One first hard step in implementing lexical scopes is to put together a lot of details that are spread over the specs. In this page we're going to sketch a model for the lexical scopes that support all the documented Perl 6 features.

Variable Lookup

The local variable lookup happens in the following order:

1. MY
2. OUTER recursive

Special cases

$_

$_ is always locally defined in any block. Routines start with an empty $_, other Code objects have a signature of:

 -> $_ is rw = OUTER::<$_>

$! and $/

These variables are compile-time aliases to $+! and $+/

Modelling

Data Structure

class LexicalScope {
  has %.entries handles postcircumfix:<{}>;
  has LexicalScope $.outer is rw;
}

Accessing the Lexical Scope

The lexical scope is an attribute of the continuation, meaning that the "current continuation" points to the "current lexical scope". The Perl 6 specs doesn't specify a macro that return the current continuation, so we have one of the SMOP Specific Macros ___CURRENT_CONTINUATION___. This should provide access to the following API:

class Continuation {
  has LexicalScope $.lexical is rw;
  has Continuation $.caller is rw;
}

This should be part of the SMOP Interpreter Implementation API.

Making a variable lookup

class LexicalScope is also {
  method lookup($name) {
    my $lexical = ___CURRENT_CONTINUATION___.lexical;
    if ($lexical.entries.exists($name)) {
      return $lexical{$name};
    } elsif ($lexical.outer) {
      return $lexical.outer;
    } else {
      fail("No variable $name in the current scope");
    }
  }
}