Question

I have the next code:

datatype expr = K of string| Number2 of expr * (expr list);
datatype number = Number1 of string | Number3 of int;
 fun append (nil, l2) = l2 
  | append (x::xs, l2) = x::append(xs, l2);
 fun map [] = []
    | map (h::t) = (What h)::(map t);
fun What (K x) = [Number1(x)]
    |What (Number2 (t,[])) = Number3(0)::What(t)
    |What (Number2 (y,a::b)) =  append(What(a), map(b));

It doesn’t recognize the function “What”.(unbound variable or constructor). How can I fix it, that it will know the function “What”?

Thanks.

Answer 1

Declarations in SML work from top to bottom, so map doesn’t see What. Switching the order won’t help, as then What wouldn’t see map, giving the same error. Instead, you need to declare the mutually recursive functions simultaneously using and:

fun map [] = []
  | map (h::t) = (What h)::(map t)
and What (K x) = [Number1(x)]
  | What (Number2 (t,[])) = Number3(0)::What(t)
  | What (Number2 (y,a::b)) =  append(What(a), map(b))