that's incorrect haskell implements the "perfect derive" behaviour that the blog post is complaining about rust's lack of. the constraint is only propagated to

that's incorrect haskell implements the "perfect derive" behaviour that the blog post is complaining about rust's lack of. the constraint is only propagated to the field types when using a haskell derive, not the parameters themselves (as in rust) so the following compiles just fine: data Foo -- not Eq data Bar a = Bar deriving Eq f :: Eq (Bar Foo) => () f = ()