Question

I would like to start some questions about simplifying different expressions in F#.

Anyone have ideas for better and/or simpler implementation of insertAt (parameters could be reordered, too). Lists or Sequences could be used.

Here is some start implementation:

let insertAt x xs n = Seq.concat [Seq.take n xs; seq [x]; Seq.skip n xs]

Answer 1

The implementation dannyasher posted is a non-tail-recursive one. In order to make the function more efficient, we’ll have to introduce an explicit accumulator parameter which makes the function tail-recursive and allows the compiler to optimize the recursion overhead away:

let insertAt =
    let rec insertAtRec acc n e list = 
        match n, list with
        | 0, _     -> (List.rev acc) @ [e] @ list
        | _, x::xs -> insertAtRec (x::acc) (n - 1) e xs
        | _        -> failwith "Index out of range"

    insertAtRec []