[ruby-core:107841] [Ruby master Bug#18620] Not possible to partially curry lambda or proc's `call` method

Issue #18620 has been updated by jeremyevans0 (Jeremy Evans).


Turns out that fixing `Method#arity` is not sufficient for this to work.  This is because `Method#curry` does the equivalent `to_proc` on the Method, which also loses the arity.  So that also needs work.  I submitted a pull request to fix this, though the approach used is suboptimal: https://github.com/ruby/ruby/pull/5640

Note that I'm not sure I would consider this a bug.  The pull request fixes this particular use case (Proc#method(:call)), but this curry issue occurs with all Method/Proc objects where the arity is not known.  I'll add this as an issue for discussion at the next developer meeting.

----------------------------------------
Bug #18620: Not possible to partially curry lambda or proc's `call` method
https://bugs.ruby-lang.org/issues/18620#change-96772

* Author: waiting_for_dev (Marc Busqu辿)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.1.1p18 (2022-02-18 revision 53f5fc4236) [x86_64-linux]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
You can curry the call method of a regular object:

``` ruby
class Foo
  def foo(a, b)
    a + b
  end
end
Foo.new.method(:foo).curry[1][2] # => 3
```

You can also curry a lambda:

```ruby
lambda { |a, b| a + b }.curry[1][2] # => 3
```

Same for a proc:

```ruby
proc { |a, b| a + b }.curry[1][2] # => 3
```

However, currying a lambda or proc's `call` method doesn't work as expected if not enough arguments are given. Returned error differs in each case:

```ruby
lambda { |a, b| a + b }.method(:call).curry[1][2] # => `block in <top (required)>': wrong number of arguments (given 1, expected 2) (ArgumentError)
```

```ruby
proc { |a, b| a + b }.method(:call).curry[1][2] # => `+': nil can't be coerced into Integer (TypeError)
```

You can however totally apply the function:

```ruby
lambda { |a, b| a + b }.method(:call).curry[1, 2] # => 3
```

```ruby
proc { |a, b| a + b }.method(:call).curry[1, 2] # => 3
```

It prevents using callable objects (responding to `#call`) and lambda/procs in a polymorphic way in regards to currying.



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