Question

I’m currently trying to extend a friend’s OCaml program. It’s a huge collection of functions needed for some data analysis.. Since I’m not really an OCaml crack I’m currently stuck on a (for me) strange List implementation:

type 'a cell = Nil     | Cons of ('a * 'a llist) and 'a llist = (unit -> 'a cell);; 

I’ve figured out that this implements some sort of ‘lazy’ list, but I have absolutely no idea how it really works. I need to implement an Append and a Map Function based on the above type. Has anybody got an idea how to do that?

Any help would really be appreciated!

Answer 1

let rec append l1 l2 =      match l1 () with         Nil -> l2 |          (Cons (a, l)) -> fun () -> (Cons (a, append l l2));;  let rec map f l =     fun () ->          match l () with             Nil -> Nil |              (Cons (a, r)) -> fun () -> (Cons (f a, map f r));; 

The basic idea of this implementation of lazy lists is that each computation is encapsulated in a function (the technical term is a closure) via fun () -> x. The expression x is then only evaluated when the function is applied to () (the unit value, which contains no information).