Language extensions: Difference between revisions
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Language extensions are used to enable language features in Haskell that may seem useful in certain cases. They can be used to loosen restrictions in the type system or add completely new language constructs to Haskell. | Language extensions are used to enable language features in Haskell that may seem useful in certain cases. They can be used to loosen restrictions in the type system or add completely new language constructs to Haskell. | ||
They can be enabled using the [http://www.haskell.org/ghc/docs/7.0.4/html/users_guide/pragmas.html#language-pragma LANGUAGE pragma] or (in GHC) using | They can be enabled using the [http://www.haskell.org/ghc/docs/7.0.4/html/users_guide/pragmas.html#language-pragma LANGUAGE pragma] | ||
<hask> | |||
{-# LANGUAGE <Extension>, <Extension> #-} | |||
</hask> | |||
or (in GHC) using [https://downloads.haskell.org/~ghc/master/users-guide/lang.html flags] <code>-X<Extension></code>. | |||
Before just using the language extension that fits your need, [[Use of language extensions|think about | Before just using the language extension that fits your need, [[Use of language extensions|think about | ||
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** [http://www.haskell.org/ghc/docs/latest/html/users_guide/deriving.html#deriving-typeable DeriveTraversable] | ** [http://www.haskell.org/ghc/docs/latest/html/users_guide/deriving.html#deriving-typeable DeriveTraversable] | ||
** [http://www.haskell.org/ghc/docs/latest/html/users_guide/deriving.html#id616911 GeneralizedNewtypeDeriving] | ** [http://www.haskell.org/ghc/docs/latest/html/users_guide/deriving.html#id616911 GeneralizedNewtypeDeriving] | ||
* [https://downloads.haskell.org/~ghc/master/users-guide/glasgow_exts.html#duplicate-record-fields DuplicateRecordFields] (GHC 8.0.1+) : Allow definition of record types with identically-named fields. | |||
* [[Existential type|ExistentialQuantification]] | * [[Existential type|ExistentialQuantification]] | ||
* [[Flexible contexts|FlexibleContexts]] | * [[Flexible contexts|FlexibleContexts]] | ||
Revision as of 09:01, 14 February 2017
Language extensions are used to enable language features in Haskell that may seem useful in certain cases. They can be used to loosen restrictions in the type system or add completely new language constructs to Haskell.
They can be enabled using the LANGUAGE pragma
{-# LANGUAGE <Extension>, <Extension> #-}
or (in GHC) using flags -X<Extension>.
Before just using the language extension that fits your need, think about when it is useful and what risk it may bring to your program.
List of language extensions by name
This list is far from complete and needs extension.
- DatatypeContexts : Add type constraints to your datatype.
- DefaultSignatures
- Derive:
- DuplicateRecordFields (GHC 8.0.1+) : Allow definition of record types with identically-named fields.
- ExistentialQuantification
- FlexibleContexts
- FlexibleInstances : Allow a type parameter to occure twice within a type class instance.
- FunctionalDependencies : Are used to constrain the parameters of type classes.
- GADTs : Generalised algebraic datatypes - A more general approach to algebraic datatypes.
- ImplicitParams
- KindSignatures
- MultiParamTypeClasses : Enable multiple type parameters in type classes.
- NoMonomorphismRestriction
- OverlappingInstances
- Rank2Types
- RankNTypes
- ScopedTypeVariables
- TemplateHaskell
- TupleSections : Allow tuples to be partially applied.
- TypeFamilies
- UndecidableInstances
- ViewPatterns