[perl #130891] [LTA] infix: always returns a Block with .arity 0 and .count Inf

[Zoffix Znet via RT]

On Tue, 28 Feb 2017 14:41:07 -0800, sml...@gmail.com wrote:
>     my &a = *+1;
>     my &b = */2;
> 
>     say &a.arity;         # 1
>     say &b.arity;         # 1
>     say (&a o &b).arity;  # 0
> 
>     say &a.count;         # 1
>     say &b.count;         # 1
>     say (&a o &b).count;  # Inf
> 
> Ideally, the result of the `o` operator should probably have the same
> signature as its right-hand argument (which gets the input "first").
> 
> Things that care about arity/count, and thus might currently behave
> differently than users expect when given a Block created by `o`,
> include:
> 
>   .sort  (comparator vs Schwartzian transform)
>   .map  (normal map vs n-at-a-time)
>   o  (interpolates input lists if count>1)


I have a patch (attached). That solves this ticket and passes all stresstests:

    cpan@perlbuild2~/CPANPRC/rakudo (nom)$ ./perl6 -e 'm: say ([o] {$_ xx 3} xx 
3)(5)'
    (((5 5 5) (5 5 5) (5 5 5)) ((5 5 5) (5 5 5) (5 5 5)) ((5 5 5) (5 5 5) (5 5 
5)))

However, it has a definite stench of an evil hack about it, so I'll leave it 
for someone who knows more about the internals to figure out whether it can be 
committed. One example of evil weirdness: the `.signature` becomes the sub's 
signature (e.g. `($a, $b)`), but the block still uses `|args` capture to pass 
around args? waaaat...

diff --git a/src/core/operators.pm b/src/core/operators.pm
index f5da866..d1661e0 100644
--- a/src/core/operators.pm
+++ b/src/core/operators.pm
@@ -726,7 +726,14 @@ multi sub trait_mod:<is>(Routine $r, Str :$looser!) {
 proto sub infix:<∘> (&?, &?) {*}
 multi sub infix:<∘> () { *.self }
 multi sub infix:<∘> (&f) { &f }
-multi sub infix:<∘> (&f, &g --> Block) { (&f).count > 1 ?? -> |args { f |g |args } !! -> |args { f g |args } }
+multi sub infix:<∘> (&f, &g --> Block) {
+    my \ret = &f.count > 1
+        ?? -> |args { f |g |args }
+        !! -> |args { f  g |args }
+    nqp::bindattr(ret, Code, '$!signature',
+        nqp::getattr(nqp::decont(&g), Code, '$!signature'));
+    ret;
+}
 my &infix:<o> := &infix:<∘>;
 
 # vim: ft=perl6 expandtab sw=4