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 DescriptionStabilityNeg Stability
Included from
Excluded fromNotesImplied byImplies
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AllowAmbiguousTypesAllow the user to write ambiguous types, and the type inference engine to infer them.ExperimentalStableWorsens type errors and usually better to use visible forall; unimplemented proposal to allow AMBIGUOUS pragmas instead
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AlternativeLayoutRuleDeprecatedDeprecatedUndocumented https://gitlab.haskell.org/ghc/ghc/-/issues/11359
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AlternativeLayoutRuleTransitional
DeprecatedDeprecatedUndocumented https://gitlab.haskell.org/ghc/ghc/-/issues/11359
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ApplicativeDoEnable Applicative do-notation desugaringExperimentalStableProposed alternative ApplicativeComprehensions https://github.com/ghc-proposals/ghc-proposals/pull/526 and looks somewhat complex/buggy https://gitlab.haskell.org/ghc/ghc/-/issues/17768
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ArrowsEnable arrow notation extensionExperimentalStableLongstanding bugs https://gitlab.haskell.org/ghc/ghc/-/issues/344
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AutoDeriveTypeableDeprecated; now default. Automatically derive Typeable for all user-defined datatypesDeprecatedDeprecatedAlwaysUndocumented. Cannot be disabled since GHC 8.2
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BangPatternsEnable bang patterns.StableStableGHC2021
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BinaryLiteralsEnable support for binary literals.StableStableGHC2021
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BlockArgumentsAllow do blocks and other constructs as function arguments.StableStableGHC20XX?
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CApiFFIEnable the CAPI calling convention.StableStableGHC20XX?
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ConstrainedClassMethodsEnable constrained class methods.StableStableGHC2021MultiParamTypeClasses
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ConstraintKindsEnable a kind of constraints.StableStableGHC2021
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CPPEnable the C preprocessor.StableStable
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CUSKsEnable detection of complete user-supplied kind signatures.LegacyStableHaskell2010StandaloneKindSignatures is a better alternative
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DataKindsEnable datatype promotion.StableStableGHC2024TypeData is a partial alternative; proposal under review to split out TypeLevelLiterals
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DatatypeContextsAllow contexts on data types.LegacyStableHaskell2010Command-line flag emits a deprecation warning already, but part of Haskell2010, so cannot be removed for now
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DeepSubsumptionEnable deep subsumptionLegacyStableHaskell98GHC2021For backwards compatibility with pre-simplified subsumption code
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DefaultSignaturesEnable default signatures.StableStableGHC20XX?
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DeriveAnyClassEnable deriving for any class.StableStableRichard's proposal excludes from Stable2024
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DeriveDataTypeableEnable deriving for the Data class.StableStableGHC2021Rename to DeriveData, since Typeable is now auto-derived?
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DeriveFoldableEnable deriving for the Foldable class.StableStableGHC2021DeriveTraversable
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DeriveFunctorEnable deriving for the Functor class.StableStableGHC2021DeriveTraversable
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DeriveGenericEnable deriving for the Generic class.StableStableGHC2021
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DeriveLiftEnable deriving for the Lift classStableStableGHC2021
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DeriveTraversableEnable deriving for the Traversable class.StableStableGHC2021DeriveFunctor, DeriveFoldable
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DerivingStrategiesEnables deriving strategies.StableStableGHC2024
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DerivingViaEnable deriving instances via types of the same runtime representation.StableStableGHC20XX?Candidate for GHC20xxDerivingStrategies
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DisambiguateRecordFieldsEnable record field disambiguation.StableStableGHC2024RecordWildCards
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DoAndIfThenElseAllow unindented if-then-else inside do notationStableLegacyHaskell2010Undocumented but part of Haskell2010 and later, so probably best to keep it (https://gitlab.haskell.org/ghc/ghc/-/issues/18631#note_540744)
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DoRecDeprecatedDeprecatedUndocumented, replaced by RecursiveDoRecursiveDo
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DuplicateRecordFieldsAllow definition of record types with identically-named fields.ExperimentalStableRichard's proposal includes in Stable2024
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EmptyCaseAllow empty case alternatives.StableStableGHC2021
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EmptyDataDeclsAllow definition of empty data types.StableStableGHC2021
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EmptyDataDerivingAllow deriving instances of standard type classes for empty data types.StableStableGHC2021
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ExistentialQuantificationAllow existentially quantified type variables in types.StableStableGHC2021
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ExplicitForAllEnable explicit universal quantification.StableStableGHC2021ScopedTypeVariables, LiberalTypeSynonyms, RankNTypes, ExistentialQuantification
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ExplicitLevelImportsExperimentalStableNew in GHC 9.14; good future direction but needs stabilising
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ExplicitNamespacesEnable using the keyword type to specify the namespace of entries in imports and exports (Explicit namespaces in import/export).StableStableGHC2024Proposals to add features (but backwards-compatible), still okay to be Stable?TypeOperators, TypeFamilies
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ExtendedDefaultRulesUse GHCi's extended default rules in a normal module.StableStableOn by default in GHCi
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ExtendedLiteralsAllows defining unboxed numeric primitive values through #Type suffixesStable?StableGHC20XX?New in GHC 9.8. Richard's proposal includes in Stable2024
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FieldSelectorsControl visibility of field selector functions.StableExperimentalHaskell98Richard proposes using NoFieldSelectors by default
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FlexibleContextsRemove some restrictions on class contextsStableStableGHC2021
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FlexibleInstancesEnable flexible instances.StableStableGHC2021TypeSynonymInstances
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ForeignFunctionInterfaceEnable foreign function interface.StableStableHaskell2010
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FunctionalDependenciesEnable functional dependencies.ExperimentalStableDark corners of FunDeps + MPTCs + overlapping instances should be Experimental because the semantics is not well-defined. But MPTCs are Stable, and overlapping instances are available without an extension, so that implies FunDeps need to be ExperimentalMultiParamTypeClasses
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GADTsEnable generalised algebraic data types.StableStableGHC2024Not formally part of GHC2021, although GADTSyntax and ExistentialQuantification areGADTSyntax, MonoLocalBinds
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GADTSyntaxEnable generalised algebraic data type syntax.StableStableGHC2021
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GeneralisedNewtypeDerivingEnable newtype deriving.StableStableGHC2021
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GHCForeignImportPrimEnable prim calling convention. Intended for internal use only.ExperimentalStableDocumentation states "This feature is not intended for use outside of the core libraries that come with GHC." Richard's proposal allows in Stable2024.
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HexFloatLiteralsEnable support for hexadecimal floating point literals.StableStableGHC2021
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ImplicitParamsEnable Implicit Parameters.StableStable
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ImplicitPreludeDon't implicitly import Prelude.StableStableHaskell98(Negatively) RebindableSyntax
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ImplicitStagePersistenceStableExperimentalHaskell98New in GHC 9.14, default enabled
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ImportQualifiedPostAllows the syntax import M qualifiedStableStableGHC2021
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ImpredicativeTypesEnable impredicative types.
Experimental->Stable?
StableGHC20XX?Is this good enough to stabilise now, or does it have negative consequences?RankNTypes
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IncoherentInstancesEnable incoherent instances.ExperimentalStableMostly replaced by INCOHERENT pragmas, but still needed for given/wanted incoherence and role annotations on classes. Should be split and only partly Deprecated. Richard's proposal allows in Stable2024.OverlappingInstances
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InstanceSigsEnable instance signatures.StableStableGHC2021
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InterruptibleFFIEnable interruptible FFI.
Stable->Experimental
StableZubin has concerns? Maybe should be reclassified Experimental?
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JavaScriptFFIExperimentalStable
Undocumented and doesn't do anything https://gitlab.haskell.org/ghc/ghc/-/issues/23940
InterruptibleFFI
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KindSignaturesEnable kind signatures.StableStableGHC2021TypeFamilies, PolyKinds
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LambdaCaseEnable lambda-case expressions.StableStableGHC2024
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LazyFieldAnnotationsStableStableGHC20XX?New, but Stable as a dependency of Strict?
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LexicalNegationUse whitespace to determine whether the minus sign stands for negation or subtraction.Stable?StableGHC20XX?
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LiberalTypeSynonymsEnable liberalised type synonyms.StableStableGHC20XX?Does this have negative consequences?
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LinearTypesEnable linear types.ExperimentalStable
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ListTuplePunsStableExperimentalNew
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MagicHashAllow # as a postfix modifier on identifiers.StableStableGHC20XX?Forces (#) as an operator to have whitespace, but good to allow a bunch of low-level stuff without extensions
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ModifiersExperimentalStableNew
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MonadComprehensionsEnable monad comprehensions.StableStable
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MonoLocalBindsDo not generalise local bindings.StableLegacyGHC2024Not part of GHC2021 although ExistentialQuantification and GADTSyntax are. NoMonoLocalBinds is discouraged but remains Legacy due to widespread useTypeFamilies, GADTs
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MonomorphismRestrictionDisable the monomorphism restriction.StableStableHaskell98Switched off in GHCi
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MultilineStringsExperimentalStableNew
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MultiParamTypeClassesEnable multi parameter type classes.StableStableGHC2021FunctionalDependenciesConstrainedClassMethods
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MultiWayIfEnable multi-way if-expressions.StableStable\cases from LambdaCase is more general, so prefer that?
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NamedDefaultsExperimentalStableNew
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NamedFieldPunsEnable record puns.StableStableGHC2021
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NamedWildCardsEnable named wildcards.StableStableGHC2021Proposal to tweak design of wildcards, but part of GHC2021 so Stable
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NegativeLiteralsEnable support for negative literals.TBC: Stable?StablePerhaps Legacy as being replaced by LexicalNegation?
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NondecreasingIndentationAllow nested contexts to be at the same indentation level as its enclosing context.LegacyStableHaskell98Haskell2010Documentation not very clear about status. Used by H98 only. Error messages include "Suggested fix: Perhaps you intended to use NondecreasingIndentation" when perhaps it should not be suggested? Richard excludes from Stable2024
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NPlusKPatternsEnable support for n+k patterns.LegacyStableHaskell98
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NullaryTypeClassesDeprecatedDeprecatedDocumentation claims "Deprecated, does nothing. nullary (no parameter) type classes are now enabled using MultiParamTypeClasses." but actually it does still enable nullary type classes.
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NumDecimalsEnable support for 'fractional' integer literals.StableStableGHC20XX?
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NumericUnderscoresEnable support for numeric underscores.StableStableGHC2021
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OrPatternsExperimentalStableNew
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OverlappingInstancesEnable overlapping instances.DeprecatedDeprecatedUse OVERLAP* pragmas instead
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OverloadedLabelsEnable overloaded labels.StableStable
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OverloadedListsEnable overloaded lists.StableStable
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OverloadedRecordDotRecord '.' syntaxStableStable
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OverloadedRecordUpdateRecord '.' syntax record updatesExperimentalStableChange to Experimental after implementation of https://github.com/ghc-proposals/ghc-proposals/pull/583
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OverloadedStringsEnable overloaded string literals.StableStableCan introduce ambiguities, and exposes users to unxpected instance behaviour (e.g. https://github.com/haskell/bytestring/issues/140)
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PackageImportsEnable package-qualified imports.DeprecatedDeprecatedBlurs boundaries between GHC and package manager. Unclear if this should be Legacy or Deprecated; see https://gitlab.haskell.org/ghc/ghc/-/issues/25025 and https://gitlab.haskell.org/ghc/ghc/-/issues/24227
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ParallelArraysDeprecatedDeprecatedUndocumented. Presumably a leftover of DPH. Only effect is enabling ParallelListComp. Enabled by base.ParallelListComp
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ParallelListCompEnable parallel list comprehensions.StableStableProbably nothing wrong with this, but not widely used or liked enough to include in GHC20xx?
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PartialTypeSignaturesEnable partial type signatures.StableStable
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PatternGuardsEnable pattern guards.StableStableHaskell98
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PatternSignaturesDeprecated synonym for ScopedTypeVariables
Experimental/Deprecated?
StableChanges proposed (https://github.com/ghc-proposals/ghc-proposals/pull/608); mark as Deprecated then change to Experimental when those changes land?
ScopedTypeVariables
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PatternSynonymsEnable pattern synonyms.StableStable
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PolyKindsEnable kind polymorphism.StableStableGHC2021KindSignatures
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PolymorphicComponentsDeprecatedDeprecatedUndocumented, synonym for RankNTypes