6.4.2. Data type contexts

DatatypeContexts
Since

7.0.1

Allow contexts on data types.

Haskell allows datatypes to be given contexts, e.g.

data Eq a => Set a = NilSet | ConsSet a (Set a)

give constructors with types:

NilSet :: Set a
ConsSet :: Eq a => a -> Set a -> Set a

This is widely considered a misfeature, and is going to be removed from the language. In GHC, it is controlled by the deprecated extension DatatypeContexts.