Bug #12671: Hash#to_proc result is not a lambda, but enforces arity

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-darwin14]
• Backport: 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN
• Tags: core, maybenotbug

[headius (Charles Nutter)]

 $ ruby23 -e 'pr = {foo:1}.to_proc; puts pr.lambda?; pr.call rescue puts $!; pr.call(1, 2) rescue puts $!'
false
wrong number of arguments (given 0, expected 1)
wrong number of arguments (given 2, expected 1)

I believe it should be marked as a lambda, since it enforces arity.

[nobu (Nobuyoshi Nakada)]

None-lambda doesn't mean that it never checks its arguments.
And if it's a lambda, it doesn't match the arity value.

As for the implementation detail, there is no room for arity in ifuncs.

[headius (Charles Nutter)]

JRuby implements Hash#to_proc as:

class Hash
  def to_proc
    method(:[]).to_proc
  end
end

This allows us to present the proc as a lambda with correct arity:

$ jruby -e "pr = {}.to_proc; puts pr.arity; puts pr.lambda?"
1
true

It works for MRI too:

$ ruby23 -e "class Hash; def to_proc; method(:[]).to_proc; end; end; pr = {}.to_proc; puts pr.arity; puts pr.lambda?"
1
true

I think this is more representative of this proc's behavior. Can MRI do it this way?

[mame (Yusuke Endoh)]

This ticket was briefly discussed at today's dev meeting, and matz said that the result of Hash#to_proc should be a lambda.

[mame (Yusuke Endoh)]

• Status changed from Open to Closed

Applied in changeset git|d514ba8e17106c6d159c3902ac5456d6269731f8.

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Proc made by Hash#to_proc should be a lambda [Bug #12671]

Like Symbol#to_proc (f0b815dc670b61eba1daaa67a8613ac431d32b16)

[marcandre (Marc-Andre Lafortune)]

• Status changed from Closed to Open

Is there a reason why the arity was not fixed to 1 (instead of -1)?

[Eregon (Benoit Daloze)]

Looking at the commit, should Hash#to_proc use rb_func_lambda_new() instead of rb_func_proc_new()? (and then rb_func_proc_new is unused)
Also it seems confusing that rb_func_proc_new() creates a lambda, so using rb_func_lambda_new() seems better here.
Code changed quite a bit though around there.

[Eregon (Benoit Daloze)]

Proposed fix (simple as it gets): https://github.com/ruby/ruby/pull/3370

[Eregon (Benoit Daloze)]

• Status changed from Open to Closed

Applied in changeset git|241244739f2b721ac7aa0961bd90d904c5e3fff6.

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Fix arity of Hash#to_proc [Bug #12671]