Perl6: Adding a sigil with Slangs

Question

I am trying to add «€» as an alias for the «$» scalar, and doing it with a Slang is the way to do it I think. But perl6.doc doesn't mention Slangs at all.

I have read the following:

And looked at the Slang::Roman and Slang::Tuxic modules.

The result is this file (ScalarEU.pm6):

use nqp;

unit module ScalarEU2;

sub EXPORT(|)
{
  my role Euscalar
  {
    token sigil:sym<$> { '€' | '$' }
  }

  my Mu $MAIN-grammar := nqp::atkey(%*LANG, 'MAIN');
  my $grammar := $MAIN-grammar.HOW.mixin($MAIN-grammar, Euscalar);

  $*LANG.define_slang('MAIN', $grammar, $*LANG.actions);

  {}
}

Then a program (called hello) using it:

use lib "lib";

use ScalarEU;

sub MAIN ($name)
{
  say "Hello, €name!";
}

But it doesn't work, or rather, doesn't do what it should:

$ ./hello Tom
Hello, €name!

(I wrote the program this way so that it doesn't crash.)

I haven't added an action class, but the way the "token sigil" is set up shouldn't require that? But that assumption is based on an 11 year article, and may be wrong.

Also, https://github.com/rakudo/rakudo/issues/2404 says that $*LANG is obsolete, and to use $?LANG instead. REPL agrees:

> $*LANG
Dynamic variable $*LANG not found

> $?LANG
(low-level object `Perl6::Grammar`)

But programs can use both, without errors. (I have tried.)

Technically a Slang is a feature of the Rakudo Perl 6 compiler. It has not been added to the Perl 6 spec because it isn't good enough yet. There is a mostly toy language named 007 which is being used to design how it should work. — Brad Gilbert
There are many literal $s sprinkled throughout the Rakudo source code, outside its grammars and actions, that are part of what makes the $ sigil work. (Similarly for @, %, etc.) It would of course be technically possible to produce a revised Rakudo that lets you do what you're trying to do. It might even be possible to do it without impacting Rakudo performance. But it would be a lot of boring little edits (again, outside of the grammars/actions of a slang) and I suspect you'd have a seriously hard time persuading core devs to merge your changes back into the Rakudo master branch. — raiph
(Continuing) Many of the literal $s relate to internal variables so those won't matter. It looks like some of the rest also won't matter or won't matter much. For example issigil. But code like %cont_info<sigil> eq '$' or $sigil eq '$', to pick a couple random examples from a single source file, looks like it will. — raiph
Ok. So it isn't possible. My first take was something like this: use MONKEY-TYPING; augment grammar Perl { token sigil:sym<$> { '€' | '$' } } my €a = 12; it would have been really neat if that had worked. But I know that BEGIN kicks in too late. — Arne Sommer
Thank you for the responses. I have just published the result at perl6.eu — Arne Sommer

Tinmarino Tinmarino · Accepted Answer · 2020-04-02T01:27:41

Briefly: You must change $!target string of the ParseShared nqp object, this changes the code at parse time.

Why: The sigil token is not a proto anymore but defined rakudo/src/Perl6/Grammar.nqp as an alternation.

So as a minimal solution: token sigil { <[$@%&€]> } but then comes new problem: the returned value can be € and is used in other grammar.

Where: So you must change $<sigil>.Str defined in nqp/src/QRegex/Cursor.nqp as:

method Str()       {
   $!pos >= $!from
        ?? nqp::substr(nqp::getattr_s($!shared, ParseShared, '$!target'),
            $!from, nqp::sub_i(self.to, $!from))
        !! '' }

<- Meaning The string in target between from and to if pos is not so low.

-> So we must change $!target between $!from and $!to in a NQPMatch.

Demo: Here is the code to embed in a slang grammar:

token sigil {
    | <[$@%&]>
    | <nogil> { say "Nogil returned: ", lk($/, 'nogil').Str; }
}

method nogil {
    # The object to return
    my $cursor := self.nogil-proxy;

    # Get from cursor
    my $shared := nqp::getattr($cursor, NQPMatch, '$!shared');
    my $from = nqp::getattr_i($cursor, NQPMatch, '$!from');
    my $target = $cursor.target;

    # Replace in target current character (€) by $
    $target.substr-rw($from, 1) = '$';

    # Set in cursor
    nqp::bindattr_s($shared, $shared.WHAT, '$!target', $target);

    # Return the "created by proxy" and modified NQPMatch
    return $cursor;
}

token nogil-proxy { '€' }

Speaking alone:_ It should work perfect in your case. In mine, (no sigil) I still have problem because the size changes during the $!target modification messes the to and from of other cursors. In which case:

I must overwrite NQPMatch.Str function (hoping it is possible).
I must list cursors (if possible) and change their $!from and $!to attribute wisely to restore peace in the galaxy or at least in the client code.

Perl6: Adding a sigil with Slangs

1 Answers