Quantified predicate must have a class or type variable head [GHC-02550]

Quantified constraints are constraints which have their own constraints. The head of a quantified constraint is the constraint on the right side of the => operator. For example, the head of the quantified constraint Ord a => Eq a is Eq a. This error message states that the head of a quantified constraint must not be a constraint resulting from, for example, a type family; it must be either a class constraint or a type variable.

Examples

Bad quantified constraint in function type

In this example, a quantified constraint occurs in the type of a binding. Because the head of the quantified constraint is computed from a type family, we get the following error message:

• Quantified predicate must have a class or type variable head:
    a => A a
• In the quantified constraint ‘a => A a’
  In the type signature: f :: (a => A a) => ()

One potential fix for this is to convert the type family into a class.

QuantifiedFunction.hs

Before

{-# LANGUAGE QuantifiedConstraints #-}
{-# LANGUAGE TypeFamilies          #-}

module QuantifiedFunction where

import Data.Kind

class C a where

type family A :: k -> Constraint

f :: (C Int => A Int) => ()
f = ()

After

{-# LANGUAGE QuantifiedConstraints #-}
{-# LANGUAGE TypeFamilies          #-}

module QuantifiedFunction where

import Data.Kind

class C a where

class A a where

f :: (C Int => A Int) => ()
f = ()