F# filter out only first occurrence from list

Question

I have a list and I want to remove an element matching some criteria but remove only one element.

let items = [1;2;3]

let predicate x =
    x >= 2

let result = items |> List.fold ...
// result = [1;3]

How to achieve method returning list with [1;3]?

Answer 1

You can use a generic recursive function

let rec removeFirst predicate = function
    | [] -> []
    | h :: t when predicate h -> t
    | h :: t -> h :: removeFirst predicate t

or a tail recursive one (if you fear a stack overflow)

let removeFirst predicate list =
    let rec loop acc = function
        | [] -> List.rev acc
        | h :: t when predicate h -> (List.rev acc) @ t
        | h :: t -> loop (h :: acc) t
    loop [] list