User-specified instance is not allowed [GHC-97044]

There are several special type classes in base which require special treatment from the compiler. For this reason they don’t permit user-specified instances.

An attempt to provide an explicit instance declaration in your library / application code, e.g.

instance Coercible A B where ...

will lead to this error being reported.

The instances for these type classes are automatically created by GHC on an as-needed basis.

This restriction applies to the following type classes:

• Coercible - This class does not have regular instances; instead they are created on-the-fly during type-checking.
• Typeable - Since GHC 7.10, all types automatically have Typeable instances derived. This is in contrast to previous releases where Typeable had to be explicitly derived using the DeriveDataTypeable language extension.
• Type equality classes ~ and ~~ defined in Data.Type.Equality
• WithDict - is used to create dictionaries for classes with a single method. It is used to implement a primitive that we cannot define in Haskell but we can write in Core. Some details can be found in a Note in GHC library
• KnownChar - This class gives the Char associated with a type-level Char literal. There are instances of the class for every concrete literal: ‘a’, ‘b’, ‘c’ etc. But instances for user-defined types are not permitted.
• KnownNat - This class gives the integer associated with a type-level natural. There are instances of the class for every concrete literal: 0, 1, 2, etc. But instances for user-defined types are not permitted.
• KnownSymbol - This class gives the string associated with a type-level symbol. There are instances of the class for every concrete literal: “hello”, etc. But instances for user-defined types are not permitted.
• Generic - This class is one of the key pieces of GHC.Generics and you usually want to derive it using DeriveGeneric extension. Hand-written instances of Generic instances are allowed by default, but disallowed when SafeHaskell extension is enabled.

Examples

UserSpecifiedInstances.hs:9:10: error: [GHC-97044]
    • Class ‘Coercible’ does not support user-specified instances.
    • In the instance declaration for ‘Coercible Foo ()’
  |
9 | instance Coercible Foo ()
  |          ^^^^^^^^^^^^^^^^

UserSpecifiedInstances.hs:11:10: error: [GHC-97044]
    • Class ‘Typeable’ does not support user-specified instances.
    • In the instance declaration for ‘Typeable Foo’
   |
11 | instance Typeable Foo
   |          ^^^^^^^^^^^^

UserSpecifiedInstances.hs:13:10: error: [GHC-97044]
    • Class ‘~’ does not support user-specified instances.
    • In the instance declaration for ‘Foo ~ Foo’
   |
13 | instance Foo ~ Foo
   |          ^^^^^^^^^

UserSpecifiedInstances.hs

module Main where

import Data.Coerce (Coercible, coerce)
import Type.Reflection (Typeable(..))
import Data.Type.Equality

newtype Foo = Foo { unFoo :: () }

instance Coercible Foo ()

instance Typeable Foo

instance Foo ~ Foo

main :: IO ()
main = do
    let foo = Foo ()
    print (coerce foo:: ())
    print (typeOf foo)
    print (Refl :: Foo :~: Foo)

module Main where

import Data.Coerce
import Data.Type.Equality
import Type.Reflection

newtype Foo = Foo { unFoo :: () }

main :: IO ()
main = do
    let foo = Foo ()
    print (coerce foo:: ())
    print (typeOf foo)
    print (Refl :: Foo :~: Foo)