Dear Café, Is there prior art/existing packages for the following? Is it maybefunctional programming folklore? Is it a sign of bad program design? Sometimes I feel the need to selectively allow or disallow alternatives in a sum type. That is, suppose we have a sum type data Foo = LeftFoo !A | RightFoo !B and at some places in the program we want the type system to enforce that only the constructor LeftFoo can be used. My solution would be to use a strict version of Data.Functor.Const and make the type higher rank: newtype Const' a b = Const' !a -- Const' Void b ~ Void -- Const' () b ~ () data Foo' f = LeftFoo' !A | RightFoo' !(f B) type Foo = Foo' Identity type LeftFoo = Foo' (Const' Void) -- can not construct a RightFoo' The advantage over defining LeftFoo as an entirely different type is that Foo and LeftFoo can share functions operating entirely on the left option. Olaf