From: Florian Frank <flori@...>
Date: 2005-10-20T04:06:32+09:00
Subject: Re: Functional with Ruby

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 [] = 0
>listSum (x : xs) = x + listSum xs
>--------------------------------
>
>Which is the best way to say it in Ruby (functional style)? Don't use
>'enum.inject', please!
>  
>

You can abuse Ruby arrays for lists:

def list_sum(l)
  case l
  when [] then 0
  else l.first + list_sum(l[1, l.size])
  end
end
list_sum([1,2,3]) # => 6

But using Array#slice for getting the "tail" of the "list" could be 
inefficient.

You could also use my lazylist gem and the do this:

require 'lazylist'
def list_sum(l)
  case l
  when LazyList::Empty then 0
  else l.head + list_sum(l.tail)
  end
end
list_sum list(1,2,3)

That way you can also create an infinite ones lazy list in Ruby with 
this nice notation:
ones = list(1) { ones } # => [1,... ]

-- 
Florian Frank