Illegal type signature in instance declaration [GHC-06202]

This error is emitted when class instance has type signature, but extension InstanceSigs is not enabled. Note that InstanceSigs is part of GHC2021 and GHC2024 editions, so you won’t encounter this error in new code unless you explicitely opt-out.

Examples

Illegal type signature in instance declaration

Error message

Type signatures are only allowed to be in instance declarations when the InstanceSigs language extension is enabled.

Example1.hs:6:13: error: [GHC-06202]
    • Illegal type signature in instance declaration:
        (==) :: A -> A -> Bool
    • In the instance declaration for ‘Eq A’
    Suggested fix: Perhaps you intended to use InstanceSigs
  |
6 |     (==) :: A -> A -> Bool

Example1.hs

Before

{-# LANGUAGE Haskell2010 #-}
module Example1 where

data A = A | B
instance Eq A where
    (==) :: A -> A -> Bool
    A == A = True
    B == B = True
    _ == _ = False

After

{-# LANGUAGE Haskell2010, InstanceSigs #-}
module Example1 where

data A = A | B
instance Eq A where
    (==) :: A -> A -> Bool
    A == A = True
    B == B = True
    _ == _ = False