From: Robert Klemme <bob.news@...>
Date: 2004-08-07T04:46:25+09:00
Subject: Re: Defining a new function by composition


"Edgardo Hames" <ehames@gmail.com> schrieb im Newsbeitrag
news:478c16ae0408061209424c2032@mail.gmail.com...
> Hi.
>
> I would like to add a method to the Array class, say Array#pop! which
> is actually split!(-1). But then, I can think of a more general
> problem: define a new function by composition of two or more
> functions.
> In Haskell, I can do something like
>
> f :: a -> b -> c
> f x y = something
>
> g::b -> c
> g = f some_value
>
> What is the Ruby equivalent of this?
>
> Regards,
> Ed
>
>

How about

class Array
  def pop!() split!(-1) end
end

Note: pop! and split! are not functions but methods.  So you always have an
implicit argument (named 'self' in Ruby).  I think this does not lend easily
to chaining the way you seek.

Of course, for the general case you could do something like this:

module Kernel
  private
  def chain(name, *funcs)
    eval "def #{name}(*a) #{funcs.map {|f| "#{f}("}}*a#{")" * funcs.size}
end"
  end
end

>> def foo(x) "<#{x}>" end
=> nil
>> def bar(x) "[#{x}]" end
=> nil
>> chain :xxx, :foo, :bar
=> nil
>> xxx 100
=> "<[100]>"

Or a more functional approach:

module Kernel
  private
  def chain2(*funcs)
    lambda {|*a| funcs.inject(a){|val, fun| send(fun, *val) } }
  end
end

>> xx2 = chain2 :bar, :foo
=> #<Proc:0x10169340@(irb):61>
>> xx2.call 100
=> "<[100]>"

Note the different order.

Regards

    robert