Can't derive instances of nullary classes [GHC-04956]

With a clever combination of stand-alone deriving and an explicit deriving strategy, it is possible to get GHC to attempt to derive a type class instance for a nullary (that is, zero-argument) typeclass using a technique that works only for typeclasses with arguments.

Examples

Deriving instances for nullary classes

Error Message

NullaryClass.hs:7:1: error: [GHC-04956]
    • Can't make a derived instance of
        ‘ThisClassHasNoParameters’ with the stock strategy:
        Cannot derive instances for nullary classes
    • In the stand-alone deriving instance for
        ‘ThisClassHasNoParameters’
  |
7 | deriving stock instance ThisClassHasNoParameters
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Explanation

The class ThisClassHasNoParameters is a nullary class, which means that it takes no type parameters. Ordinarily, deriving only works for classes with a single parameter, and that check is carried out by a different part of the compiler that emits a different message. However, StandaloneDeriving allows the specification of type class derivation tasks that take an arbitrary number of arguments.

Ordinarily, the derived instance of ThisClassHasNoParameters would be performed using DeriveAnyClass, which fills in the instance body with zero method definitions. The deriving strategy stock pushes GHC to use a different method for instance derivation that requires non-nullary classes, resulting in this error.

NullaryClass.hs

Before

{-# LANGUAGE DeriveAnyClass #-}
{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE StandaloneDeriving #-}
module NullaryClass where

class ThisClassHasNoParameters
deriving stock instance ThisClassHasNoParameters

After

{-# LANGUAGE DeriveAnyClass #-}
{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE StandaloneDeriving #-}
module NullaryClass where

class ThisClassHasAParameter a
deriving instance ThisClassHasAParameter Int