From: Louis J Scoras <louis.j.scoras@...>
Date: 2005-10-20T06:06:37+09:00
Subject: Re: Functional with Ruby

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On 10/19/05, Florian Frank <flori@nixe.ping.de> wrote:
>
>
> I think the point is, that you can use foldr, a function that is defined
> in Haskell only in terms of recursion. But Ruby's inject isn't defined
> in Ruby, it's a method of Array (a stateful data structure) written in C
> and it is actually implemented iterative. The Ruby version isn't really
> functional, it's only very similar, while the Haskell foldr is a
> recursivley defined function. So if you want to play with functional
> style programming in Ruby, using Array#inject is a bit fishy.
>
> On the other hand, if you want to write a reasonable fast program in
> Ruby, I would avoid recursion as much as possible, go for iteration, and
> concentrate on using methods implemented in C instead. But it seemed to
> me, that the OP was more interested in learning about functional
> programming (using Ruby as well as Haskell), than writing fast programs.



Hehe. You got me there. How about this then?

class Array
def fold(start =3D nil, &block)
start =3D self[0] unless start
return start if self.length =3D=3D 0
yield(self[1..-1].fold(start, &block), self[0])
end
def my_sum; fold {|total, i| total + i} ; end
def my_length; fold(0) {|length, i| length + 1} ; end
def my_to_s; fold('') {|buffer, i| buffer << i.to_s }.reverse ; end
end

x =3D [1,3,5,2,3]
puts x.my_sum
puts x.my_length
puts x.my_to_s

Or is the problem that there are method calls at all? Isn't Haskell defined
in C? Eventually at some level you're going to have to maintain state. We
have C because of good old Von Neumann.

Again, I'm very rusty, but doesn't this boil down to not having
side-effects? Is inject referentially transparent if you consider the
reciever to be an implicit parameter?


--
Lou

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