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Parametric types and recursion lead to attribute or accessor confusion #5813

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p6rt opened this issue Nov 26, 2016 · 1 comment
Open

Parametric types and recursion lead to attribute or accessor confusion #5813

p6rt opened this issue Nov 26, 2016 · 1 comment

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@p6rt
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p6rt commented Nov 26, 2016

Migrated from rt.perl.org#130183 (status was 'new')

Searchable as RT130183$

@p6rt
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p6rt commented Nov 26, 2016

From @moritz

In this small example, I get an exception where I don't expect one​:

role LL[​::T] {
  has T $.item;
  has LL[T] $.next;
  method visit(&c) {
  c($.item);
  $.next.visit(&c) if $.next;
  }
}

my $t = LL[Int].new(
  item => 5,
  next => LL[Int].new( item => 4 ),
);
say $t.perl;
$t.visit(&say);

Output​:

LL[Int].new(item => 5, next => LL[Int].new(item => 4, next => LL[T]))
5
No such method 'visit' for invocant of type 'Int'
  in method visit at role-param-bug.p6 line 6
  in block <unit> at role-param-bug.p6 line 15

It calls method visit on an Int, but $.next is a LL[Int], as the .perl
output confirms.

When I add a 'dd $.next' before the dying recursion call, it reports
$.next as Int 4. Which the type constraint shouldn't even allow as value
for $.next.

Other observations​:

* changing the name of the attribute doesn't make a difference
* changing $.next.visit(&amp;c) to $!next.visit(&c) makes the error go away.

Cheers,
Moritz

--
Moritz Lenz
https://deploybook.com/ -- https://perlgeek.de/ -- https://perl6.org/

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