.. _class-method-types:
Constrained class method types
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.. extension:: ConstrainedClassMethods
:shortdesc: Enable constrained class methods.
:since: 6.8.1
:status: Included in :extension:`GHC2021`
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
:extension:`ConstrainedClassMethods`. The restriction is a pretty stupid one in
the first place, so :extension:`ConstrainedClassMethods` is implied by
:extension:`MultiParamTypeClasses`.