How to state that a type variable in a newtype statement is of a type that belongs to some type class?

Suppose that I have this newtype:

newtype SomeType a = SomeType { foo :: OtherType a }

I want to ensure that a is showable (belongs to the type class Show x).

How do I ensure that? (Is it even possible?)

Bonus points: am I using the terminology correctly?

Solution

It is possible with the DatatypeContexts extension, but it is strongly discouraged:

newtype Show a => SomeType a = SomeType { foo :: Maybe a }

It is recommended to put the constraint on the functions that use SomeType or use GADTs. See the answers to these questions for more information.

Basically, it doesn't add anything useful and it makes you have to put constraints where they wouldn't otherwise be necessary.