georgefst/Haskell-breaking-changes

This document describes a list of upcoming/proposed changes to Haskell core/de facto libraries.

Upcoming: Remove `Control.Monad.Trans.List`

Summary: Control.Monad.Trans.List is gone because it's not a monad transformer.
ETA: GHC 9.4 / transformers-0.6
Expected breakage: medium
Tracking issue: https://hub.darcs.net/ross/transformers/issue/75
Patch: https://hub.darcs.net/ross/transformers/patch/7c809b6f9db019ed761c971d10c86fa004f60d89
How to fix: remove instance declarations for transformer's ListT (deprecated). Users should switch to lawful alternatives such as pipes.

Summary: Control.Monad.Trans.Error is removed in favour of Control.Monad.Trans.Except.
ETA: GHC 9.4 / transformers-0.6
Expected breakage: medium
Tracking issue: N/A
Patch: https://hub.darcs.net/ross/transformers/patch/7c809b6f9db019ed761c971d10c86fa004f60d89
How to fix: remove instance declarations for ErrorT and switch to ExceptT

Summary: Foldable/Traversable-polymorphic functions in Data.List get specialised to []
ETA: GHC 9.6 / base-4.18?
Proposal: https://groups.google.com/g/haskell-core-libraries/c/q3zHLmzBa5E/m/OrHHKaJNAQAJ?pli=1
Tracking issue: #20025
Merge request: !5304 https://gitlab.haskell.org/ghc/ghc/-/merge_requests/5107
Expected breakage: significant
Relevant warnings: -Wcompat-unqualified-imports (implied by -Wcompat)
How to fix: import Data.List qualified or explicitly import functions that don't conflict with Foldable/Traversable variants

Summary: return gets removed from the Monad class and becomes a top-level function defined as return = pure
Proposal: GHC wiki: monad-of-no-return, related: Enable -Wnoncanonical-monad-instances and -Wnoncanonical-monoid-instances by default
ETA: Unknown
Tracking issue: N/A
Merge request: N/A
Expected breakage: minor (canonical definition of return will be ignored)
Relevant warnings: -Wnoncanonical-monad-instances (enabled by default as of GHC 9.2)
How to fix: Remove definitions of return (compatible up to GHC 7.10)

Summary: return gets removed from the Monad class and becomes a top-level function defined as mappend = (<>)
Proposal: GHC wiki: semigroup-monoid, related: Enable -Wnoncanonical-monad-instances and -Wnoncanonical-monoid-instances by default
ETA: Unknown
Tracking issue: N/A
Merge request: N/A
Expected breakage: major (without an update to the warning)
Relevant warnings: -Wnoncanonical-monoid-instances (enabled by default as of GHC 9.2)
How to fix: Remove definitions of mappend (compatible up to GHC 8.4)
TODO: make -Wnoncanonical-monoid-instances warn manual declarations of mappend

Summary: (/=) gets removed from the Eq class and becomes a top-level function defined as a /= b = not (a == b)
Proposal: haskell/core-libraries-committee#3
ETA: Unknown
Tracking issue: N/A
Merge request: N/A
Expected breakage: 131 packages according to the proposal
How to fix: Remove definitions of (/=) (backward-compatible)