $P0 = open "myfile.txt", "r"   # Same!

The open opcode internally creates a new filehandle PMC and calls the 'open'() method on it. So even though the above two code snippets act in an identical way, the later one is a little more concise to write. The caveat is that the open opcode creates a new PMC for every call, while the 'open'() method call can reuse an existing filehandle PMC for a new file.

$P0.'isatty'()

Returns a boolean value whether the filehandle is a TTY terminal

$P0.'close'()

Closes the filehandle. Can be reopened with .'open' later.

  $P0.'close'()

  close $P0   # Same

The close opcode calls the 'close'() method on the Filehandle PMC internally, so these two calls are equivalent.

$P0.'is_closed'()

Returns true if the filehandle is closed, false if it is opened.

$P0.'read'(INTVAL length)

Reads length bytes from the filehandle.

  $S0 = read $P0, 10

  $P0.'read'(10)

The two calls are equivalent, and the read opcode calls the 'read'() method internally.

$P0.'readline'()

Reads an entire line (up to a newline character or EOF) from the filehandle.

$P0.'readline_interactive'(STRING prompt)

Displays the string prompt and then reads a line of input.

$P0.'readall'(STRING name)

Reads the entire file name into a string. If the filehandle is closed, it will open the file given by name, read the entire file, and then close the handle. If the filehandle is already open, name should not be passed (it is an optional parameter).

$P0.'flush'()

Flushes the buffer

$P0.'print'(PMC to_print)

Prints the given value to the filehandle. The print opcode uses the 'print'() method internally.

  print "Hello"

  $P0 = getstdout
  print $P0, "Hello!"    # Same

  $P0.'print'("Hello!")  # Same

$P0.'puts'(STRING to_print)

Prints the given string value to the filehandle

$P0.'buffer_type'(STRING new_type)

If new_type is given, changes the buffer to the new type. If it is not, returns the current type. Acceptable types are:

  unbuffered
  line-buffered
  full-buffered

$P0.'buffer_size'(INTVAL size)

If size is given, set the size of the buffer. If not, returns the size of the current buffer.

$P0.'mode'()

Returns the current file access mode.

$P0.'encoding'(STRING encoding)

Sets the filehandle's string encoding to encoding if given, returns the current encoding otherwise.

$P0.'eof'()

Returns true if the filehandle is at the end of the current file, false otherwise.

$P0.'get_fd'()

Returns the integer file descriptor of the current file, but only on operating systems that use file descriptors. Returns -1 on systems that do not support this.

Exceptions

Parrot includes a robust exception mechanism that is not only used internally to implement a variety of control flow constructs, but is also available for use directly from PIR code. Exceptions, in as few words as possible, are error conditions in the program. Exceptions are thrown when an error occurs, and they can be caught by special routines called handlers. This enables Parrot to recover from errors in a controlled way, instead of crashing and terminating the process entirely.

Exceptions, like most other data objects in Parrot, are PMCs. They contain and provide access to a number of different bits of data about the error, such as the location where the error was thrown (including complete backtraces), any annotation information from the file, and other data.

Throwing Exceptions

Many exceptions are used internally in Parrot to indicate error conditions. Opcodes such as die and warn throw exceptions internally to do what they are supposed to do. Other opcodes such as div throw exceptions only when an error occurs, such as an attempted division by zero.

Exceptions can also be thrown manually using the throw opcode. Here's an example:

  $P0 = new 'Exception'
  throw $P0

This throws the exception object as an error. If there are any available handlers in scope, the interpreter will pass the exception object to the handler and continue execution there. If there are no handlers available, Parrot will exit.

Exception Attributes

Since Exceptions are PMC objects, they can contain a number of useful data items. One such data item is the message:

  $P0 = new 'Exception'
  $P1 = new 'String'
  $P1 = "this is an error message for the exception"
  $P0["message"] = $P1

Another is the severity and the type:

  $P0["severity"] = 1   # An integer value
  $P0["type"] = 2       # Also an Integer

Finally, there is a spot for additional data to be included:

  $P0["payload"] = $P2  # Any arbitrary PMC

Exception Handlers

Exception handlers are labels in PIR code that can be jumped to when an exception is thrown. To list a label as an exception handler, the push_eh opcode is used. All handlers exist on a stack. Pushing a new handler adds it to the top of the stack, and using the pop_eh opcode pops the handler off the top of the stack.

  push_eh my_handler
    # something that might cause an error

  my_handler:
    # handle the error here

Catching Exceptions

The exception PMC that was thrown can be caught using the .get_results() directive. This returns the Exception PMC object that was thrown from inside the handler:

  my_handler:
    .local pmc err
    .get_results(err)

With the exception PMC available, the various attributes of that PMC can be accessed and analyzed for additional information about the error.

Exception Handler PMCs

Like all other interesting data types in Parrot, exception handlers are a PMC type. When using the syntax above with push_eh LABEL, the handler PMC is created internally by Parrot. However, you can create it explicitly too if you want:

  $P0 = new 'ExceptionHandler'
  set_addr $P0, my_handler
  push_eh $P0
  ...

  my_handler:
    ...

Rethrowing and Exception Propagation

Exception handlers are nested and are stored in a stack. This is because not all handlers are intended to handle all exceptions. If a handler cannot deal with a particular exception, it can rethrow the exception to the next handler in the stack. Exceptions propagate through the handler stack until it reaches the default handler which causes Parrot to exit.

Annotations

Annotations are pieces of metadata that can be stored in a bytecode file to give some information about what the original source code looked like. This is especially important when dealing with high-level languages. We'll go into detail about annotations and their use in Chapter 10.

Annotations are created using the c<.annotation> keyword. Annotations consist of a key/value pair, where the key is a string and the value is an integer, a number, or a string. Since annotations are stored compactly as constants in the compiled bytecode, PMCs cannot be used.

  .annotation 'file', 'mysource.lang'
  .annotation 'line', 42
  .annotation 'compiletime', 0.3456

Annotations exist, or are "in force" throughout the entire compilation unit, or until they are redefined. Creating a new annotation with the same name as an old one overwrites it with the new value. The current hash of annotations can be retrieved with the annotations opcode:

  .annotation 'line', 1
  $P0 = annotations # {'line' => 1}
  .annotation 'line', 2
  $P0 = annotations # {'line' => 2}

Or, to retrieve a single annotation by name, you can write:

  $I0 = annotations 'line'

Annotations in Exceptions

Exception objects contain information about the annotations that were in force when the exception was thrown. These can be retrieved with the 'annotation'() method of the exception PMC object:

  $I0 = $P0.'annotations'('line')  # only the 'line' annotation
  $P1 = $P0.'annotations'()        # hash of all annotations

Exceptions can also give out a backtrace to try and follow where the program was exactly when the exception was thrown:

  $P1 = $P0.'backtrace'()

The backtrace PMC is an array of hashes. Each element in the array corresponds to a function in the current call stack. Each hash has two elements: 'annotation' which is the hash of annotations that were in effect at that point, and 'sub' which is the Sub PMC of that function.

Copyright © 2002-2009, Parrot Foundation.