Haskell-breaking-changes

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

• Ongoing: the change is already rolled out.
• Upcoming: the change is made upstream, and will be released in near future.
• Planned: the change has been approved by the committee. Implementation is not necessaryly available

Ongoing: Word8#

• Summary: Word{8,16,32} are now defined in terms of the corresponding primitive types instead of Word#.
• Reason: Small word types used to store Word# inside, making record types containing them space-inefficient.
• Since: GHC 9.2
• Breakage: major
• Proposal: https://ghc-proposals.readthedocs.io/en/latest/proposals/0074-small-primitives.html
• Merge request: https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4390
• How to fix: insert word*ToWord* (https://hackage.haskell.org/package/ghc-prim-0.8.0/docs/GHC-Prim.html#v:word8ToWord-35-) conditionally (cf. Fix build on GHC 9.2 by josephcsible · Pull Request #354 · haskell-crypto/cryptonite) using CPP

Upcoming: Remove Control.Monad.Trans.List

• Summary: Control.Monad.Trans.List is gone
• Reason: ListT is a monad only if the underlying monad is commutative; therefore it is not a valid 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.

Upcoming: Remove Control.Monad.Trans.Error

• Summary: Control.Monad.Trans.Error is removed in favour of Control.Monad.Trans.Except.
• Reason: ErrorT imposes an Error constraint, making it difficult to use as a general-purpose Either transformer.
• 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

Upcoming: Monomorphise Data.List

• Summary: Foldable/Traversable-polymorphic functions in Data.List get specialised to []
• Reason: There's a custom importing Data.List unqualified, making additions like singleton and uncons controversial. Making the list API monomorphic and encouraging to import it qualified make it more consistent with other container modules.
• 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

Planned: forall becomes a keyword

• Summary: 'forall' will become a keyword at the term level
• Reason: Making Type-level and term-level syntax more consistent is a way forward to introduce dependent types
• Proposal: Visible 'forall' in types of terms
• ETA: GHC 9.10?
• Tracking issue: N/A
• Merge request: N/A
• Expected breakage: minor (authors of the affected packages were notified)
• Relevant warnings: -Wforall-identifier (introduced and enabled by default in GHC 9.4)
• How to fix: Rename identifiers

Planned: remove return from Monad

• Summary: return gets removed from the Monad class and becomes a top-level function defined as return = pure
• Reason: Now that Monad is a subclass of Applicative, return is completely redundant (it doesn't make sense if return is not 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)

Planned: remove mappend from Monoid

• Summary: mappend gets removed from the Monoid class and becomes a top-level function defined as mappend = (<>)
• Reason: Since Semigroup is a superclass of Monoid, mappend does not have to be defined.
• 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

Planned: remove (/=) from the the Eq class

• Summary: (/=) gets removed from the Eq class and becomes a top-level function defined as a /= b = not (a == b)
• Reason: The only valid semantics of x /= y is not (x == y); unlike compare vs (<=), there's no room for any performance improvements.
• Proposal: https://github.com/haskell/core-libraries-committee/issues/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)

Planned: disable StarIsType by default

• Summary: * will no longer be parsed as a kind of value types (Type).
• Reason: Special treatment to * complicates the syntax and prevents the use of it as a type operator.
• Proposal: https://github.com/ghc-proposals/ghc-proposals/pull/143
• ETA: Unknown
• Tracking issue: N/A
• Merge request: N/A
• Expected breakage: medium
• Relevant warnings: -Wstar-is-type
• How to fix: Replace * with Type (from Data.Kind)

See also

• deprecation-mechanisms/type-class-methods (not implemented; it would be nice to have method deprecation)
• Warnings and sanity-checking
• Approved core library proposals
• Accepted GHC proposals
• GHC Boot Library Version History: useful for updating version constraints