list solution

list.fs

type symbol = Number of int | Plus | Minus

let merge (i1: int) (i2:int)  = (int)(sprintf "%d%d" i1 i2)

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

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

let rec prettyPrint (s:symbol list) :string =
    match s with
        | [] -> ""
        | Number n :: rest -> n.ToString() + prettyPrint rest
        | Plus :: rest -> "+" + prettyPrint rest
        | Minus :: rest -> "-" + prettyPrint rest

genSolutions 1 9 |> Seq.filter (fun x -> 100 = calc x) |> 
    Seq.map((prettyPrint)) |> Seq.toList |> printfn "%A"