Split list into two equal lists in F#

Question

I\'m really new to F#, and I need a bit of help with an F# problem.

I need to implement a cut function that splits a list in half so that the output would be...

Answer 1

(I didn't like my previous answer so I deleted it)

The first place to start when attacking list problems is to look at the List module which is filled with higher order functions which generalize many common problems and can give you succinct solutions. If you can't find anything suitable there, then you can look at the Seq module for solutions like @BrokenGlass demonstrated (but you can run into performance issues there). Next you'll want to consider recursion and pattern matching. There are two kinds of recursion you'll have to consider when processing lists: tail and non-tail. There are trade-offs. Tail-recursive solutions involve using an accumulator to pass state around, allowing you to place the recursive call in the tail position and avoid stack-overflows with large lists. But then you'll typically end up with a reversed list! For example,

Tail-recursive gencut solution:

let gencutTailRecursive n input =
    let rec gencut cur acc = function
        | hd::tl when cur < n ->
            gencut (cur+1) (hd::acc) tl
        | rest -> (List.rev acc), rest //need to reverse accumulator!
    gencut 0 [] input

Non-tail-recursive gencut solution:

let gencutNonTailRecursive n input =
    let rec gencut cur = function
        | hd::tl when cur < n ->
            let x, y = gencut (cur+1) tl //stackoverflow with big lists!
            hd::x, y
        | rest -> [], rest
    gencut 0 input

Once you have your gencut solution, it's really easy to define cut:

let cut input = gencut ((List.length input)/2) input