foldWhere.fsx

let rec foldWhere (f:'State -> 'T -> 'State) acc p list =
    match list with
    | [] -> acc
    | head::tail ->
        let acc = if p head then (f acc head) else acc
        foldWhere f acc p tail

//let countWhere<'T> = foldWhere (fun s (x:'T) -> s + 1) 0

let countWhere p l = List.fold (fun s x -> if p x then s + 1 else s) 0 l

countWhere ((>) 5) [1..100];;

countWhere (fun x -> x.ToString().Contains("1")) [1..100];;

List.filter (fun x -> x.ToString().Contains("1")) [1..100];;

List.fold (fun s x -> if x.ToString().Contains("1") then s + 1 else s) 0 [1..100]

(*

How about a foldWhere function with signature

    foldWhere :
      f:('State -> 'T -> 'State) ->
        acc:'State -> p:('T -> bool) -> C<'T> -> 'State

You can get countWhere from this with

    let countWhere<'T> = foldWhere (fun s (x:'T) -> s + 1) 0


Of course then we'll need foldRightWhere, scanWhere, reduceWhere, ...

*)