1-9 to 100 with FParsec

numbs.fs

open FParsec

let rec permute xs = seq {
  match xs with 
    | [] -> yield ""
    | [x] -> yield string x
    | h::t -> 
      let s = string h
      for foo in permute t do
        yield s + foo
        yield s + "+" + foo
        yield s + "-" + foo
}

let eval str =
  let parse =
    let pp op = pstring op >>. pint32
    let ppos = pp "+"
    let pneg = pp "-" |>> (fun x -> -x)
    pipe2 pint32 (many (ppos <|> pneg)) (fun x y -> List.sum (x::y))
  match run parse str with
    | Success(n, _, _) -> n
    | _ -> 0

let solutions = Seq.filter (fun x -> eval x = 100)
	
[1..9] |> permute |> solutions |> Seq.toList |> printfn "%A"