orin% poly
Poly/ML 5.7.1 Release
> signature FOO = sig type foo val foo : foo end;
signature FOO = sig val foo: foo type foo end
> structure Foo : FOO = struct type foo = int val foo = 0 end;
structure Foo: FOO
> Foo.foo = 0;
val it = true: bool
> structure Foo :> FOO = struct type foo = int val foo = 0 end;
structure Foo: FOO
> Foo.foo = 0;
poly: : error: Type error in function application.
   Function: = : ''a * ''a -> bool
   Argument: (Foo.foo, 0) : Foo.foo * int
   Reason: Can't unify ''a to Foo.foo (Requires equality type)
Found near Foo.foo = 0
Static Errors
>