Issue #6253 has been updated by mame (Yusuke Endoh).

Status changed from Open to Assigned
Assignee set to matz (Yukihiro Matsumoto)

Hello,

2012/4/4 jballanc (Joshua Ballanco) <jballanc / gmail.com>:
> The example below demonstrates a Rack-like system where a curried proc is passed to a helper, then after partial application of the arguments, the returned curried proc is stored and later evaluated:
*snip*


I think Proc#curry is not an appropriate answer to your problem.
You should just create a new lambda:

    class NightGreeting
      def initialize(app)
        @app = app
      end
      def call(sender)
        proc do |receiver|
          @app.call(sender, receiver, greeting: "Goodnight")
        end #.curry # needed when the proc accepts multiple parameters
      end
    end


> Currying is really only useful when it can be used with partial application. However, Ruby currently limits how you can achieve partial application of curried procs. In particular, there is no way to manage partial application of parameters with default values. As such, it is not surprising that Proc#curry does not seem to have been adopted very widely. In my personal survey of ~600 gems that I use in various projects, I did not find any usage of Proc#curry.


In a sense, the situation is just as I expected :-)

I'm the person who made the proposal of Proc#curry in [ruby-dev:33676].
I suggested it with purely theoretical interest, or, just for the fun
of functional programming, such as playing with SKI combinator [1].
I did not think it would be used widely in real-world case, and even
I think it should not be used that way.

[1] http://en.wikipedia.org/wiki/SKI_combinator_calculus


Usage example in my products:

  - An compiler for Grass (esoteric functional language) [2]
  - An interpreter for Unlambda (yet another esoteric functional
    language) [3]
  - An interpreter for SKI-based joke language [4]

[2] https://github.com/mame/grass-misc/blob/master/yagc.rb
[3] http://d.hatena.ne.jp/ku-ma-me/20090203/p1 (in Japanese)
[4] https://github.com/mame/ikamusume


In addition,  I had another (ironical) intent; there is a lot of
confusion between curry and partial application [5].
I wanted to preempt the name "curry" in a correct terminology.

[5] http://lambda-the-ultimate.org/node/2266

-- 
Yusuke Endoh <mame / tsg.ne.jp>
----------------------------------------
Feature #6253: Implement a way to pass keyword options to curried procs
https://bugs.ruby-lang.org/issues/6253#change-25652

Author: jballanc (Joshua Ballanco)
Status: Assigned
Priority: Normal
Assignee: matz (Yukihiro Matsumoto)
Category: core
Target version: 2.0.0


=begin
(See original discussion in issue #4610)

With the introduction of keyword arguments in Ruby 2.0, it would be useful to have a way to set a keyword-based option on a curried proc. The example below demonstrates a Rack-like system where a curried proc is passed to a helper, then after partial application of the arguments, the returned curried proc is stored and later evaluated:

    class NicknameTranslator
      def initialize(app)
        @app = app
      end
    
      def call(name)
        case name
        when 'Robert'
          @app.call('Bob')
        when 'James'
          @app.call('Jimmy')
        else
          @app.call(name)
        end
      end
    end
    
    class NightGreeting
      def initialize(app)
        @app = app
        @app.pass_option(greeting: 'Goodnight')
      end
    
      def call(name)
        @app.call(name)
      end
    end
    
    class Greeter
      def initialize(helper)
        @helper = helper
        @app = lambda do |sender, receiver, greeting: 'Hello'|
          puts "#{sender} says \"#{greeting}\" to #{receiver}"
        end.curry
        @greetings = {}
      end
    
      def call(sender, receiver)
        @greetings[sender] ||= @helper.new(@app).call(sender)
        @greetings[sender].call(receiver)
      end
    end
    
    Greeter.new(NicknameTranslator).call('Josh', 'Joe')
    Greeter.new(NicknameTranslator).call('Robert', 'Joe')
    Greeter.new(NicknameTranslator).call('Josh', 'Robert')

If we wanted, however, to be able to set a keyword-based option in the helper, there is currently no way in Ruby 2.0 to do so. Currently, keyword arguments can only be used at the same time as the final non-keyword, non-default, non-rest argument to the proc is applied. So, for example, there is no way to do the above with NightGreeting in place of NicknameTranslator.

Currying is really only useful when it can be used with partial application. However, Ruby currently limits how you can achieve partial application of curried procs. In particular, there is no way to manage partial application of parameters with default values. As such, it is not surprising that Proc#curry does not seem to have been adopted very widely. In my personal survey of ~600 gems that I use in various projects, I did not find any usage of Proc#curry.

So, I would request a method like Proc#pass_option (or some other, better name) that allows for setting keyword arguments on a curried proc at any time.
=end



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