How to skip type parameters in function signature

[Benny]

@heliosdrm

mymethod(x::Foo{A}) where {A}

You’re not skipping B there either, it’s a shorthand for a where clause: ::Foo{A,Bi} where Bi. Julia didn’t used to print shorthands of UnionAll types, so it’s worth checking with typeof now.

@heliosdrm

Is there also a way of telling “I don’t need the first type parameter, but I do want the second”?

You can do something similar to omit the 1st parameter from the method’s where clause, but it must be made clear there is a type parameter there rather than some other type from the global scope: mymethod(x::Foo{Ai, B} where Ai) where {B}. Ai will not be available as a variable in the method body, it’s limited to the parametric type expression.

@heliosdrm

it’s an “all-underscore identifier used as rvalue” syntax error.

_ is just a variable, not some placeholder token, and all-underscore variables are intended solely for throwaway assignments, so they can only show up on the left hand side of =. If you replaced it with a variable Ai, you’d hit an undefined variable error because Ai isn’t assigned in the outer scope or specified in a where clause.