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.. _options-language:
Controlling extensions
----------------------
.. index::
single: language; option
single: options; language
single: extensions; options controlling
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 :ref:`language-pragma`).
.. extension:: GHC2021
:shortdesc: Use GHC’s set of default language extensions from 2021
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``.
Note that, because GHC2021 includes a number of non-standardized
extensions, the stability guarantees it provides are not quite as strong as
those provided by, e.g., :extension:`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 :extension:`RankNTypes`).
The ``GHC2021`` language set comprises the following extensions:
.. hlist::
* :extension:`BangPatterns`
* :extension:`BinaryLiterals`
* :extension:`ConstrainedClassMethods`
* :extension:`ConstraintKinds`
* :extension:`DeriveDataTypeable`
* :extension:`DeriveFoldable`
* :extension:`DeriveFunctor`
* :extension:`DeriveGeneric`
* :extension:`DeriveLift`
* :extension:`DeriveTraversable`
* :extension:`DoAndIfThenElse`
* :extension:`EmptyCase`
* :extension:`EmptyDataDecls`
* :extension:`EmptyDataDeriving`
* :extension:`ExistentialQuantification`
* :extension:`ExplicitForAll`
* :extension:`FieldSelectors`
* :extension:`FlexibleContexts`
* :extension:`FlexibleInstances`
* :extension:`ForeignFunctionInterface`
* :extension:`GADTSyntax`
* :extension:`GeneralisedNewtypeDeriving`
* :extension:`HexFloatLiterals`
* :extension:`ImplicitPrelude`
* :extension:`ImportQualifiedPost`
* :extension:`InstanceSigs`
* :extension:`KindSignatures`
* :extension:`MonomorphismRestriction`
* :extension:`MultiParamTypeClasses`
* :extension:`NamedFieldPuns`
* :extension:`NamedWildCards`
* :extension:`NumericUnderscores`
* :extension:`PatternGuards`
* :extension:`PolyKinds`
* :extension:`PostfixOperators`
* :extension:`RankNTypes`
* :extension:`RelaxedPolyRec`
* :extension:`ScopedTypeVariables`
* :extension:`StandaloneDeriving`
* :extension:`StandaloneKindSignatures`
* :extension:`StarIsType`
* :extension:`TraditionalRecordSyntax`
* :extension:`TupleSections`
* :extension:`TypeApplications`
* :extension:`TypeOperators`
* :extension:`TypeSynonymInstances`
.. extension:: Haskell2010
:shortdesc: Use the Haskell 2010 language variant.
Compile Haskell 2010 language variant. Enables the
following language extensions:
.. hlist::
* :extension:`CUSKs`
* :extension:`DatatypeContexts`
* :extension:`DoAndIfThenElse`
* :extension:`EmptyDataDecls`
* :extension:`FieldSelectors`
* :extension:`ForeignFunctionInterface`
* :extension:`ImplicitPrelude`
* :extension:`MonomorphismRestriction`
* :extension:`PatternGuards`
* :extension:`RelaxedPolyRec`
* :extension:`StarIsType`
* :extension:`TraditionalRecordSyntax`
.. extension:: Haskell98
:shortdesc: Use the Haskell 98 language variant.
Compile using Haskell 98 language variant. Enables the
following language extensions:
.. hlist::
* :extension:`CUSKs`
* :extension:`DatatypeContexts`
* :extension:`FieldSelectors`
* :extension:`ImplicitPrelude`
* :extension:`MonomorphismRestriction`
* :extension:`NPlusKPatterns`
* :extension:`NondecreasingIndentation`
* :extension:`StarIsType`
* :extension:`TraditionalRecordSyntax`
Although not recommended, the deprecated :ghc-flag:`-fglasgow-exts` flag enables
a large swath of the extensions supported by GHC at once.
.. ghc-flag:: -fglasgow-exts
:shortdesc: Deprecated. Enable most language extensions;
see :ref:`options-language` for exactly which ones.
:type: dynamic
:reverse: -fno-glasgow-exts
:category: misc
The flag ``-fglasgow-exts`` is equivalent to enabling the following extensions:
.. include:: ../what_glasgow_exts_does.rst
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.
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