Too many overloaded symbols error

[FissoreD]

type none (X -> o) -> list X -> o.

main :- (X = none; X = some 2).

In the code above, the type checker complains that there are too many
types that could be assigned to X. However, the term
X = some 2 fixes the type of X, removing the
ambiguity.

Should the type checker be more clever?

[FissoreD]

I don't remember if there is a syntax to explicitly type variables, such as
main :- ((X:option _) = none; X = some 2).

[gares]

Yes, the type cast should work