How to use infix operator from a SML module?

Question

I have the following to code and I use SML/NJ:

signature STACK=
sig

    type 'a Stack

    val empty :'a Stack
    val isEmpty : 'a Stack -> bool

    val cons : 'a*'a Stack -> 'a Stack
    val head : 'a Stack ->'a
    val tail : 'a Stack -> 'a Stack
    val ++ : 'a Stack * 'a Stack -> 'a Stack
end
structure List : STACK = 
 struct
 infix 9 ++
type 'a Stack = 'a list

val empty = []
fun isEmpty s = null s

fun cons (x,s) = x::s
fun head s = hd s
fun tail s = tl s
fun xs ++ ys = if isEmpty xs then ys else cons(head xs, tail xs ++ ys)    

end

I want to use the ++ operator from the interpreter but when I write s1 List.++ s2 where s1 and s2 stack types I get the message that operator is not a function.

Thanks.

Answer 1

You've declared ++ as infix inside the structure, and that declaration is restricted to the scope of the structure (inside struct...end). You can declare it as infix at the top-level, or use it as prefix, but in SML infix declarations aren't part of the signature.

- List.++ ([1], [2,3]);
val it = [1,2,3] : int Stack

- infix 9 ++;
infix 9 ++
- open List;
...
- [1] ++ [2,3];
val it = [1,2,3] : int Stack

Check this out for some interesting hacks: http://www.mlton.org/InfixingOperators