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 separator: "::"

In this document, "::" indicates namespace nesting. For example, "a::b" means "the namespace 'b' inside the namespace 'a'". In Parrot, nesting is actually denoted by other means (e.g. multidimensional hash keys), but writing ["a"; "b"] is harder to both write and read.

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 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 a namespace named with the lowercased name of the HLL. For example, Perl 5's CGI module must be named "perl5::CGI". This eliminates any accidental collisions between languages.; 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.; HLLs should use hierarchical namespaces to the extent practical. A single flat namespace can be made to work, of course, 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: perl6::namespace.add_var may choose to check which parts of the variable interface are implemented by $P0 so it can decide on an appropriate sigil.
del_namespace($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: perl6::namespace.find_var should 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, don't have both our $A and our @A.
export_to($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: Perl 6 use tcl:Some::Module 'c*' will import all the commands that start with 'c' from the given Tcl namespace into the current Perl namespace. Regardless of whether 'c*' is a Perl 6 style export pattern, it is a valid Tcl export pattern.; 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 = name(): Returns the name of the namespace as an array of strings. So perl5:Some::Module would return an array containing "perl5", "Some", "Module".; NOTE: This is a naive method. It does not account for any aliasing.

Compiler PMC API

Methods

load_library($P0): Ask this compiler to load a library/module named by the elements of the array in $P0. So perl5:Some::Module should be loaded using (in pseudo Perl 6): perl5.load_library(["Some", "Module"]).

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

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 the variable $S0 in $P1 or the current 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 $P1 or the current 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 "Foo::" should be also created as a Perl 5 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 ["perl6"; "Foo"]
    # A smart perl6 compiler would emit this,
    # because it knows that Foo is a perl6 namespace:
    #   $P1["&bar"] = $P0
    # But a naive one would emit this:
    $P1.add_sub("bar", $P0)
    end
  .end

  .sub foo
    ...
  .end

Cross-language Exportation

Perl:

  #!/usr/bin/perl
  use tcl:Some::Module 'w*';
  write("this is a tcl command");

PIR:

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

ATTACHMENTS

None.

FOOTNOTES

None.

REFERENCES

None.

NAME

VERSION

DESCRIPTION

DEFINITIONS

"HLL"

"current namespace"

namespace separator: "::"

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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