From: keystonelemur@...
Date: 2018-04-20T18:34:28+00:00
Subject: [ruby-core:86632] [Ruby trunk Feature#6284] Add composition for	procs

Issue #6284 has been updated by baweaver (Brandon Weaver).


yuroyoro (TOMOHITO Ozaki) wrote:
> Most languages do not define function composition in built-in operators, but provide them as function or method such as `compose`.
> 
> F.Y.I) https://rosettacode.org/wiki/Function_composition
> 
> In some few languages defined function composition operator as following.
> 
>   haskell : `.`
>   F# : `<<` and  `>>`
>   Groovy : `<<` and  `>>`
> 
> I think Ruby should proivide `compose` method (and `and_then` as forward composition), and alias some operator (like `#<<` or `#*) to them.
> 
> In my opinion, +1 for `<<` and `>>` instead of `*`. 
> Because there is no language define function composition as `*` and `+`, but F# and groovy defined as `<<` and `>>`. It is intutive.
> 
> By the way, It is useful if `Symbol#<<` (or `Symbol#+`) is shortcut method to `sym.to_proc.compose(other_proc)`.
> 
> ```
> arr.map(&:upcase.to_proc.compose(:to_s.to_proc))
> arr.map(&:to_s >> :upcase)
> ```
> It is more visually and intuitive.
> 
> My reference implemenation of composition is following.
> 
> https://github.com/yuroyoro/ruby/pull/7

I do like the idea of using shovel as it's already fairly common in the language, and present in others. I'd be interested in potentially making a container variant though:

~~~ ruby
arr.map(&:to_s >> :upcase)

# becomes
arr.map(&[:to_s, :upcase])
~~~

Though that would involve introducing the idea of Array#to_proc. I've used a similar technique in a few gems with `#[]()`, so a vague implementation may look like:

```ruby
class Array
  def to_proc
    self[1..].reduce(self[0].to_proc, :compose)
  end
end
```

Wherein it'd be nice if `#compose` tried to coerce its argument:

```ruby
def compose(sym)
  fn = sym.is_a?(Proc) ? sym : sym.to_proc

  ...
end
```

Currently planning on releasing a gem this weekend that will do a similar thing, it could serve as inspiration for an implementation, but my C is no where near good enough to try at the Ruby core level.

Aside: Have we ever considered implicitly `to_proc`ing anything passed to a method expecting a block? e.g. `arr.map(&:to_s)` becomes `arr.map(:to_s)`

----------------------------------------
Feature #6284: Add composition for procs
https://bugs.ruby-lang.org/issues/6284#change-71589

* Author: pabloh (Pablo Herrero)
* Status: Feedback
* Priority: Normal
* Assignee: matz (Yukihiro Matsumoto)
* Target version: 
----------------------------------------
=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

---Files--------------------------------
0001-proc.c-Implement-Proc-for-Proc-composition.patch (3.65 KB)
0002-proc.c-Implement-Method-for-Method-composition.patch (2.67 KB)
0003-proc.c-Support-any-callable-when-composing-Procs.patch (3.97 KB)


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