6.8.3. Undecidable (or recursive) superclasses

UndecidableSuperClasses
Since

8.0.1

Allow all superclass constraints, including those that may result in non-termination of the typechecker.

The language 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 (#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 UndecidableSuperClasses GHC lifts all restrictions on superclass constraints. If there really is a loop, GHC will only expand it to finite depth.