1-9 and things

nums.sml

datatype exp = Num of int
             | Plus of exp * int
             | Minus of exp * int

fun eval (Num n) = n 
  | eval (Plus (a, b)) = eval a + b
  | eval (Minus (a, b)) = eval a - b

fun expstr (Num n) = Int.toString n
  | expstr (Plus (a, b)) = expstr a ^ "+" ^ Int.toString b
  | expstr (Minus (a, b)) = expstr a ^ "-" ^ Int.toString b

fun join (n, (Num x)) = Num ((x * 10) + n)
  | join (n, (Plus (a, b))) = Plus (a, (b * 10) + n)
  | join (n, (Minus (a, b))) = Minus (a, (b * 10) + n)

fun next (exp, n) =
    let val p = Plus (exp, n)
        val m = Minus (exp, n)
        val j = join (n, exp)
        fun foo x = (x, n + 1)
    in [foo p, foo m, foo j]
    end

val res =
    let fun generate (exp, n) =
        if n = 10
        then if eval exp = 100 then [exp] else [] 
        else List.concat (map generate (next (exp, n)))
    in generate (Num 1, 2)
    end

val resstr = map expstr res