error: incompatible types in assignment
--> main.sml:3:20
     |
   3 | FooUnwrapped: str = ResolveOptional(optional_value=Foo)
     |                     ^ has type `int`, expected `str`
--> (1) main.sml:3:14
     |
   3 | FooUnwrapped: str = ResolveOptional(optional_value=Foo)
     |               ^ expected `str` due to this