I half agree with you. I think the “abstraction” your parent is talking about would correspond to typeclasses rather than polymorphic functions. That is, I thin

I half agree with you. I think the “abstraction” your parent is talking about would correspond to typeclasses rather than polymorphic functions. That is, I think that this would be a reasonable type for a function even if it is only called once: Eq a => a -> MyObj a -> Maybe Foo Whereas I think the following would not be ok (probably even if you had a lot of instances) class HasFoo o where getFoo :: Eq a => a -> o -> Maybe Foo instance HasFoo MyObj where ... I think the rule should be that one should write the most general code that minimises the entropy/size of the source code. That way one can prefer polymorphic functions (as they need less type-signature entropy and bytes, unless they have loads of constraints, in which case one should consider wrapping those constraints together), while still preferring not making crazy single-instance typeclasses.