.. _superclass-rules:
Undecidable (or recursive) superclasses
---------------------------------------
.. extension:: UndecidableSuperClasses
:shortdesc: Allow all superclass constraints, including those that may
result in non-termination of the typechecker.
:since: 8.0.1
Allow all superclass constraints, including those that may result in
non-termination of the typechecker.
The language extension :extension:`UndecidableSuperClasses` allows much more flexible
constraints in superclasses.
A class cannot generally have itself as a superclass. So this is illegal ::
class C a => D a where ...
class D a => C a where ...
GHC implements this test conservatively when type functions, or type variables,
are involved. For example ::
type family F a :: Constraint
class F a => C a where ...
GHC will complain about this, because you might later add ::
type instance F Int = C Int
and now we'd be in a superclass loop. Here's an example involving a type variable ::
class f (C f) => C f
class c => Id c
If we expanded the superclasses of ``C Id`` we'd get first ``Id (C Id)`` and
thence ``C Id`` again.
But superclass constraints like these are sometimes useful, and the conservative
check is annoying where no actual recursion is involved.
Moreover genuinely-recursive superclasses are sometimes useful. Here's a real-life
example (:ghc-ticket:`10318`) ::
class (Frac (Frac a) ~ Frac a,
Fractional (Frac a),
IntegralDomain (Frac a))
=> IntegralDomain a where
type Frac a :: Type
Here the superclass cycle does terminate but it's not entirely straightforward
to see that it does.
With the language extension :extension:`UndecidableSuperClasses` GHC lifts all restrictions
on superclass constraints. If there really *is* a loop, GHC will only
expand it to finite depth.