Syntax for module option

Question

I'm trying to make a type with an optional map:

module CharMap = Map.Make(Char)
type trie = bool * CharMap.t option

But this results in a syntax error:

Error: The type constructor CharMap.t expects 1 argument(s),
       but is here applied to 0 argument(s)

What am I doing wrong?

Answer 1

CharMap.t is a map from char to 'a, so actually its type is 'a Charmap.t, so you forget to specify the polymorphic argument. So you should write:

type 'a trie = bool * 'a CharMap.t option

If you want your map to be monomorphic (for instance char -> int) you can just write:

type trie = bool * int CharMap.t option