Language/Haskell/Extension.hs

    {-# LANGUAGE DeriveDataTypeable #-}
    {-# LANGUAGE DeriveGeneric #-}

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Language.Haskell.Extension
    -- Copyright   :  Isaac Jones 2003-2004
    -- License     :  BSD3
    --
    -- Maintainer  :  libraries@haskell.org
    -- Portability :  portable
    --
    -- Haskell language dialects and extensions

    module Language.Haskell.Extension (
            Language(..),
            knownLanguages,
            classifyLanguage,

            Extension(..),
            KnownExtension(..),
            knownExtensions,
            deprecatedExtensions,
            classifyExtension,
      ) where

    import Prelude ()
    import Distribution.Compat.Prelude

    import Distribution.Text
    import qualified Distribution.Compat.ReadP as Parse

    import qualified Text.PrettyPrint as Disp
    import Data.Array (Array, accumArray, bounds, Ix(inRange), (!))

    -- ------------------------------------------------------------
    -- * Language
    -- ------------------------------------------------------------

    -- | This represents a Haskell language dialect.
    --
    -- Language 'Extension's are interpreted relative to one of these base
    -- languages.
    --
    data Language =

      -- | The Haskell 98 language as defined by the Haskell 98 report.
      -- <http://haskell.org/onlinereport/>
         Haskell98

      -- | The Haskell 2010 language as defined by the Haskell 2010 report.
      -- <http://www.haskell.org/onlinereport/haskell2010>
      | Haskell2010

      -- | An unknown language, identified by its name.
      | UnknownLanguage String
      deriving (Generic, Show, Read, Eq, Typeable, Data)

    instance Binary Language

    knownLanguages :: [Language]
    knownLanguages = [Haskell98, Haskell2010]

    instance Text Language where
      disp (UnknownLanguage other) = Disp.text other
      disp other                   = Disp.text (show other)

      parse = do
        lang <- Parse.munch1 isAlphaNum
        return (classifyLanguage lang)

    classifyLanguage :: String -> Language
    classifyLanguage = \str -> case lookup str langTable of
        Just lang -> lang
        Nothing   -> UnknownLanguage str
      where
        langTable = [ (show lang, lang)
                    | lang <- knownLanguages ]

    -- ------------------------------------------------------------
    -- * Extension
    -- ------------------------------------------------------------

    -- Note: if you add a new 'KnownExtension':
    --
    -- * also add it to the Distribution.Simple.X.languageExtensions lists
    --   (where X is each compiler: GHC, JHC, LHC, UHC, HaskellSuite)
    --
    -- | This represents language extensions beyond a base 'Language' definition
    -- (such as 'Haskell98') that are supported by some implementations, usually
    -- in some special mode.
    --
    -- Where applicable, references are given to an implementation's
    -- official documentation.

    data Extension =
      -- | Enable a known extension
        EnableExtension KnownExtension

      -- | Disable a known extension
      | DisableExtension KnownExtension

      -- | An unknown extension, identified by the name of its @LANGUAGE@
      -- pragma.
      | UnknownExtension String

      deriving (Generic, Show, Read, Eq, Ord, Typeable, Data)

    instance Binary Extension

    data KnownExtension =

      -- | Allow overlapping class instances, provided there is a unique
      -- most specific instance for each use.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensions.html#instance-overlap>
        OverlappingInstances

      -- | Ignore structural rules guaranteeing the termination of class
      -- instance resolution.  Termination is guaranteed by a fixed-depth
      -- recursion stack, and compilation may fail if this depth is
      -- exceeded.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensions.html#undecidable-instances>
      | UndecidableInstances

      -- | Implies 'OverlappingInstances'.  Allow the implementation to
      -- choose an instance even when it is possible that further
      -- instantiation of types will lead to a more specific instance
      -- being applicable.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensions.html#instance-overlap>
      | IncoherentInstances

      -- | /(deprecated)/ Allow recursive bindings in @do@ blocks, using the @rec@
      -- keyword. See also 'RecursiveDo'.
      | DoRec

      -- | Allow recursive bindings using @mdo@, a variant of @do@.
      -- @DoRec@ provides a different, preferred syntax.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#recursive-do-notation>
      | RecursiveDo

      -- | Provide syntax for writing list comprehensions which iterate
      -- over several lists together, like the 'zipWith' family of
      -- functions.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#parallel-list-comprehensions>
      | ParallelListComp

      -- | Allow multiple parameters in a type class.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensions.html#multi-param-type-classes>
      | MultiParamTypeClasses

      -- | Enable the dreaded monomorphism restriction.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/monomorphism.html>
      | MonomorphismRestriction

      -- | Allow a specification attached to a multi-parameter type class
      -- which indicates that some parameters are entirely determined by
      -- others. The implementation will check that this property holds
      -- for the declared instances, and will use this property to reduce
      -- ambiguity in instance resolution.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensions.html#functional-dependencies>
      | FunctionalDependencies

      -- | Like 'RankNTypes' but does not allow a higher-rank type to
      -- itself appear on the left of a function arrow.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#universal-quantification>
      | Rank2Types

      -- | Allow a universally-quantified type to occur on the left of a
      -- function arrow.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#universal-quantification>
      | RankNTypes

      -- | Allow data constructors to have polymorphic arguments.  Unlike
      -- 'RankNTypes', does not allow this for ordinary functions.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#universal-quantification>
      | PolymorphicComponents

      -- | Allow existentially-quantified data constructors.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/data-type-extensions.html#existential-quantification>
      | ExistentialQuantification

      -- | Cause a type variable in a signature, which has an explicit
      -- @forall@ quantifier, to scope over the definition of the
      -- accompanying value declaration.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#scoped-type-variables>
      | ScopedTypeVariables

      -- | Deprecated, use 'ScopedTypeVariables' instead.
      | PatternSignatures

      -- | Enable implicit function parameters with dynamic scope.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#implicit-parameters>
      | ImplicitParams

      -- | Relax some restrictions on the form of the context of a type
      -- signature.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#flexible-contexts>
      | FlexibleContexts

      -- | Relax some restrictions on the form of the context of an
      -- instance declaration.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensions.html#instance-rules>
      | FlexibleInstances

      -- | Allow data type declarations with no constructors.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/data-type-extensions.html#nullary-types>
      | EmptyDataDecls

      -- | Run the C preprocessor on Haskell source code.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/options-phases.html#c-pre-processor>
      | CPP

      -- | Allow an explicit kind signature giving the kind of types over
      -- which a type variable ranges.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#kinding>
      | KindSignatures

      -- | Enable a form of pattern which forces evaluation before an
      -- attempted match, and a form of strict @let@/@where@ binding.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/bang-patterns.html>
      | BangPatterns

      -- | Allow type synonyms in instance heads.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensions.html#flexible-instance-head>
      | TypeSynonymInstances

      -- | Enable Template Haskell, a system for compile-time
      -- metaprogramming.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/template-haskell.html>
      | TemplateHaskell

      -- | Enable the Foreign Function Interface.  In GHC, implements the
      -- standard Haskell 98 Foreign Function Interface Addendum, plus
      -- some GHC-specific extensions.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/ffi.html>
      | ForeignFunctionInterface

      -- | Enable arrow notation.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/arrow-notation.html>
      | Arrows

      -- | /(deprecated)/ Enable generic type classes, with default instances defined in
      -- terms of the algebraic structure of a type.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/generic-classes.html>
      | Generics

      -- | Enable the implicit importing of the module "Prelude".  When
      -- disabled, when desugaring certain built-in syntax into ordinary
      -- identifiers, use whatever is in scope rather than the "Prelude"
      -- -- version.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#rebindable-syntax>
      | ImplicitPrelude

      -- | Enable syntax for implicitly binding local names corresponding
      -- to the field names of a record.  Puns bind specific names, unlike
      -- 'RecordWildCards'.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#record-puns>
      | NamedFieldPuns

      -- | Enable a form of guard which matches a pattern and binds
      -- variables.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#pattern-guards>
      | PatternGuards

      -- | Allow a type declared with @newtype@ to use @deriving@ for any
      -- class with an instance for the underlying type.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/deriving.html#newtype-deriving>
      | GeneralizedNewtypeDeriving

      -- | Enable the \"Trex\" extensible records system.
      --
      -- * <http://cvs.haskell.org/Hugs/pages/users_guide/hugs-only.html#TREX>
      | ExtensibleRecords

      -- | Enable type synonyms which are transparent in some definitions
      -- and opaque elsewhere, as a way of implementing abstract
      -- datatypes.
      --
      -- * <http://cvs.haskell.org/Hugs/pages/users_guide/restricted-synonyms.html>
      | RestrictedTypeSynonyms

      -- | Enable an alternate syntax for string literals,
      -- with string templating.
      --
      -- * <http://cvs.haskell.org/Hugs/pages/users_guide/here-documents.html>
      | HereDocuments

      -- | Allow the character @#@ as a postfix modifier on identifiers.
      -- Also enables literal syntax for unboxed values.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#magic-hash>
      | MagicHash

      -- | Allow data types and type synonyms which are indexed by types,
      -- i.e. ad-hoc polymorphism for types.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/type-families.html>
      | TypeFamilies

      -- | Allow a standalone declaration which invokes the type class
      -- @deriving@ mechanism.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/deriving.html#stand-alone-deriving>
      | StandaloneDeriving

      -- | Allow certain Unicode characters to stand for certain ASCII
      -- character sequences, e.g. keywords and punctuation.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#unicode-syntax>
      | UnicodeSyntax

      -- | Allow the use of unboxed types as foreign types, e.g. in
      -- @foreign import@ and @foreign export@.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/ffi.html#id681687>
      | UnliftedFFITypes

      -- | Enable interruptible FFI.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/ffi.html#ffi-interruptible>
      | InterruptibleFFI

      -- | Allow use of CAPI FFI calling convention (@foreign import capi@).
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/ffi.html#ffi-capi>
      | CApiFFI

      -- | Defer validity checking of types until after expanding type
      -- synonyms, relaxing the constraints on how synonyms may be used.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/data-type-extensions.html#type-synonyms>
      | LiberalTypeSynonyms

      -- | Allow the name of a type constructor, type class, or type
      -- variable to be an infix operator.
      | TypeOperators

      -- | Enable syntax for implicitly binding local names corresponding
      -- to the field names of a record.  A wildcard binds all unmentioned
      -- names, unlike 'NamedFieldPuns'.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#record-wildcards>
      | RecordWildCards

      -- | Deprecated, use 'NamedFieldPuns' instead.
      | RecordPuns

      -- | Allow a record field name to be disambiguated by the type of
      -- the record it's in.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#disambiguate-fields>
      | DisambiguateRecordFields

      -- | Enable traditional record syntax (as supported by Haskell 98)
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#traditional-record-syntax>
      | TraditionalRecordSyntax

      -- | Enable overloading of string literals using a type class, much
      -- like integer literals.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensions.html#overloaded-strings>
      | OverloadedStrings

      -- | Enable generalized algebraic data types, in which type
      -- variables may be instantiated on a per-constructor basis. Implies
      -- 'GADTSyntax'.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/data-type-extensions.html#gadt>
      | GADTs

      -- | Enable GADT syntax for declaring ordinary algebraic datatypes.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/data-type-extensions.html#gadt-style>
      | GADTSyntax

      -- | Make pattern bindings monomorphic.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/monomorphism.html#id630981>
      | MonoPatBinds

      -- | Relax the requirements on mutually-recursive polymorphic
      -- functions.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#typing-binds>
      | RelaxedPolyRec

      -- | Allow default instantiation of polymorphic types in more
      -- situations.
      --
      -- * <http://downloads.haskell.org/~ghc/latest/docs/html/users_guide/ghci.html#type-defaulting-in-ghci>
      | ExtendedDefaultRules

      -- | Enable unboxed tuples.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/primitives.html#unboxed-tuples>
      | UnboxedTuples

      -- | Enable @deriving@ for classes 'Data.Typeable.Typeable' and
      -- 'Data.Generics.Data'.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/deriving.html#deriving-typeable>
      | DeriveDataTypeable

      -- | Enable @deriving@ for 'GHC.Generics.Generic' and 'GHC.Generics.Generic1'.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/deriving.html#deriving-typeable>
      | DeriveGeneric

      -- | Enable support for default signatures.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensions.html#class-default-signatures>
      | DefaultSignatures

      -- | Allow type signatures to be specified in instance declarations.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensions.html#instance-sigs>
      | InstanceSigs

      -- | Allow a class method's type to place additional constraints on
      -- a class type variable.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensions.html#class-method-types>
      | ConstrainedClassMethods

      -- | Allow imports to be qualified by the package name the module is
      -- intended to be imported from, e.g.
      --
      -- > import "network" Network.Socket
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#package-imports>
      | PackageImports

      -- | /(deprecated)/ Allow a type variable to be instantiated at a
      -- polymorphic type.
      --
      -- * <http://www.haskell.org/ghc/docs/6.12.3/html/users_guide/other-type-extensions.html#impredicative-polymorphism>
      | ImpredicativeTypes

      -- | /(deprecated)/ Change the syntax for qualified infix operators.
      --
      -- * <http://www.haskell.org/ghc/docs/6.12.3/html/users_guide/syntax-extns.html#new-qualified-operators>
      | NewQualifiedOperators

      -- | Relax the interpretation of left operator sections to allow
      -- unary postfix operators.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#postfix-operators>
      | PostfixOperators

      -- | Enable quasi-quotation, a mechanism for defining new concrete
      -- syntax for expressions and patterns.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/template-haskell.html#th-quasiquotation>
      | QuasiQuotes

      -- | Enable generalized list comprehensions, supporting operations
      -- such as sorting and grouping.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#generalised-list-comprehensions>
      | TransformListComp

      -- | Enable monad comprehensions, which generalise the list
      -- comprehension syntax to work for any monad.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#monad-comprehensions>
      | MonadComprehensions

      -- | Enable view patterns, which match a value by applying a
      -- function and matching on the result.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#view-patterns>
      | ViewPatterns

      -- | Allow concrete XML syntax to be used in expressions and patterns,
      -- as per the Haskell Server Pages extension language:
      -- <http://www.haskell.org/haskellwiki/HSP>. The ideas behind it are
      -- discussed in the paper \"Haskell Server Pages through Dynamic Loading\"
      -- by Niklas Broberg, from Haskell Workshop '05.
      | XmlSyntax

      -- | Allow regular pattern matching over lists, as discussed in the
      -- paper \"Regular Expression Patterns\" by Niklas Broberg, Andreas Farre
      -- and Josef Svenningsson, from ICFP '04.
      | RegularPatterns

      -- | Enable the use of tuple sections, e.g. @(, True)@ desugars into
      -- @\x -> (x, True)@.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#tuple-sections>
      | TupleSections

      -- | Allow GHC primops, written in C--, to be imported into a Haskell
      -- file.
      | GHCForeignImportPrim

      -- | Support for patterns of the form @n + k@, where @k@ is an
      -- integer literal.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#n-k-patterns>
      | NPlusKPatterns

      -- | Improve the layout rule when @if@ expressions are used in a @do@
      -- block.
      | DoAndIfThenElse

      -- | Enable support for multi-way @if@-expressions.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#multi-way-if>
      | MultiWayIf

      -- | Enable support lambda-@case@ expressions.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#lambda-case>
      | LambdaCase

      -- | Makes much of the Haskell sugar be desugared into calls to the
      -- function with a particular name that is in scope.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#rebindable-syntax>
      | RebindableSyntax

      -- | Make @forall@ a keyword in types, which can be used to give the
      -- generalisation explicitly.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#explicit-foralls>
      | ExplicitForAll

      -- | Allow contexts to be put on datatypes, e.g. the @Eq a@ in
      -- @data Eq a => Set a = NilSet | ConsSet a (Set a)@.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/data-type-extensions.html#datatype-contexts>
      | DatatypeContexts

      -- | Local (@let@ and @where@) bindings are monomorphic.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#mono-local-binds>
      | MonoLocalBinds

      -- | Enable @deriving@ for the 'Data.Functor.Functor' class.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/deriving.html#deriving-typeable>
      | DeriveFunctor

      -- | Enable @deriving@ for the 'Data.Traversable.Traversable' class.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/deriving.html#deriving-typeable>
      | DeriveTraversable

      -- | Enable @deriving@ for the 'Data.Foldable.Foldable' class.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/deriving.html#deriving-typeable>
      | DeriveFoldable

      -- | Enable non-decreasing indentation for @do@ blocks.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/bugs-and-infelicities.html#infelicities-syntax>
      | NondecreasingIndentation

      -- | Allow imports to be qualified with a safe keyword that requires
      -- the imported module be trusted as according to the Safe Haskell
      -- definition of trust.
      --
      -- > import safe Network.Socket
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/safe-haskell.html#safe-imports>
      | SafeImports

      -- | Compile a module in the Safe, Safe Haskell mode -- a restricted
      -- form of the Haskell language to ensure type safety.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/safe-haskell.html#safe-trust>
      | Safe

      -- | Compile a module in the Trustworthy, Safe Haskell mode -- no
      -- restrictions apply but the module is marked as trusted as long as
      -- the package the module resides in is trusted.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/safe-haskell.html#safe-trust>
      | Trustworthy

      -- | Compile a module in the Unsafe, Safe Haskell mode so that
      -- modules compiled using Safe, Safe Haskell mode can't import it.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/safe-haskell.html#safe-trust>
      | Unsafe

      -- | Allow type class/implicit parameter/equality constraints to be
      -- used as types with the special kind constraint.  Also generalise
      -- the @(ctxt => ty)@ syntax so that any type of kind constraint can
      -- occur before the arrow.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/constraint-kind.html>
      | ConstraintKinds

      -- | Enable kind polymorphism.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/kind-polymorphism.html>
      | PolyKinds

      -- | Enable datatype promotion.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/promotion.html>
      | DataKinds

      -- | Enable parallel arrays syntax (@[:@, @:]@) for /Data Parallel Haskell/.
      --
      -- * <http://www.haskell.org/haskellwiki/GHC/Data_Parallel_Haskell>
      | ParallelArrays

      -- | Enable explicit role annotations, like in (@type role Foo representational representational@).
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/roles.html>
      | RoleAnnotations

      -- | Enable overloading of list literals, arithmetic sequences and
      -- list patterns using the 'IsList' type class.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensions.html#overloaded-lists>
      | OverloadedLists

      -- | Enable case expressions that have no alternatives. Also applies to lambda-case expressions if they are enabled.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#empty-case>
      | EmptyCase

      -- | Triggers the generation of derived 'Typeable' instances for every
      -- datatype and type class declaration.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/deriving.html#auto-derive-typeable>
      | AutoDeriveTypeable

      -- | Desugars negative literals directly (without using negate).
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#negative-literals>
      | NegativeLiterals

      -- | Allow the use of binary integer literal syntax (e.g. @0b11001001@ to denote @201@).
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#binary-literals>
      | BinaryLiterals

      -- | Allow the use of floating literal syntax for all instances of 'Num', including 'Int' and 'Integer'.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#num-decimals>
      | NumDecimals

      -- | Enable support for type classes with no type parameter.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-extensions.html#nullary-type-classes>
      | NullaryTypeClasses

      -- | Enable explicit namespaces in module import/export lists.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#explicit-namespaces>
      | ExplicitNamespaces

      -- | Allow the user to write ambiguous types, and the type inference engine to infer them.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#ambiguity>
      | AllowAmbiguousTypes

      -- | Enable @foreign import javascript@.
      | JavaScriptFFI

      -- | Allow giving names to and abstracting over patterns.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#pattern-synonyms>
      | PatternSynonyms

      -- | Allow anonymous placeholders (underscore) inside type signatures.  The
      -- type inference engine will generate a message describing the type inferred
      -- at the hole's location.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#partial-type-signatures>
      | PartialTypeSignatures

      -- | Allow named placeholders written with a leading underscore inside type
      -- signatures.  Wildcards with the same name unify to the same type.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#named-wildcards>
      | NamedWildCards

      -- | Enable @deriving@ for any class.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/other-type-extensions.html#derive-any-class>
      | DeriveAnyClass

      -- | Enable @deriving@ for the 'Language.Haskell.TH.Syntax.Lift' class.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/deriving.html#deriving-lift>
      | DeriveLift

      -- | Enable support for 'static pointers' (and the @static@
      -- keyword) to refer to globally stable names, even across
      -- different programs.
      --
      -- * <http://www.haskell.org/ghc/docs/latest/html/users_guide/static-pointers.html>
      | StaticPointers

      -- | Switches data type declarations to be strict by default (as if
      -- they had a bang using @BangPatterns@), and allow opt-in field
      -- laziness using @~@.
      | StrictData

      -- | Switches all pattern bindings to be strict by default (as if
      -- they had a bang using @BangPatterns@), ordinary patterns are
      -- recovered using @~@. Implies @StrictData@.
      | Strict

      -- | Allows @do@-notation for types that are @'Applicative'@ as well
      -- as @'Monad'@. When enabled, desugaring @do@ notation tries to use
      -- @(<*>)@ and @'fmap'@ and @'join'@ as far as possible.
      | ApplicativeDo

      -- | Allow records to use duplicated field labels for accessors.
      | DuplicateRecordFields

      -- | Enable explicit type applications with the syntax @id \@Int@.
      | TypeApplications

      -- | Dissolve the distinction between types and kinds, allowing the compiler
      -- to reason about kind equality and therefore enabling GADTs to be promoted
      -- to the type-level.
      | TypeInType

      -- | Allow recursive (and therefore undecideable) super-class relationships.
      | UndecidableSuperClasses

      -- | A temporary extension to help library authors check if their
      -- code will compile with the new planned desugaring of fail.
      | MonadFailDesugaring

      -- | A subset of @TemplateHaskell@ including only quasi-quoting.
      | TemplateHaskellQuotes

      -- | Allows use of the @#label@ syntax.
      | OverloadedLabels

      -- | Allow functional dependency annotations on type families to declare them
      -- as injective.
      | TypeFamilyDependencies

      deriving (Generic, Show, Read, Eq, Ord, Enum, Bounded, Typeable, Data)

    instance Binary KnownExtension

    {-# DEPRECATED knownExtensions
       "KnownExtension is an instance of Enum and Bounded, use those instead." #-}
    knownExtensions :: [KnownExtension]
    knownExtensions = [minBound..maxBound]

    -- | Extensions that have been deprecated, possibly paired with another
    -- extension that replaces it.
    --
    deprecatedExtensions :: [(Extension, Maybe Extension)]
    deprecatedExtensions =
      [ (EnableExtension RecordPuns, Just (EnableExtension NamedFieldPuns))
      , (EnableExtension PatternSignatures, Just (EnableExtension ScopedTypeVariables))
      ]
    -- NOTE: when adding deprecated extensions that have new alternatives
    -- we must be careful to make sure that the deprecation messages are
    -- valid. We must not recommend aliases that cannot be used with older
    -- compilers, perhaps by adding support in Cabal to translate the new
    -- name to the old one for older compilers. Otherwise we are in danger
    -- of the scenario in ticket #689.

    instance Text Extension where
      disp (UnknownExtension other) = Disp.text other
      disp (EnableExtension ke)     = Disp.text (show ke)
      disp (DisableExtension ke)    = Disp.text ("No" ++ show ke)

      parse = do
        extension <- Parse.munch1 isAlphaNum
        return (classifyExtension extension)

    instance Text KnownExtension where
      disp ke = Disp.text (show ke)

      parse = do
        extension <- Parse.munch1 isAlphaNum
        case classifyKnownExtension extension of
            Just ke ->
                return ke
            Nothing ->
                fail ("Can't parse " ++ show extension ++ " as KnownExtension")

    classifyExtension :: String -> Extension
    classifyExtension string
      = case classifyKnownExtension string of
        Just ext -> EnableExtension ext
        Nothing ->
            case string of
            'N':'o':string' ->
                case classifyKnownExtension string' of
                Just ext -> DisableExtension ext
                Nothing -> UnknownExtension string
            _ -> UnknownExtension string

    -- | 'read' for 'KnownExtension's is really really slow so for the Text
    -- instance
    -- what we do is make a simple table indexed off the first letter in the
    -- extension name. The extension names actually cover the range @'A'-'Z'@
    -- pretty densely and the biggest bucket is 7 so it's not too bad. We just do
    -- a linear search within each bucket.
    --
    -- This gives an order of magnitude improvement in parsing speed, and it'll
    -- also allow us to do case insensitive matches in future if we prefer.
    --
    classifyKnownExtension :: String -> Maybe KnownExtension
    classifyKnownExtension "" = Nothing
    classifyKnownExtension string@(c : _)
      | inRange (bounds knownExtensionTable) c
      = lookup string (knownExtensionTable ! c)
      | otherwise = Nothing

    knownExtensionTable :: Array Char [(String, KnownExtension)]
    knownExtensionTable =
      accumArray (flip (:)) [] ('A', 'Z')
        [ (head str, (str, extension))
        | extension <- [toEnum 0 ..]
        , let str = show extension ]