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

    {- All rights reserved.

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    module Language.Haskell.Extension (
            Extension(..),
            knownExtensions
      ) where

    import Distribution.Text (Text(..))
    import qualified Distribution.Compat.ReadP as Parse
    import qualified Text.PrettyPrint as Disp
    import qualified Data.Char as Char (isAlphaNum)
    import Data.Array (Array, accumArray, bounds, Ix(inRange), (!))

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

    -- Note: if you add a new 'Extension':
    --
    -- * also add it to the Distribution.Simple.X.languageExtensions lists
    --   (where X is each compiler: GHC, JHC, Hugs, NHC)
    --
    -- * also to the 'knownExtensions' list below.
    --
    -- * If the first character of the new extension is outside the range 'A' - 'V'
    --   (ie 'W'-'Z' or any non-uppercase-alphabetical char) then update the bounds
    --   of the 'extensionTable' below.

    -- |This represents language extensions beyond Haskell 98 that are
    -- supported by some implementations, usually in some special mode.

    data Extension
      = OverlappingInstances
      | UndecidableInstances
      | IncoherentInstances
      | RecursiveDo
      | ParallelListComp
      | MultiParamTypeClasses
      | NoMonomorphismRestriction
      | FunctionalDependencies
      | Rank2Types
      | RankNTypes
      | PolymorphicComponents
      | ExistentialQuantification
      | ScopedTypeVariables
      | ImplicitParams
      | FlexibleContexts
      | FlexibleInstances
      | EmptyDataDecls
      | CPP

      | KindSignatures
      | BangPatterns
      | TypeSynonymInstances
      | TemplateHaskell
      | ForeignFunctionInterface
      | Arrows
      | Generics
      | NoImplicitPrelude
      | NamedFieldPuns
      | PatternGuards
      | GeneralizedNewtypeDeriving

      | ExtensibleRecords
      | RestrictedTypeSynonyms
      | HereDocuments
      | MagicHash
      | TypeFamilies
      | StandaloneDeriving

      | UnicodeSyntax
      | PatternSignatures
      | UnliftedFFITypes
      | LiberalTypeSynonyms
      | TypeOperators
    --PArr -- not ready yet, and will probably be renamed to ParallelArrays
      | RecordWildCards
      | RecordPuns
      | DisambiguateRecordFields
      | OverloadedStrings
      | GADTs
      | NoMonoPatBinds
      | RelaxedPolyRec
      | ExtendedDefaultRules
      | UnboxedTuples
      | DeriveDataTypeable
      | ConstrainedClassMethods

      -- | Allow imports to be qualified by the package name that the module
      -- is intended to be imported from, e.g.
      --
      -- > import "network" Network.Socket
      | PackageImports

      | ImpredicativeTypes
      | NewQualifiedOperators
      | PostfixOperators
      | QuasiQuotes
      | TransformListComp
      | ViewPatterns

      | UnknownExtension String
      deriving (Show, Read, Eq)

    knownExtensions :: [Extension]
    knownExtensions =
      [ OverlappingInstances
      , UndecidableInstances
      , IncoherentInstances
      , RecursiveDo
      , ParallelListComp
      , MultiParamTypeClasses
      , NoMonomorphismRestriction
      , FunctionalDependencies
      , Rank2Types
      , RankNTypes
      , PolymorphicComponents
      , ExistentialQuantification
      , ScopedTypeVariables
      , ImplicitParams
      , FlexibleContexts
      , FlexibleInstances
      , EmptyDataDecls
      , CPP

      , KindSignatures
      , BangPatterns
      , TypeSynonymInstances
      , TemplateHaskell
      , ForeignFunctionInterface
      , Arrows
      , Generics
      , NoImplicitPrelude
      , NamedFieldPuns
      , PatternGuards
      , GeneralizedNewtypeDeriving

      , ExtensibleRecords
      , RestrictedTypeSynonyms
      , HereDocuments
      , MagicHash
      , TypeFamilies
      , StandaloneDeriving

      , UnicodeSyntax
      , PatternSignatures
      , UnliftedFFITypes
      , LiberalTypeSynonyms
      , TypeOperators
    --PArr -- not ready yet, and will probably be renamed to ParallelArrays
      , RecordWildCards
      , RecordPuns
      , DisambiguateRecordFields
      , OverloadedStrings
      , GADTs
      , NoMonoPatBinds
      , RelaxedPolyRec
      , ExtendedDefaultRules
      , UnboxedTuples
      , DeriveDataTypeable
      , ConstrainedClassMethods
      , PackageImports
      , ImpredicativeTypes
      , NewQualifiedOperators
      , PostfixOperators
      , QuasiQuotes
      , TransformListComp
      , ViewPatterns
      ]

    instance Text Extension where
      disp (UnknownExtension other) = Disp.text other
      disp other                    = Disp.text (show other)

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

    -- | 'read' for 'Extension'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'-'U'@
    -- 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.
    --
    classifyExtension :: String -> Extension
    classifyExtension string@(c:_)
      | inRange (bounds extensionTable) c
      = case lookup string (extensionTable ! c) of
          Just extension    -> extension
          Nothing           -> UnknownExtension string
    classifyExtension string = UnknownExtension string

    extensionTable :: Array Char [(String, Extension)]
    extensionTable =
      accumArray (flip (:)) [] ('A', 'V')
        [ (head str, (str, extension))
        | extension <- knownExtensions
        , let str = show extension ]