From @drforr

OS​: Ubuntu Linux 14.04 LTS in VirtualBox
Host​: Windows 8
Rakudo version​: git pull from 25/3/2015

Issue 1​: Are we missing the line​:

------> token TOP { <term>⏏ % \n }

in the error message?

Issue 2​: Should the quantifier be required?

Assume you're debugging this grammar fragment​: " token top-level {
<list>* % \, } token list { [ <list> | <term> ]* % \, } " - A fairly
simple (albeit left-recursive) fragment of a common grammar pattern.
If you're allowed to temporarily delete the quantifiers, $/<top-level>
is an array reference to a single $/<list> match, and your actions get
what they're expecting, an array.
If you're not allowed to delete the quantifiers, you get " token
top-level { <list> } token list { [ <list> | <term> ] } " and
$/&lt;top-level&gt; is simply a $/<list>, and not $/<top-level>[0] as your
actions expect. So you have to change the actions as well.

Error​:
--cut here--
perl6 ~/bug.t
===SORRY!=== Error while compiling /home/jgoff/bug.t
Missing quantifier on the left argument of %
at /home/jgoff/bug.t​:3
--cut here--

--cut here--
grammar Bug {
  token term { a }
  token TOP { <term> % \n }
}
Bug.parse( 'a' );
--cut here--

@coke - Status changed from 'new' to 'open'

From @coke

On Tue Mar 31 12​:50​:50 2015, drforr@​pobox.com wrote​:

OS​: Ubuntu Linux 14.04 LTS in VirtualBox
Host​: Windows 8
Rakudo version​: git pull from 25/3/2015

Issue 1​: Are we missing the line​:

------> token TOP { <term>⏏ % \n }

in the error message?

Issue 2​: Should the quantifier be required?

Assume you're debugging this grammar fragment​: " token top-level {
<list>* % \, } token list { [ <list> | <term> ]* % \, } " - A fairly
simple (albeit left-recursive) fragment of a common grammar pattern.
If you're allowed to temporarily delete the quantifiers, $/<top-level>
is an array reference to a single $/<list> match, and your actions get
what they're expecting, an array.
If you're not allowed to delete the quantifiers, you get " token
top-level { <list> } token list { [ <list> | <term> ] } " and
$/&lt;top-level&gt; is simply a $/<list>, and not $/<top-level>[0] as your
actions expect. So you have to change the actions as well.

Error​:
--cut here--
perl6 ~/bug.t
===SORRY!=== Error while compiling /home/jgoff/bug.t
Missing quantifier on the left argument of %
at /home/jgoff/bug.t​:3
--cut here--

--cut here--
grammar Bug {
token term { a }
token TOP { <term> % \n }
}
Bug.parse( 'a' );
--cut here--

This is fixed, and closable with tests.

$ perl6 /tmp/a.p6
===SORRY!=== Error while compiling /tmp/a.p6
Missing quantifier on the left argument of %
at /tmp/a.p6​:3
------> token TOP { <term> %⏏ \n }

--
Will "Coke" Coleda

From @zoffixznet

Tests added in Raku/roast@d633c0411c

@zoffixznet - Status changed from 'open' to 'resolved'