[ruby-core:49182] [ruby-trunk - Feature #6284] Add composition for procs

["duerst (Martin Dürst)" <duerst@it.aoyama.ac.jp>]

Issue #6284 has been updated by duerst (Martin Dürst).


marcandre (Marc-Andre Lafortune) wrote:
> +1 for #*
> 
> The symbol used in mathematics for function composition is a circle (∘); the arrows are for the definitions of functions (like lambdas) only, so #<- or whatever make no sense to me.

Very good point.

> Finally, the f ∘ g(x) is defined as f(g(x)), so there is no argument there either.

Not true. Depending on which field of mathematics you look at, either (f ∘ g)(x) is either f(g(x)), or it is g(f(x)). The later is in particular true in work involving relations, see e.g. http://en.wikipedia.org/wiki/Composition_of_relations#Definition.

Speaking from a more programming-related viewpoint, f(g(x)) is what is used e.g. in Haskell, and probably in many other functional languages, and so may be familiar with many programmers.

However, we should take into account that a functional language writes e.g. reverse(sort(array)), so it makes sense to define revsort = reverse * sort (i.e. (f ∘ g)(x) is f(g(x))). But in Ruby, it would be array.sort.reverse, so revsort = sort * reverseve may feel much more natural (i.e. (f ∘ g)(x) is g(f(x))). 

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Feature #6284: Add composition for procs
https://bugs.ruby-lang.org/issues/6284#change-32728

Author: pabloh (Pablo Herrero)
Status: Feedback
Priority: Normal
Assignee: matz (Yukihiro Matsumoto)
Category: 
Target version: 2.0.0


=begin
It would be nice to be able to compose procs like functions in functional programming languages:

    to_camel = :capitalize.to_proc
    add_header = ->val {"Title: " + val}

    format_as_title = add_header << to_camel << :strip

instead of:

    format_as_title = lambda {|val| "Title: " + val.strip.capitalize }


It's pretty easy to implement in pure ruby:

  class Proc
    def << block
      proc { |*args| self.call( block.to_proc.call(*args) ) }
    end
  end
=end


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