typebased

types.fs

type symbol =  Number of int | Plus of int * symbol | Minus of int * symbol | Join of int * symbol

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

let rec flattenJoin ((i,s): (int*symbol)) =
    match s with
        | Number n -> Number (merge i n)
        | Plus (n,t) -> Plus (merge i n,t)
        | Minus (n,t) -> Minus (merge i n,t)
        | Join (n,t) -> flattenJoin (merge i n,t)

let rec calc (s: symbol) : int =
        match s with
            | Number x -> x
            | Plus (x,s) -> x + calc(s)
            | Minus (x,s) -> x- calc(s)
            | Join (x,s) -> calc(flattenJoin (x, s)) 
let rec genSolutions (s:int) (stopp: int) = seq {
    if s = stopp then yield Number s
    else for i in genSolutions (s+1) stopp do
             yield Plus (s,i) 
             yield Minus (s,i)
             yield Join (s,i)
}

let rec prettyPrint (s:symbol) :string =
    match s with
        | Number n -> sprintf "%d" n
        | Join (n,t) -> sprintf "%d" n + prettyPrint t
        | Plus (n,t) -> sprintf "%d" n + "+" + prettyPrint t
        | Minus (n,t) -> sprintf "%d" n + "-" + prettyPrint t

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