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

    {- All rights reserved.

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    modification, are permitted provided that the following conditions are
    met:

        * Redistributions of source code must retain the above copyright
          notice, this list of conditions and the following disclaimer.

        * Redistributions in binary form must reproduce the above
          copyright notice, this list of conditions and the following
          disclaimer in the documentation and/or other materials provided
          with the distribution.

        * Neither the name of Isaac Jones nor the names of other
          contributors may be used to endorse or promote products derived
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    module Language.Haskell.Extension (
            Language(..),
            knownLanguages,

            Extension(..),
            knownExtensions,
            deprecatedExtensions
      ) 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), (!))

    -- ------------------------------------------------------------
    -- * 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 (Show, Read, Eq)

    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 Char.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 '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.

    -- | 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, e.g. \"GHC &#xa7; 7.2.1\" for an extension
    -- documented in section 7.2.1 of the GHC User's Guide.

    data Extension =

      -- | [GHC &#xa7; 7.6.3.4] Allow overlapping class instances,
      -- provided there is a unique most specific instance for each use.
        OverlappingInstances

      -- | [GHC &#xa7; 7.6.3.3] 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.
      | UndecidableInstances

      -- | [GHC &#xa7; 7.6.3.4] 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.
      | IncoherentInstances

      -- | [GHC &#xa7; 7.3.8] Allows recursive bindings in @do@ blocks,
      -- using the @rec@ keyword.
      | DoRec

      -- | [GHC &#xa7; 7.3.8.2] Deprecated in GHC.  Allows recursive bindings
      -- using @mdo@, a variant of @do@.  @DoRec@ provides a different,
      -- preferred syntax.
      | RecursiveDo

      -- | [GHC &#xa7; 7.3.9] Provide syntax for writing list
      -- comprehensions which iterate over several lists together, like
      -- the 'zipWith' family of functions.
      | ParallelListComp

      -- | [GHC &#xa7; 7.6.1.1] Allow multiple parameters in a type class.
      | MultiParamTypeClasses

      -- | [GHC &#xa7; 7.17] Disable the dreaded monomorphism restriction.
      | NoMonomorphismRestriction

      -- | [GHC &#xa7; 7.6.2] 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.
      | FunctionalDependencies

      -- | [GHC &#xa7; 7.8.5] Like 'RankNTypes' but does not allow a
      -- higher-rank type to itself appear on the left of a function
      -- arrow.
      | Rank2Types

      -- | [GHC &#xa7; 7.8.5] Allow a universally-quantified type to occur on
      -- the left of a function arrow.
      | RankNTypes

      -- | [GHC &#xa7; 7.8.5] Allow data constructors to have polymorphic
      -- arguments.  Unlike 'RankNTypes', does not allow this for ordinary
      -- functions.
      | PolymorphicComponents

      -- | [GHC &#xa7; 7.4.4] Allow existentially-quantified data constructors.
      | ExistentialQuantification

      -- | [GHC &#xa7; 7.8.7] Cause a type variable in a signature, which has an
      -- explicit @forall@ quantifier, to scope over the definition of the
      -- accompanying value declaration.
      | ScopedTypeVariables

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

      -- | [GHC &#xa7; 7.8.3] Enable implicit function parameters with dynamic
      -- scope.
      | ImplicitParams

      -- | [GHC &#xa7; 7.8.2] Relax some restrictions on the form of the context
      -- of a type signature.
      | FlexibleContexts

      -- | [GHC &#xa7; 7.6.3.2] Relax some restrictions on the form of the
      -- context of an instance declaration.
      | FlexibleInstances

      -- | [GHC &#xa7; 7.4.1] Allow data type declarations with no constructors.
      | EmptyDataDecls

      -- | [GHC &#xa7; 4.10.3] Run the C preprocessor on Haskell source code.
      | CPP

      -- | [GHC &#xa7; 7.8.4] Allow an explicit kind signature giving the kind of
      -- types over which a type variable ranges.
      | KindSignatures

      -- | [GHC &#xa7; 7.11] Enable a form of pattern which forces evaluation
      -- before an attempted match, and a form of strict @let@/@where@
      -- binding.
      | BangPatterns

      -- | [GHC &#xa7; 7.6.3.1] Allow type synonyms in instance heads.
      | TypeSynonymInstances

      -- | [GHC &#xa7; 7.9] Enable Template Haskell, a system for compile-time
      -- metaprogramming.
      | TemplateHaskell

      -- | [GHC &#xa7; 8] Enable the Foreign Function Interface.  In GHC,
      -- implements the standard Haskell 98 Foreign Function Interface
      -- Addendum, plus some GHC-specific extensions.
      | ForeignFunctionInterface

      -- | [GHC &#xa7; 7.10] Enable arrow notation.
      | Arrows

      -- | [GHC &#xa7; 7.16] Enable generic type classes, with default instances
      -- defined in terms of the algebraic structure of a type.
      | Generics

      -- | [GHC &#xa7; 7.3.11] Disable the implicit importing of the module
      -- @Prelude@.  When desugaring certain built-in syntax into ordinary
      -- identifiers, use whatever is in scope rather than the @Prelude@
      -- version.
      | NoImplicitPrelude

      -- | [GHC &#xa7; 7.3.15] Enable syntax for implicitly binding local names
      -- corresponding to the field names of a record.  Puns bind specific
      -- names, unlike 'RecordWildCards'.
      | NamedFieldPuns

      -- | [GHC &#xa7; 7.3.5] Enable a form of guard which matches a pattern and
      -- binds variables.
      | PatternGuards

      -- | [GHC &#xa7; 7.5.4] Allow a type declared with @newtype@ to use
      -- @deriving@ for any class with an instance for the underlying type.
      | GeneralizedNewtypeDeriving

      -- | [Hugs &#xa7; 7.1] Enable the \"Trex\" extensible records system.
      | ExtensibleRecords

      -- | [Hugs &#xa7; 7.2] Enable type synonyms which are transparent in
      -- some definitions and opaque elsewhere, as a way of implementing
      -- abstract datatypes.
      | RestrictedTypeSynonyms

      -- | [Hugs &#xa7; 7.3] Enable an alternate syntax for string literals,
      -- with string templating.
      | HereDocuments

      -- | [GHC &#xa7; 7.3.2] Allow the character @#@ as a postfix modifier on
      -- identifiers.  Also enables literal syntax for unboxed values.
      | MagicHash

      -- | [GHC &#xa7; 7.7] Allow data types and type synonyms which are
      -- indexed by types, i.e. ad-hoc polymorphism for types.
      | TypeFamilies

      -- | [GHC &#xa7; 7.5.2] Allow a standalone declaration which invokes the
      -- type class @deriving@ mechanism.
      | StandaloneDeriving

      -- | [GHC &#xa7; 7.3.1] Allow certain Unicode characters to stand for
      -- certain ASCII character sequences, e.g. keywords and punctuation.
      | UnicodeSyntax

      -- | [GHC &#xa7; 8.1.1] Allow the use of unboxed types as foreign types,
      -- e.g. in @foreign import@ and @foreign export@.
      | UnliftedFFITypes

      -- | [GHC &#xa7; 7.4.3] Defer validity checking of types until after
      -- expanding type synonyms, relaxing the constraints on how synonyms
      -- may be used.
      | LiberalTypeSynonyms

      -- | [GHC &#xa7; 7.4.2] Allow the name of a type constructor, type class,
      -- or type variable to be an infix operator.
      | TypeOperators

    --PArr -- not ready yet, and will probably be renamed to ParallelArrays

      -- | [GHC &#xa7; 7.3.16] Enable syntax for implicitly binding local names
      -- corresponding to the field names of a record.  A wildcard binds
      -- all unmentioned names, unlike 'NamedFieldPuns'.
      | RecordWildCards

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

      -- | [GHC &#xa7; 7.3.14] Allow a record field name to be disambiguated
      -- by the type of the record it's in.
      | DisambiguateRecordFields

      -- | [GHC &#xa7; 7.6.4] Enable overloading of string literals using a
      -- type class, much like integer literals.
      | OverloadedStrings

      -- | [GHC &#xa7; 7.4.6] Enable generalized algebraic data types, in
      -- which type variables may be instantiated on a per-constructor
      -- basis.  Enables \"GADT syntax\" which can be used to declare
      -- GADTs as well as ordinary algebraic types.
      | GADTs

      -- | [GHC &#xa7; 7.17.2] Allow pattern bindings to be polymorphic.
      | NoMonoPatBinds

      -- | [GHC &#xa7; 7.8.8] Relax the requirements on mutually-recursive
      -- polymorphic functions.
      | RelaxedPolyRec

      -- | [GHC &#xa7; 2.4.5] Allow default instantiation of polymorphic
      -- types in more situations.
      | ExtendedDefaultRules

      -- | [GHC &#xa7; 7.2.2] Enable unboxed tuples.
      | UnboxedTuples

      -- | [GHC &#xa7; 7.5.3] Enable @deriving@ for classes
      -- @Data.Typeable.Typeable@ and @Data.Generics.Data@.
      | DeriveDataTypeable

      -- | [GHC &#xa7; 7.6.1.3] Allow a class method's type to place
      -- additional constraints on a class type variable.
      | ConstrainedClassMethods

      -- | [GHC &#xa7; 7.3.18] Allow imports to be qualified by the package
      -- name the module is intended to be imported from, e.g.
      --
      -- > import "network" Network.Socket
      | PackageImports

      -- | [GHC &#xa7; 7.8.6] Deprecated in GHC 6.12 and will be removed in
      -- GHC 7.  Allow a type variable to be instantiated at a
      -- polymorphic type.
      | ImpredicativeTypes

      -- | [GHC &#xa7; 7.3.3] Change the syntax for qualified infix
      -- operators.
      | NewQualifiedOperators

      -- | [GHC &#xa7; 7.3.12] Relax the interpretation of left operator
      -- sections to allow unary postfix operators.
      | PostfixOperators

      -- | [GHC &#xa7; 7.9.5] Enable quasi-quotation, a mechanism for defining
      -- new concrete syntax for expressions and patterns.
      | QuasiQuotes

      -- | [GHC &#xa7; 7.3.10] Enable generalized list comprehensions,
      -- supporting operations such as sorting and grouping.
      | TransformListComp

      -- | [GHC &#xa7; 7.3.6] Enable view patterns, which match a value by
      -- applying a function and matching on the result.
      | 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

      -- | Enables the use of tuple sections, e.g. @(, True)@ desugars into
      -- @\x -> (x, True)@.
      | TupleSections

      -- | Allows 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.
      | NPlusKPatterns

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

      -- | Makes much of the Haskell sugar be desugared into calls to the
      -- function with a particular name that is in scope.
      | RebindableSyntax

      -- | Make @forall@ a keyword in types, which can be used to give the
      -- generalisation explicitly.
      | ExplicitForAll

      -- | Allow contexts to be put on datatypes, e.g. the @Eq a@ in
      -- @data Eq a => Set a = NilSet | ConsSet a (Set a)@.
      | DatatypeContexts

      -- | Local (@let@ and @where@) bindings are monomorphic.
      | MonoLocalBinds

      -- | Enable @deriving@ for the @Data.Functor.Functor@ class.
      | DeriveFunctor

      -- | Enable @deriving@ for the @Data.Traversable.Traversable@ class.
      | DeriveTraversable

      -- | Enable @deriving@ for the @Data.Foldable.Foldable@ class.
      | DeriveFoldable

      -- | An unknown extension, identified by the name of its @LANGUAGE@
      -- pragma.
      | UnknownExtension String
      deriving (Show, Read, Eq)

    -- | Extensions that have been deprecated, possibly paired with another
    -- extension that replaces it.
    --
    deprecatedExtensions :: [(Extension, Maybe Extension)]
    deprecatedExtensions =
      [ (RecordPuns, Just NamedFieldPuns)
      , (PatternSignatures, Just ScopedTypeVariables)
      ]

    knownExtensions :: [Extension]
    knownExtensions =
      [ OverlappingInstances
      , UndecidableInstances
      , IncoherentInstances
      , DoRec
      , 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
      , XmlSyntax
      , RegularPatterns

      , TupleSections
      , GHCForeignImportPrim
      , NPlusKPatterns
      , DoAndIfThenElse
      , RebindableSyntax
      , ExplicitForAll
      , DatatypeContexts
      , MonoLocalBinds
      , DeriveFunctor
      , DeriveTraversable
      , DeriveFoldable
      ]

    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'-'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.
    --
    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', 'Z')
        [ (head str, (str, extension))
        | extension <- knownExtensions
        , let str = show extension ]