6.1.1. Controlling editions and extensions

GHC supports multiple language editions: Haskell98, Haskell2010, GHC2021 and GHC2024. Each language edition consists of a collection of language extensions, and there are many other language extensions not currently part of a language edition but that can be enabled explicitly.

Currently, GHC2021 is used by default if no other language edition is explicitly requested, for backwards compatibility purposes. Since later versions of GHC may use a different language edition by default, users are advised to declare a language edition explicitly. Using GHC2024 is recommended for new code.

A language edition can be selected:

• at the package level, e.g. using default-language: GHC2024 in a .cabal file;

• with a command-line flag prefixed by “-X...” (e.g. -XGHC2024); or

• for an individual module using the LANGUAGE pragma, e.g. {-# LANGUAGE GHC2024 #-}.

Selecting a language edition overrides any previous selection. It is not possible to disable a language edition.

Similarly, language extensions can be controlled (either enabled or disabled):

• at the package level, e.g. using default-extensions: TemplateHaskell in a .cabal file;

• with command-line flags, switched on by a command-line flag “-X...” (e.g. -XTemplateHaskell), and switched off by the flag “-XNo...”; (e.g. -XNoTemplateHaskell);

• for an individual module using the LANGUAGE pragma, e.g. {-# LANGUAGE TemplateHaskell #-} or {-# LANGUAGE NoTemplateHaskell #-}.

GHC2024

Since:

9.10.1

GHC blesses a number of extensions, beyond Haskell 2010, to be suitable to turned on by default. These extensions are considered to be stable and conservative.

Note that, because GHC2024 includes a number of non-standardized extensions, the stability guarantees it provides are not quite as strong as those provided by, e.g., Haskell2010. While GHC does take pains to avoid changing the semantics of these extensions, changes may still happen (e.g. the simplified subsumption change introduced in GHC 9.0 which caused GHC to reject some programs using RankNTypes).

The GHC2024 language edition includes the following extensions:

BangPatterns BinaryLiterals ConstrainedClassMethods ConstraintKinds DataKinds DeriveDataTypeable DeriveFoldable DeriveFunctor DeriveGeneric DeriveLift DeriveTraversable DerivingStrategies DisambiguateRecordFields DoAndIfThenElse EmptyCase EmptyDataDecls EmptyDataDeriving ExistentialQuantification ExplicitForAll ExplicitNamespaces FieldSelectors FlexibleContexts FlexibleInstances ForeignFunctionInterface GADTs GADTSyntax GeneralisedNewtypeDeriving HexFloatLiterals

ImplicitPrelude ImportQualifiedPost InstanceSigs KindSignatures LambdaCase MonoLocalBinds MonomorphismRestriction MultiParamTypeClasses NamedFieldPuns NamedWildCards NumericUnderscores PatternGuards PolyKinds PostfixOperators RankNTypes RelaxedPolyRec RoleAnnotations ScopedTypeVariables StandaloneDeriving StandaloneKindSignatures StarIsType TraditionalRecordSyntax TupleSections TypeApplications TypeOperators TypeSynonymInstances ImplicitStagePersistence

GHC2021

Since:

9.2.1

See GHC2024 for general comments about GHC20xx language editions.

Also note that due to a minor oversight, enabling this edition behaves slightly differently than enabling each of its constituent extensions. Specifically, while TypeOperators implies ExplicitNamespaces, ExplicitNamespaces is not included in GHC2021. Moreover, while GADTs is not part of GHC2021, the combination of GADTSyntax and ExistentialQuantification is enough to define and use GADTs.

The GHC2021 language edition includes the following extensions:

BangPatterns BinaryLiterals ConstrainedClassMethods ConstraintKinds DeriveDataTypeable DeriveFoldable DeriveFunctor DeriveGeneric DeriveLift DeriveTraversable DoAndIfThenElse EmptyCase EmptyDataDecls EmptyDataDeriving ExistentialQuantification ExplicitForAll FieldSelectors FlexibleContexts FlexibleInstances ForeignFunctionInterface GADTSyntax GeneralisedNewtypeDeriving HexFloatLiterals ImplicitPrelude

ImportQualifiedPost InstanceSigs KindSignatures MonomorphismRestriction MultiParamTypeClasses NamedFieldPuns NamedWildCards NumericUnderscores PatternGuards PolyKinds PostfixOperators RankNTypes RelaxedPolyRec ScopedTypeVariables StandaloneDeriving StandaloneKindSignatures StarIsType TraditionalRecordSyntax TupleSections TypeApplications TypeOperators TypeSynonymInstances NoExplicitNamespaces ImplicitStagePersistence

Haskell2010

Compile using the Haskell 2010 language edition, as specified by the Haskell 2010 report. GHC aims to behave mostly as a Haskell 2010 compiler, but there are a few known deviations from the standard (see Haskell standards vs. Glasgow Haskell: language non-compliance).

The Haskell2010 language edition includes the following language extensions:

CUSKs DatatypeContexts DeepSubsumption DoAndIfThenElse EmptyDataDecls FieldSelectors ForeignFunctionInterface

ImplicitPrelude MonomorphismRestriction PatternGuards RelaxedPolyRec StarIsType TraditionalRecordSyntax ImplicitStagePersistence

Haskell98

Compile using the Haskell 98 language edition, as specified by the Haskell 98 report. GHC aims to behave mostly as a Haskell 98 compiler, but there are a few known deviations from the standard (see Haskell standards vs. Glasgow Haskell: language non-compliance).

The Haskell98 language edition includes the following language extensions:

CUSKs DatatypeContexts DeepSubsumption FieldSelectors ImplicitPrelude MonomorphismRestriction

NPlusKPatterns NondecreasingIndentation StarIsType TraditionalRecordSyntax ImplicitStagePersistence

Although not recommended, the deprecated -fglasgow-exts flag enables a large swath of the extensions supported by GHC at once.

-fglasgow-exts

The flag -fglasgow-exts is equivalent to enabling the following extensions:

ConstrainedClassMethods DeriveDataTypeable DeriveFoldable DeriveFunctor DeriveGeneric DeriveTraversable EmptyDataDecls ExistentialQuantification ExplicitNamespaces FlexibleContexts FlexibleInstances ForeignFunctionInterface FunctionalDependencies GeneralizedNewtypeDeriving ImplicitParams InterruptibleFFI

KindSignatures LiberalTypeSynonyms MagicHash MultiParamTypeClasses ParallelListComp PatternGuards PostfixOperators RankNTypes RecursiveDo ScopedTypeVariables StandaloneDeriving TypeOperators TypeSynonymInstances UnboxedTuples UnicodeSyntax UnliftedFFITypes

Enabling these options is the only effect of -fglasgow-exts. We are trying to move away from this portmanteau flag, and towards enabling features individually.