From: "trans (Thomas Sawyer)" <transfire@...>
Date: 2012-05-24T20:32:17+09:00
Subject: [ruby-core:45211] [ruby-trunk - Feature #6483] parametric map


Issue #6483 has been updated by trans (Thomas Sawyer).


=begin

  p %w[12 45 32].map(&.to_i(9)).map(& * 2)

That's pretty neat. I wonder about its implementation, so basically `&` becomes a special object that returns a proc when method is called on it?

  class Ampersand < BasicObject
    def method_missing(s, *a, &b)
      ::Proc.new{ |x| x.public_send(s, *a, &b) }
    end
  end

  & = Ampersand.new

This is also interesting in that it has an appearance similar to an anaphora (default block argument):

  p %w[12 45 32].map{it.to_i(9)}.map{it * 2}

But despite appearances they are very different in nature.
=end

----------------------------------------
Feature #6483: parametric map
https://bugs.ruby-lang.org/issues/6483#change-26796

Author: prijutme4ty (Ilya Vorontsov)
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 


I found very common use-case for map: map with parameters and &:meth syntax. For example:
matrix =[[1,2,3],[4,5,6]]
matrix.pmap(' ',&:join).join("\n") # => "1 2 3\n4 5 6
[1,2,3,4,5].pmap(2, &:to_s) # ['1', '10', '11', '100', '101']

[1,2,3,4,5].pmap(&:to_s) # ['1', '2, '3', '4', '5'] # empty parameter list behaves as usual map

Isn't it much better than ugly and verbose code: 
matrix.map{|line| line.join(' ')}.join("\n")

I can write simple implementation 
class Proc
  def curry_except_self(*args)
    Proc.new{|slf| curry[slf,*args] }
  end
end

module Enumerable
  def pmap!(*args,&block)
    map! &block.curry_except_self(*args)
  end
  def pmap(*args,&block)
    dup.pmap!(*args, &block)
  end
end

Use-cases can be rewritten as tests (I can send my own unit-test if needed)


Also I've two related things to discuss.
1) First is &-syntax. Is it possible to change ruby-interpeter in such a way that &:proc could be at any place. matrix.pmap(&:join,' ') is much prettier than matrix.join(' ',&:join) What is the reason behind this restriction? And if one can remove this restriction, we'd have new nice syntax.
2) I'm not very experience in functional programming with curry etc, but it seems to me that currying proc without supplying first argument(self) can be common task when &:meth syntax is in play. If so, may be my curry_except_self(*args) also should be included in ruby.


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