NAME

docs/pdds/pdd21_namespaces.pod - Parrot Namespaces

VERSION

$Revision$

DESCRIPTION

- Namespaces should be stored under first-level namespaces corresponding to the HLL language name
- Namespaces should be hierarchical
- Add a get_namespace opcode (that takes an array or a multidimensional hash index)
- Namespaces follow the semantics of the HLL in which they're defined
- exports follow the semantics of the library's language
- Two interfaces: typed and generic

DEFINITIONS

"HLL"

A High Level Language, such as Perl, Python, or Tcl, in contrast to PIR, which is a low-class language.

The current namespace at runtime is the namespace associated with the currently executing subroutine. Pasm assigns each subroutine a namespace when compilation of the subroutine begins. Don't change the associated namespace of a subroutine unless you're prepared for weird consequences.

(Pasm also has its own separate concept of current namespace which is used to initialize the runtime current namespace as well as determine where to store compiled symbols.)

namespace indexing syntax

Namespaces are denoted in Parrot as either simple strings or (potentially) multimentional hash keys.

A non-nested namespace may appear in Parrot source code as the string "a" or the key ["a"].

A nested namespace "b" inside the namespace "a" will appear as the key ["a"; "b"].

(There is no limit to namespace nesting.)

IMPLEMENTATION

Namespace PMC API

There are many different ways to implement a namespace and Parrot's target languages display a wide variety of them. By implementing an API, it should be possible to allow interoperability while still allowing each one choose the best internal representation.

Naming Conventions

HLL Root Namespaces: Each HLL must store public items in a namespace named with the lowercased name of the HLL. This is the HLL root namespace. For instance, Tcl's user- created namespaces should live in the tcl namespace. This eliminates any accidental collisions between languages.; This namespace must be stored at the first level in parrot's namespace hierarchy. These top-level namespaces should also be specified in a standard unicode encoding. The reasons for these restrictions is to allow compilers to remain completely ignorant of each other.
True Root Namespace: The true root namespace is available only via introspection with the interpinfo opcode, e.g. $P0 = interpinfo .INTERPINFO_NAMESPACE_ROOT.
HLL Implementation Namespaces: Each HLL must store implementation internals (private items) in a namespace named with an underscore and the lowercased name of the HLL. For instance, Tcl's implementation internals should live in the _tcl namespace.
HLL User-Created Namespaces: Each HLL must store all user-created namespaces under the HLL root namespace. It is suggested that HLLs use hierarchical namespaces to the extent practical. A single flat namespace can be made to work, but it complicates symbol exportation.

Namespace PMC Interfaces: Generic and Typed

Most languages leave their symbols plain, which makes lookups quite straightforward. Others use sigils or other mangling techniques, complicating the problem of interoperability.

Parrot namespaces assist with interoperability by providing two interface subsets: the raw interface and the typed interface.

Raw Interface

Each HLL may, when working with its own namespace objects, use the raw interface, which allows direct naming in the native style of the namespace's HLL.

This interface consists of standard Parrot hash interface, with all its keys, values, lookups, deletions, etc. Just treat the namespace like a hash. (It probably is one, really, deep down.)

It's kind of an anticlimax, isn't it, giving a fancy name like "raw interface" to a hash? "It's a just a hash," you say. Oh well. I'll try to live with the shame.

Typed Interface

When a given namespace's HLL is either different from the current HLL or unknown, an HLL should generally use only the language-agnostic namespace interface. This interface isolates HLLs from each others' naming quirks. It consists of add_foo(), find_foo(), and del_foo() methods, for values of "foo" including "sub" (something executable), "namespace" (something in which to find more names), and "var" (anything).

NOTE: The job of the typed interface is to bridge naming differences, and only naming differences. Therefore:

(1) It does not enforce, nor even notice, the interface requirements of "sub" or "namespace": e.g. execution of add_sub("foo", $P0) does not automatically guarantee that $P0 is an invokable subroutine; and: (2) It does not prevent overwriting one type with another

add_namespace($S0, $P0): Store $P0 as a namespace under the name of $S0.
add_sub($S0, $P0): Store $P0 as a subroutine with the name of $S0.
add_var($S0, $P0): Store $P0 as a variable under the name of $S0.; IMPLEMENTATION NOTE: Perl namespace implementations may choose to implement add_var() by checking what which parts of the variable interface are implemented by $P0 (scalar, array, and/or hash) so it can decide on an appropriate sigil.
del_namespace($S0)
del_sub($S0)
del_var($S0): Delete the sub, namespace, or variable named $S0.
$P0 = find_namespace($S0)
$P0 = find_sub($S0)
$P0 = find_var($S0): Find the sub, namespace, or variable named $S0.; IMPLEMENTATION NOTE: Perl namespace implementations should implement find_var() to check all variable sigils, but the order is not to be counted on by users. If you're planning to let Python code see your module, you should avoid exporting both our $A and our @A. (Well, you might want to consider not exporting variables at all, but that's a style issue.)
export_to($P0, $P1): Export items from the current namespace into the namespace in $P0. The items to export are named in the array $P1; a null $P1 requests the 'default' set of items. The interpretation of the array in $P1 always follows the conventions of the source (exporting) namespace.; The base Parrot namespace export_to() function interprets item names as literals -- no wildcards or other special meaning. There is no default list of items to export, so $P1 of null and $P1 of an empty array have the same behavior.; NOTE: Exportation may entail non-obvious, odd, or even mischievious behavior. For example, Perl's pragmata are implemented as exports, and they don't actually export anything.; IMPLEMENTATION EXAMPLES: Suppose a Perl program were to import some Tcl module with an import pattern of "c*" -- something that might be expressed in Perl 6 as use tcl:Some::Module 'c*'. This operation would import all the commands that start with 'c' from the given Tcl namespace into the current Perl namespace. This is so because, regardless of whether 'c*' is a Perl 6 style export pattern, it is a valid Tcl export pattern.; {XXX - Is the ':' for HLL approved or just proposed? Somebody familiar with Perl 6 please fix the example in the preceding paragraph to use the currently planned syntax for importing modules from other languages.}; IMPLEMENTATION NOTE: Most namespace export_to implementations will restrict themselves to using the typed interface on the target namespace. However, they may also decide to check the type of the target namespace and, if it turns out to be of a compatible type, to use same-language shortcuts.; DESIGN TODO: Figure out a good convention for a default export list in the base namespace PMC. Maybe a standard method "expand_export_list()"?

Non-interface Methods

These methods don't belong to either the typed or the generic interface.

$P0 = get_name(): Gets the name of the namespace as an array of strings. For example, if the current language is Perl 5 and the current Perl 5 namespace is "Some::Module" (that's Perl 5 syntax), then get_name() on that namespace returns an array of "perl5", "Some", "Module".; NOTE: Due to aliasing, this value may be wrong, even if nobody's been playing around with it.

Compiler PMC API

Methods

get_namespace($P0): Ask this compiler to find its namespace named by the elements of the array in $P0. Returns namespace PMC on success and null PMC on failure. Note that a null PMC or an empty array requests the HLL's base namespace.; This method allows other HLLs to know one name (the HLL) and then work with that HLL's modules without having to know the name it chose for its namespace tree. (If you really want to know the name, the name() method should work on the returned namespace PMC.); Note that this method is basically a convenience and/or performance hack, as it does the equivalent of interpinfo .INTERPINFO_NAMESPACE_ROOT followed by zero or more calls to <namespace>.find_namespace(). However, any compiler is free to cheat if it doesn't get caught, e.g. to use the untyped namespace interface if the language doesn't mangle namespace names.
load_library($P0, $P1): Ask this compiler to load a library/module named by the elements of the array in $P0, with optional control information in $P1.; For example, Perl 5's module named "Some::Module" should be loaded using (in pseudo Perl 6): perl5.load_library(["Some", "Module"], null).; The meaning of $P1 is compiler-specific. The only universal legal value is Null, which requests a "normal" load. The meaning of "normal" varies, but the ideal would be to perform only the minimal actions required.; On failure, an exception is thrown. {XXX - I'd settle for a universal error pmc interface. Hm, sounds like a new pdd. >>TODO}

Subroutine PMC API

Some information must be available about subroutines to implement the correct behavior about namespaces.

Methods

get_namespace: The namespace where the subroutine was defined. (As opposed to namespace(s) that it may have been exported to.)

Namespace Opcodes

Note that all namespace opcodes operate from the local HLL root namespace. Navigating outside one's own HLL namespace requires either the interpinfo .INTERPINFO_NAMESPACE_ROOT opcode or the get_namespace() compiler PMC method.

add_namespace $P0, $P1: Add the namespace PMC $P1 as the namespace $P0 (an array of names or a multidimensional hash index).
del_namespace $P0: Delete the namespace $P0 (an array of names or a multidimensional hash index).
$P0 = find_global $P1, $S0
$P0 = find_global $S0: Find $P0 as the variable $S0 in the current namespace. or in $P1, relative to the HLL root namespace.
$P0 = get_namespace $P1
$P0 = get_namespace: Get the namespace $P1 (an array of names or a multidimensional hash index) or the current namespace. To get the "Foo::Bar" namespace, one would use this:; or this:
store_global $P1, $S0, $P0
store_global $S0, $P0: Store $P0 as the variable $S0 in the current namespace, or in $P1, relative to the HLL root namespace.

HLL Namespace Mapping

In order to make this work, Parrot must somehow figure out what type of namespace PMC to create.

Default Namespace

The default namespace PMC will implement Parrot's current behavior.

Compile-time Creation

This perl:

  #!/usr/bin/perl
  package Foo;
  $x = 5;

should map roughly to this PIR:

  .HLL "Perl5", "perl5_group"
  .namespace [ "Foo" ]
  .sub main :main
    $P0 = new .PerlInt
    $P0 = 5
    store_global "$x", $P0
  .end

In this case, the main sub would be tied to Perl5 by the .HLL directive, so a Perl5 namespace would be created.

Run-time Creation

Consider the following Perl5 program:

  #!/usr/bin/perl
  $a = 'x';
  ${"Foo::$a"} = 5;

The Foo:: namespace is created at run-time (without any optimizations). In these cases, Parrot should create the namespace based on the HLL of the PIR subroutine that calls the store function.

  .HLL "Perl5", "perl5_group"
  .sub main :main
    # $a = 'x';
    $P0 = new .PerlString
    $P0 = "x"
    store_global "$a", a
    # ${"Foo::$a"} = 5;
    $P1 = new PerlString
    $P1 = "Foo::"
    $P1 .= $P0
    $S0 = $P1
    $P2 = split "::", $S0
    $S0 = pop $P2
    $S0 = "$" . $S0
    $P3 = new .PerlInt
    $P3 = 5
    store_global $P2, $S0, $P3
  .end

In this case, store_global should see that it was called from "main", which is in a Perl5 namespace, so it will create the "Foo" namespace as a Perl5 namespace.

LANGUAGE NOTES

Perl 6

Sigils

Perl6 may wish to be able to access the namespace as a hash with sigils. That is certainly possible, even with subroutines and methods. It's not important that a HLL use the typed namespace API, it is only important that it provides it for others to use.

So Perl6 may implement get_keyed and set_keyed VTABLE slots that allow the namespace PMC to be used as a hash. The find_sub method would, in this case, would append a "&" sigil to the front of the sub/method name and search in the internal hash.

Python

Importing from Python

Since functions and variables overlap in Python's namespaces, when exporting to another HLL's namespace, the Python namespace PMC's export_to method should use introspection to determine whether x should be added using add_var or add_sub. $I0 = does $P0, "Sub" may be enough to decide correctly.

  #!/usr/bin/perl6
  sub foo {...}
  %Foo::{"&bar"} = &foo;

PIR:

  .sub main :main
    $P0 = find_name "&foo"
    $P1 = get_namespace ["Foo"]

    # A smart perl6 compiler would emit this,
    # because it knows that Foo is a perl6 namespace:
    $P1["&bar"] = $P0

    # But a naive perl6 compiler would emit this:
    $P1.add_sub("bar", $P0)

  .end

  .sub foo
    ...
  .end

Cross-language Exportation

Perl5:

  #!/usr/bin/perl
  use tcl:Some::Module 'w*';   # XXX - is ':' after HLL standard Perl 6?
  write("this is a tcl command");

PIR (without error checking):

  .sub main :main
    .local pmc tcl
    .local pmc ns
    tcl = compreg "tcl"
    ns = new .Array
    ns = 2
    ns[0] = "Some"
    ns[1] = "Module"
    null $P0
    tcl.load_library(ns, $P0)
    $P0 = tcl.get_namespace(ns)
    $P1 = get_namespace
    $P0.export_to($P1, 'w*')
    "write"("this is a tcl command")
  .end

ATTACHMENTS

None.

FOOTNOTES

None.

REFERENCES

None.

NAME

VERSION

DESCRIPTION

DEFINITIONS

"HLL"

"current namespace"

namespace indexing syntax

IMPLEMENTATION

Namespace PMC API

Naming Conventions

Namespace PMC Interfaces: Generic and Typed

Raw Interface

Typed Interface

Non-interface Methods

Compiler PMC API

Methods

Subroutine PMC API

Methods

Namespace Opcodes

HLL Namespace Mapping

Default Namespace

Compile-time Creation

Run-time Creation

LANGUAGE NOTES

Perl 6

Sigils

Python

Importing from Python

Subroutines and Namespaces

Examples

Aliasing

Cross-language Exportation

ATTACHMENTS

FOOTNOTES

REFERENCES

parrotcode: Parrot Namespaces
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