F# undefined type error when annotating variable

Am I doing something fundamentally wrong?

This code looks like it expects mutable values:

        if Array.exists (fun (c,i) -> c = head) result then
            let index = Array.findIndex (fun (c,i) -> if c = head then true else false) result;
            Array.set result index (result.[index].[1]+1);
        else
            // new array value will be discarded
            Array.append result [|(head,1)|];
        _check tail result; // result will be unchanged

Array.appendReturns the modified array value; it does not change the value of the input array. result.[index].[1]+1It looks like you're trying to get the second item from the tuple using the array index syntax ( use sndinstead ) , but even if the function is fixed, there's still a problem because it will always recurse with the same/unchanged value result.

Also, you don't need to end every line with a semicolon in F#.

Here's a version of your function that can be used with minimal changes and no extra type annotations:

let letterCheck (a:string) =
    let aList = a |> Seq.toList
    let rec _check charList result = 
        match charList with
        | head :: tail -> 
            if Array.exists (fun (c,i) -> c = head) result then
                let index = Array.findIndex (fun (c,i) -> c = head) result
                Array.set result index (head, (snd(result.[index])+1))
                _check tail result
            else
                _check tail (Array.append result [|(head,1)|])
        | [] -> result
    _check aList [||]

> letterCheck "hello";;
val it : (char * int) array = [|('h', 1); ('e', 1); ('l', 2); ('o', 1)|]

Or have all the fun with:

"hello" |> Seq.countBy id