From @sjn

Please inspect the attached example test.

The following error appears when calling .perl on the Match object returned by a subclassed JSON​::Tiny Grammar parser.

$ perl6 Match.perl-error.t
1..3
  ok 1 - myparse accepts proper JSON text
  ok 2 - Parsing result is a Match object
This Seq has already been iterated, and its values consumed
(you might solve this by adding .cache on usages of the Seq, or
by assigning the Seq into an array)
  in block <unit> at Match.perl-error.t line 28

From @sjn

# vim​: ft=perl6

use v6;
use JSON​::Tiny​::Grammar;
use JSON​::Tiny​::Actions;
use Test;

grammar A​::B​::Grammar is JSON​::Tiny​::Grammar { }
class A​::B​::Actions is JSON​::Tiny​::Actions { }

module A​::B {
  proto myparse ($) is export { * };
  multi myparse (Str​:D $json) {
  my $a = A​::B​::Actions.new;
  A​::B​::Grammar.parse( $json, :actions($a));
  }
}

import A​::B; # we need the &myparse sub

plan 3;

subtest {
  my $result = myparse $=finish; # Read from POD
  ok $result, 'myparse accepts proper JSON text';
  isa-ok $result, 'Match', 'Parsing result is a Match object';

  ok $result.perl, 'We can call .perl on the Match object';

  #note "##### Match.gist is " ~ $result.gist; # This works
  #note "##### Match.perl is " ~ $result.perl; # THIS FAILS

}, "Parse JSON file and print stuff";

=finish

{
  "name" : "JSON​::Tiny",
  "version" : "*",
  "description" : "A minimal JSON (de)serializer",
  "depends" : [ ],
  "provides" : {
  "JSON​::Tiny" : "lib/JSON/Tiny.pm",
  "JSON​::Tiny​::Actions" : "lib/JSON/Tiny/Actions.pm",
  "JSON​::Tiny​::Grammar" : "lib/JSON/Tiny/Grammar.pm"
  },
  "source-url" : "git​://github.com/moritz/json.git"
}

From @moritz

Golfed even further​:

# vim​: ft=perl6

use v6;
use JSON​::Tiny​::Grammar;
use JSON​::Tiny​::Actions;

my $result = JSON​::Tiny​::Grammar.parse( '{}', :actions(JSON​::Tiny​::Actions));
say $result.perl;

From @moritz

The underlying culprit is that deep down in that Match object there's a .made that's a Seq that has been iterated already​:

# vim​: ft=perl6

use v6;
use JSON​::Tiny​::Grammar;
use JSON​::Tiny​::Actions;

my $result = JSON​::Tiny​::Grammar.parse( '{}', :actions(JSON​::Tiny​::Actions));
say $result<value><object><pairlist>.made.^name; # Seq
say $result<value><object><pairlist>.made.perl; # dies with "Seq has already been iterated"

Maybe the real bug here is that (already iterated Seq).perl throws an exception instead of reproducing a Seq that already been iterated :-)

From @moritz

Fixed with rakudo/rakudo@8ca26cebb4 and tested with Raku/roast@760dbe8465

@moritz - Status changed from 'new' to 'resolved'