smlsmlnj

SML function call doesn't parse arguments as arguments

I'm getting what seems to be a precedence issue in calling an SML function, substitute:

fun substitute v new (typevar q) = ...

And I am calling this from another function:

fun new_type_vars (typevar v) non_gens = 
  substitute v new_var() (typevar v)

But I get an error:

C:/sml/type_checker.sml:22.48-23.44 Error: operator and operand don't agree [tycon mismatch]
  operator domain: type_exp
  operand:         unit -> string
  in expression:
    (substitute v) new_var

Which seems to suggest that it's trying to call (substitute v), and then call the result of that call with argument new_var.

I've tried adding parentheses around the whole thing, but that doesn't help, and when I add parenthesis around the arguments like (v new_var...) it thinks that v is a function application on new_var. What's going on with this function call?

Solution

  • You probably want to change

    substitute v new_var() (typevar v)

    to

    substitute v (new_var()) (typevar v)

    (boutta's answer explains why.)