From: Edward Faulkner <ef@...>
Date: 2005-10-20T04:00:39+09:00
Subject: Re: Functional with Ruby

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On Thu, Oct 20, 2005 at 03:41:09AM +0900, Andreas Semt wrote:
> here a really simple Haskell function, which sums all elements of a list
> (from the book "The Haskell School of Expression"):
> --------------------------------
> listSum [] =3D 0
> listSum (x : xs) =3D x + listSum xs
> --------------------------------
>=20
> Which is the best way to say it in Ruby (functional style)? Don't use
> 'enum.inject', please!

But inject *is* functional!  It's equivalent to Lisp's accumulate.  It
doesn't modify the list at all or have any side-effects. =20

Perhaps you're asking for a function that takes the list as an
argument, rather than a method that operates on the lisp.  For
example:

def listsum(x)
  x.size =3D=3D 0 ? 0 : x[0] + listsum(x[1..-1])
end

This looks more like the Haskell, but it's not idiomatic for Ruby.  It
only looks like a standalone function, it's really a method on Object.
Ruby can be used this way, but it's going against the grain.

The Ruby way is definitely x.inject {|a,b| a + b}, or if you'll be
using it a lot:

class Array
  def sum
    inject {|a,b| a+b}
  end
end

regards,
Ed

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