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 usingRankNTypes
).The
GHC2024
language edition includes the following extensions:
-
GHC2021
¶ Since: 9.2.1 See
GHC2024
for general comments aboutGHC20xx
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
impliesExplicitNamespaces
,ExplicitNamespaces
is not included inGHC2021
. Moreover, whileGADTs
is not part ofGHC2021
, the combination ofGADTSyntax
andExistentialQuantification
is enough to define and use GADTs.The
GHC2021
language edition includes the following extensions:
-
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:
-
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:
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: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.