How to quit an iteration in OCaml?

Question

I try to go through a list xl by List.fold_left, I would like to quit the iteration when some condition is satisfied:

List.fold_left
  (fun x acc -> 
     if x = 5 then STOP THE ITERATION
     else x + acc)
xl

Could anyone tell me how to express STOP THE ITERATION here? Thank you

Edit1: By the code above, I would like to say we do not stop accumulation until we meet the first 5.

Answer 1

You cannot do this with the built in folds without an exception or some no-op being flagged in the accumulator for the remaining calls --as phimuemue mentions. Alternatively, you can just write a very simple tail-recursive function to take care of returning early,

(** Fold left on a list with function [f] until predicate [p] is satisfied **)
let rec fold_until f p acc = function
    | x :: xs when p x -> acc
    | x :: xs -> fold_until f p (f acc x) xs
    | [] -> acc

let accum_until_five =
    fold_until (fun acc x -> x + acc) (fun x -> x = 5) 0

Answer 2

Therefore, you'll have to encode the condition into your x argument of the inner function:

let t l =
    let (a,b) = 
        List.fold_left
            (fun ((accx, condition) as acc) x ->
                if condition then acc else (x+accx, x=5))
            (0,false) l
    in
    a;;

That is, you have to tell your function that - once the 5 has been seen - it should simply return the stuff it has done up to now.