6.1.1. Controlling extensions

Language extensions can be controlled (i.e. allowed or not) in two ways:

• Every language extension can be switched on by a command-line flag “-X...” (e.g. -XTemplateHaskell), and switched off by the flag “-XNo...”; (e.g. -XNoTemplateHaskell).
• Language extensions can also be enabled using the LANGUAGE pragma, thus {-# LANGUAGE TemplateHaskell #-} (see LANGUAGE pragma).

GHC2021

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.

GHC2021 is used by GHC if neither Haskell98 nor Haskell2010 is turned on explicitly. Since later versions of GHC may use a later GHC20xx by default, users are advised to declare the language set explicitly with -XGHC2021.

The GHC2021 language set comprises the following extensions:

ImplicitPrelude StarIsType MonomorphismRestriction TraditionalRecordSyntax EmptyDataDecls ForeignFunctionInterface PatternGuards DoAndIfThenElse RelaxedPolyRec BangPatterns BinaryLiterals ConstrainedClassMethods ConstraintKinds DeriveDataTypeable DeriveFoldable DeriveFunctor DeriveGeneric DeriveLift DeriveTraversable EmptyCase EmptyDataDeriving ExistentialQuantification ExplicitForAll

FlexibleContexts FlexibleInstances GADTSyntax GeneralisedNewtypeDeriving HexFloatLiterals ImportQualifiedPost InstanceSigs KindSignatures MultiParamTypeClasses NamedFieldPuns NamedWildCards NumericUnderscores PolyKinds PostfixOperators RankNTypes ScopedTypeVariables StandaloneDeriving StandaloneKindSignatures TupleSections TypeApplications TypeOperators TypeSynonymInstances

Haskell2010

Compile Haskell 2010 language variant. Enables the following language extensions:

ImplicitPrelude StarIsType CUSKs MonomorphismRestriction DatatypeContexts TraditionalRecordSyntax

FieldSelectors EmptyDataDecls ForeignFunctionInterface PatternGuards DoAndIfThenElse RelaxedPolyRec

Haskell98

Compile using Haskell 98 language variant. Enables the following language extensions:

ImplicitPrelude StarIsType CUSKs MonomorphismRestriction NPlusKPatterns

DatatypeContexts TraditionalRecordSyntax FieldSelectors NondecreasingIndentation

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.