From: John Carter <john.carter@...>
Date: 2009-01-14T08:11:17+09:00
Subject: Point free (pointless) programming in ruby?

I'm very fond of the notion of Concatenative Languages such as Joy,
Factor...
   http://en.wikipedia.org/wiki/Joy_(programming_language)
   http://www.latrobe.edu.au/philosophy/phimvt/joy/jp-joyjoy.html
   http://factorcode.org/

There is a supreme simplicity about them.

So while reading what the current state of them is, I stumbled across
the notion of Point free (sometimes called Pointless) programming.

The idea of sequences of function compositions that elide the
arguments that they will be applied to.

http://www.haskell.org/haskellwiki/Pointfree

One can easily define a sequence of methods in Ruby...

fun_seq =  [:a, :b, :c]

as a sequence of symbols.

Which one could apply to an arbitrary argument..

irb
> f = [:to_s, :succ]
=> [:to_s, :succ]
> f.inject(4,:send)
=> "5"

Oooh! Looky! That's sneaky of me!
   symbol_sequence.inject(arg,:send)

   [:a, :b, :c].inject(arg,:send)
that's equivalent to
   arg.a.b.c

Not quite function composition. Cute, but let's try...

irb
> def a(a)
>  p ["a",a]
> a+"a"
> end
=> nil
> def b(b)
>  p ["b",b]
> b+"b"
> end
=> nil
>  def c(c)
> p ["c",c]
> c+"c"
> end
=> nil
> f=[:a,:b,:c]
=> [:a, :b, :c]
> f.inject("foo"){|memo,obj|send(obj,memo)}
["a", "foo"]
["b", "fooa"]
["c", "fooab"]
=> "fooabc"

[:a, :b, :c].inject(arg){|memo,obj|send(obj,memo)}
  is equivalent to
c(b(a(arg)))

> c(b(a("bah")))
["a", "bah"]
["b", "baha"]
["c", "bahab"]
=> "bahabc"

Or how about working with lambda's or Proc objects...

> a1 = lambda {|x| x+"a"}
=> #<Proc:0xb7d9f744@(irb):41>
> b1 = lambda {|x| x+"b"}
=> #<Proc:0xb7e0feb8@(irb):42>
> c1 = lambda {|x| x+"c"}
=> #<Proc:0xb7dfa5e0@(irb):43>
> [a1, b1, c1]
=> [#<Proc:0xb7d9f744@(irb):41>, #<Proc:0xb7e0feb8@(irb):42>, #<Proc:0xb7dfa5e0@(irb):43>]
> [a1, b1, c1].inject("foo"){|memo,proc| proc.call(memo)}
=> "fooabc"


Anyhoo! Clearly in principle one can do Pointfree programming Ruby.

Questions for the Group:

1) Is there a neater way of expressing a sequence of function
    compositions in Ruby?

2) Which Ruby Pointfree sequences are actually useful?



John Carter                             Phone : (64)(3) 358 6639
Tait Electronics                        Fax   : (64)(3) 359 4632
PO Box 1645 Christchurch                Email : john.carter@tait.co.nz
New Zealand