initial

initial.fs

type symbol = Number of int | Plus | Minus

let rec calc (s: symbol list) : int =
    match s with
        | [] -> 0
        | [Number x] -> x
        | Number x :: Number y :: rest -> calc(Number ((int)(sprintf "%d%d"  x y)) :: rest)
        | Number x :: rest -> x + calc(rest)
        | Plus :: rest -> + calc(rest)
        | Minus :: rest -> - calc(rest)

let rec genPossibleSolutions (start: int) (stop: int) = seq {
    if start = stop then yield [Number stop]
    else for n in start .. start do
            for i in genPossibleSolutions (start + 1) stop do
              yield Number n :: i
              yield Number n :: Minus :: i
              yield Number n :: Plus:: i
}

genPossibleSolutions 1 9 |> Seq.filter (fun x -> 100 = calc x) |> Seq.toList 
    |> List.length |> printfn "Is %d long"