Silent Whatevercode genning error

[lizmat]

The Problem

subset WalkCb of Callable where *.signature ~~ :(Int $);
my WalkCb $x = sub (Int $a) { return $a }

fails with:

Type check failed in assignment to $x; expected WalkCb but got Sub (sub (Int $a) { #`(Sub..

Expected Behavior

subset WalkCb of Callable where { .signature ~~ :(Int $) }
my WalkCb $x = sub (Int $a) { return $a }

works as expected.

Environment

• Operating system:
• Compiler version (perl6 -v):

This is Rakudo version 2018.04.1-91-gb8318b8 built on MoarVM version 2018.04.1-115-ga607c42

[lizmat]

Some more data points:

my constant A = :(Int $);
===SORRY!===␤QAST::Block with cuid 1 has not appeared

[b2gills]

A where constraint can take a block (lambda) or a statement.

… where { $_ == 42 }; # block
… where    * == 42;   # lambda
… where   $_ == 42;   # statement

The thing is that ~~ doesn't participate in the creation of WhateverCode lambdas (*)
So what you are really doing is creating a lambda of *.signature inside of a larger where statement (not lambda).

constant $lambda = *.signature;
subset WalkCb of Callable where $lambda ~~ :(Int $);

Which will of course never return True as the lambda has a signature of :(;; $ is raw)

---

So instead just write the statement form. (using implicit $_)

subset WalkCb of Callable where .signature ~~ :(Int $);
my WalkCb $x = sub (Int $a) { return $a }
say $x(42); # 42␤

---

It could be argued that ~~ should participate in WhateverCode lambdas, but that is a separate issue.

[lizmat]

Thanks for the explanation, also on SO! Closing now