6.8.6. Nullary type classes

NullaryTypeClasses
Since:7.8.1

Allows the use definition of type classes with no parameters. This extension has been replaced by MultiParamTypeClasses.

Nullary (no parameter) type classes are enabled with MultiParamTypeClasses; historically, they were enabled with the (now deprecated) NullaryTypeClasses. Since there are no available parameters, there can be at most one instance of a nullary class. A nullary type class might be used to document some assumption in a type signature (such as reliance on the Riemann hypothesis) or add some globally configurable settings in a program. For example,

class RiemannHypothesis where
  assumeRH :: a -> a

-- Deterministic version of the Miller test
-- correctness depends on the generalised Riemann hypothesis
isPrime :: RiemannHypothesis => Integer -> Bool
isPrime n = assumeRH (...)

The type signature of isPrime informs users that its correctness depends on an unproven conjecture. If the function is used, the user has to acknowledge the dependence with:

instance RiemannHypothesis where
  assumeRH = id