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[perl #130859] [WEIRD] WhateverCode subscript inside WhateverCodeinside block doesn't properly close over lexical

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From:
Zoffix Znet via RT
Date:
February 5, 2018 00:44
Subject:
[perl #130859] [WEIRD] WhateverCode subscript inside WhateverCodeinside block doesn't properly close over lexical
Message ID:
rt-4.0.24-17058-1517791476-1071.130859-15-0@perl.org
On Sat, 25 Feb 2017 06:55:02 -0800, smls75@gmail.com wrote:
> A WhateverCode containing an array subscript that is itself a
> WhateverCode, usually works fine... e.g. `*.[*-1]` always returns the
> last positional element of its argument.
> 
> A WhateverCode closing over a lexical variable usually works fine,
> e.g. `* - $x` always subtracts the *current* value of $x from its
> argument.
> 
> But when *both* of those features are combined, *and* the variable
> that is closed over is scoped to an outer block or routine that is
> called repeatedly, then it misbehaves:
> 
> sub f { my $x = ++$; (*.[* - $x])(<a b c>) }
> say (f,f,f);                                # (c c c)
> 
> The same, using `map`:
> 
> say (1..3).map: { (*.[* - $_])(<a b c>) };  # (c c c)
> 
> The expected output would be `(c b a)`. For some reason, it seems to
> use the first value of `$x` or `$_` for all three iterations.
> 
> ----------
> 
> Demonstration that the particular circumstances described above have
> to align for the bug to appear:
> 
> 1) If the WhateverCode subscript is replaced with a static expression,
> the bug disappears:
> 
> say (1..3).map: { (*.[3 - $_])(<a b c>) };  # (c b a)
> 
> 2) If the outer WhateverCode is removed by "inlining" it, the bug
> disappears:
> 
> say (1..3).map: { <a b c>[* - $_] };        # (c b a)
> 
> 3) If the `map` is unrolled so that there is no outer block that gets
> re-entered between invocations of the WhateverCode's, the bug
> disappears:
> 
> my &f = *.[* - $_];
> $_ = 1; say f <a b c>;  # c
> $_ = 2; say f <a b c>;  # b
> $_ = 3; say f <a b c>;  # a
> 
> ----------
> This is Rakudo version 2017.02-95-g0be724727 built on MoarVM version
> 2017.02-7-g3d859008 implementing Perl 6.c.


Thank you for the report. This is now fixed.

Fix:  https://github.com/rakudo/rakudo/commit/e8c6c259ce72fb5
Test: https://github.com/perl6/roast/commit/501f51b56c77fec06

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