role to execute a matching sub in raku

matching-with-macros.raku

use v6.d;
use experimental :macros;

role mmm {
    method ACCEPTS(Code:D: |c) {
        dd self;dd c; # debugging!
        return False unless c ~~ self.signature;
        self.(|c);
        return True;
    }
}

macro matches($xxx) {
    quasi { when ({{{$xxx}}} but mmm) { }; };
}

class Point {
    has $.x;
    has $.y;
}

given Point.new(:5x,:3y) {
    matches -> (:$a) {
        say "something a=$a";
    };
    matches -> Point:D (:$x, :$y) {
        say "point $x $y";
    };
    matches -> (:$x,:$y) {
        say "other $x $y";
    };
};

matching.raku

use v6.d;

role mmm {
    method ACCEPTS(Code:D: |c) {
        return False unless c ~~ self.signature;
        self.(|c);
        return True;
    }
}

class Point {
    has $.x;
    has $.y;
}

given Point.new(:5x,:3y) {
    when -> (:$a) {
        say "something a=$a";
    } but mmm {};
    when -> Point:D (:$x, :$y) {
        say "point $x $y";
    } but mmm {};
    when -> (:$x,:$y) {
        say "other $x $y";
    } but mmm {};
};

notes.md

the first program works

the second program… really doesn't:

-> $ (:$a) { #`(Block+{mmm}|2199162782080) ... }
\(AST.new)
Use of uninitialized value element of type Mu in string context.
Methods .^name, .raku, .gist, or .say can be used to stringify it to something meaningful.
  in block  at /tmp/d.raku line 24
something a=
-> Point:D $ (:$x, :$y) { #`(Block+{mmm}|2199162862416) ... }
\(AST.new)
-> $ (:$x, :$y) { #`(Block+{mmm}|2199162862488) ... }
\(AST.new)
Use of uninitialized value element of type Mu in string context.
Methods .^name, .raku, .gist, or .say can be used to stringify it to something meaningful.
  in block  at /tmp/d.raku line 30
Use of uninitialized value element of type Mu in string context.
Methods .^name, .raku, .gist, or .say can be used to stringify it to something meaningful.
  in block  at /tmp/d.raku line 30
other
When invoking 8 '', provided outer frame 0x20003c2ed00 (32 '') does not match expected static frame 0x20003c2e600 (26 '')
  in any  at /tmp/d.raku line 14
  in block <unit> at /tmp/d.raku line 29