no-noneq-in-eq.md

What

CLC has approved the proposal to remove (/=) from class Eq. It means that class Eq will contain a single member (==) and (/=) will be just a normal function available from Prelude.

Before:

class Eq a where
  (==) :: a -> a -> Bool
  (/=) :: a -> a -> Bool

After:

class Eq a where
  (==) :: a -> a -> Bool

(/=) :: Eq a => a -> a -> Bool

When

The proposal was approved conditionally, pending a resolution of -Winline-rule-shadowing: Different for single and multi method classes? issue. At the very earliest the change may appear in GHC 9.8.

How

This is a breaking change, but of limited scope. If you import Prelude in full and either derive Eq or define only (==) in your instances, there is nothing to change. An impact analysis showed that only 45 package of current Stackage subset require updates, and thanks to @phadej we have all patches at hand: https://gist.github.com/phadej/0e810832685cb4e70e3ca3a61e2f33d1

The migration policy for this change is backward-compatible: you can migrate already and still retain compatibility with existing GHCs. Because of this CLC suggests applying patches at your earliest convenience.

1. If your code used to import Eq(..) from Prelude explicitly, please change

import Prelude (Eq(..))

to

import Prelude (Eq, (==), (/=))

2. If your code used to import Eq(..) directly from Data.Eq, please change

import Data.Eq (Eq(..))

to

import Data.Eq (Eq, (==), (/=))

3. If your code used to define both members of Eq, just remove the definition of (/=), so that

instance Eq Foo where
  x == y = foo x y
  x /= y = bar x y

becomes

instance Eq Foo where
  x == y = foo x y

4. If your code used to define only (/=), implement (==) instead, so that

instance Eq Foo where
  x /= y = foo x y

becomes

instance Eq Foo where
  x == y = not (foo x y)

That's it!

PR template

Here is a template, which you can use when raising PRs against affected libraries.

Title: Future-proof class Eq

CLC has approved the proposal to remove (/=) from class Eq (#3). It means that class Eq will contain a single member (==) and (/=) will be just a normal function.

The implementation of the proposal is delayed at least to GHC 9.4 and likely to GHC 9.6, but one can already future-proof code to be compliant with this change in a backwards-compatible way. No CPP required.

Migration guide and more info: https://github.com/haskell/core-libraries-committee/blob/main/guides/no-noneq-in-eq.md