1-9 to 100 directly

numbss.fs

let solve lim total =
  let rec gen (n::t) s =
    let m = (n % 10) % lim
    if m = 0 then 
      if n = lim then [(List.head (List.rev (n::t)), s)] else []
    else
      match t with
        | [] ->
          gen ((m+1)::n::t) (string n) @
          gen ((10 * n + m + 1)::t) s
        | sum::r ->
          gen ((m+1)::(sum + n)::r) (s + "+" + string n) @
          gen ((m+1)::(sum - n)::r) (s + "-" + string n) @
          gen ((10 * n + m + 1)::t) s
  gen [1] "" |> List.filter (fun (sum, s) -> sum = total) |> List.map (fun (sum, s) -> s)

solve 10 100 |> printfn "%A"