[ruby-core:109000] [Ruby master Feature#18644] Coerce anything callable to a Proc

Issue #18644 has been updated by [e-mail] (Joel Drapper).


I really like the first option but unfortunately it makes every object respond to `to_proc` even when they don't respond to `call`. Perhaps a third option is for the & operator to try to coerce using `to_proc` and failing that, try `method(:call).to_proc` if the object responds to `call`.

Alternatively there might be a way to provide a default definition for `to_proc` on only objects that respond to `call`. Here's a hacky way to do that in Ruby.

```ruby
class Object
  def method_missing(name, ...)
    if name == :to_proc && respond_to?(:call)
      if respond_to?(:define_method)
        define_method(name) { method(:call).to_proc }
      else
        define_singleton_method(name) { method(:call).to_proc }
      end

      method(:call).to_proc
    else
      super
    end
  end

  def respond_to_missing?(name, ...)
    name == :to_proc && respond_to?(:call, ...) || super
  end
end
```

----------------------------------------
Feature #18644: Coerce anything callable to a Proc
https://bugs.ruby-lang.org/issues/18644#change-98112

* Author: waiting_for_dev (Marc Busqu辿)
* Status: Open
* Priority: Normal
----------------------------------------
Functional objects are increasingly popular in Ruby. Having objects that respond to `#call` makes them interchangeable with a `Proc`.

However, when you need to perform some Proc-specific operation, like currying, you have to break the abstraction and ask for the type of object. Example:

```ruby
(callable.is_a?(Proc) ? callable : callable.method(:call)).curry[value]
```

Because of https://bugs.ruby-lang.org/issues/18620, it's not possible to make them polymorphic by taking the `:call` method:

```ruby
callable.method(:call).curry[value] # won't work!
```

Consequently, I propose adding a built-in Ruby way to coerce anything callable to a proc (examples in Ruby):

### Option 1: `Object#to_proc`

```ruby
class Object
  def to_proc
    return method(:call).to_proc if respond_to?(:call)
  
    raise "Needs to respond to :call"
  end
end

class Proc
  def to_proc
    self
  end
end

callable.to_proc.curry[value]
```

### Option 2. `Kernel#Proc`

```ruby
class Kernel
  def Proc(value)
    if value.is_a?(::Proc)
      value
    elsif value.respond_to?(:call)
      value.method(:call).to_proc
    else
      raise "Needs to implement :call"
    end
  end
end

Proc(callable).curry[value]
```



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