From: "zverok (Victor Shepelev) via ruby-core" <ruby-core@...>
Date: 2023-10-24T20:53:06+00:00
Subject: [ruby-core:115157] [Ruby master Misc#19971] Confusing arity of a Proc with implicit rest parameter

Issue #19971 has been updated by zverok (Victor Shepelev).





@shan I don't think this documentation definition covers the case described by OP. 



`proc { |a,| }` is not the same as `proc { |a| }`, and is semantically equivalent of `proc { |a,*| }`. I think it is reasonable to expect its `arity`���and, by the way, `parameters`, would behave as in latter, not as in former.



I think this would be reasonable (and might be useful for metaprogramming code that checks the signature of the passed callable object):

```ruby

proc { |a| a }.call([1, 2, 3]) #=> [1, 2, 3]

proc { |a| }.arity #=> 1

proc { |a| }.parameters #=> [[:opt, :a]]



proc { |a, *| a }.call([1, 2, 3]) #=> 1

proc { |a, *| }.arity #=> -2

proc { |a, *| }.parameters #=> [[:opt, :a], [:rest, :*]]



proc { |a,| a }.call([1, 2, 3]) #=> 1, like the latter

proc { |a,| }.arity #=> 1, should be -2

proc { |a,| }.parameters #=> [[:opt, :a]], should be [[:opt, :a], [:rest, :*]]

```



Though, experimenting a bit more with it, there is a funny edge case:

```ruby

define_method(:m1, &proc { |a| })

define_method(:m2, &proc { |a, *| })

define_method(:m3, &proc { |a,| })



p method(:m1)  #<Method: Object#m1(a)>

p method(:m2)  #<Method: Object#m2(a, *)>

p method(:m3)  #<Method: Object#m3(a)> --- like 1, not like 2

```

...which is probably related to reported arity/parameters.





----------------------------------------

Misc #19971: Confusing arity of a Proc with implicit rest parameter

https://bugs.ruby-lang.org/issues/19971#change-105064



* Author: andrykonchin (Andrew Konchin)

* Status: Open

* Priority: Normal

----------------------------------------

I've noticed that such proc `proc { |a,| }` has arity `1` that means 1 required parameter, but behaves like there is a rest parameter (`proc { |a, *| }`) that has arity `-2` (that means 1 required parameter + rest parameter).



So I am wondering whether it's intentional behaviour and what the rational is behind it.







-- 

https://bugs.ruby-lang.org/

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