6.8.3. Constrained class method types

ConstrainedClassMethods

Since:6.8.1 Status:Included in GHC2024, GHC2021 Implied by:MultiParamTypeClasses

Allows the definition of further constraints on individual class methods.

Haskell 98 prohibits class method types to mention constraints on the class type variable, thus:

class Seq s a where
  fromList :: [a] -> s a
  elem     :: Eq a => a -> s a -> Bool

The type of elem is illegal in Haskell 98, because it contains the constraint Eq a, which constrains only the class type variable (in this case a). More precisely, a constraint in a class method signature is rejected if

• The constraint mentions at least one type variable. So this is allowed:

  class C a where
    op1 :: HasCallStack => a -> a
    op2 :: (?x::Int) => Int -> a
  

• All of the type variables mentioned are bound by the class declaration, and none is locally quantified. Examples:

  class C a where
    op3 :: Eq a => a -> a    -- Rejected: constrains class variable only
    op4 :: D b => a -> b     -- Accepted: constrains a locally-quantified variable `b`
    op5 :: D (a,b) => a -> b -- Accepted: constrains a locally-quantified variable `b`
  

GHC lifts this restriction with language extension ConstrainedClassMethods. The restriction is a pretty stupid one in the first place, so ConstrainedClassMethods is implied by MultiParamTypeClasses.