    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Data.Generics.Basics
    -- Copyright   :  (c) The University of Glasgow, CWI 2001--2004
    -- License     :  BSD-style (see the file libraries/base/LICENSE)
    --
    -- Maintainer  :  libraries@haskell.org
    -- Stability   :  experimental
    -- Portability :  non-portable (local universal quantification)
    --
    -- \"Scrap your boilerplate\" --- Generic programming in Haskell.
    -- See <http://www.cs.vu.nl/boilerplate/>. This module provides
    -- the 'Data' class with its primitives for generic programming.
    --
    -----------------------------------------------------------------------------

    module Data.Generics.Basics (

            -- * Module Data.Typeable re-exported for convenience
            module Data.Typeable,

            -- * The Data class for processing constructor applications
            Data(
                    gfoldl,         -- :: ... -> a -> c a
                    gunfold,        -- :: ... -> Constr -> c a
                    toConstr,       -- :: a -> Constr
                    dataTypeOf,     -- :: a -> DataType
                    dataCast1,      -- mediate types and unary type constructors
                    dataCast2,      -- mediate types and binary type constructors
                    -- Generic maps defined in terms of gfoldl
                    gmapT,
                    gmapQ,
                    gmapQl,
                    gmapQr,
                    gmapQi,
                    gmapM,
                    gmapMp,
                    gmapMo
                ),

            -- * Datatype representations
            DataType,       -- abstract, instance of: Show
            -- ** Constructors
            mkDataType,     -- :: String   -> [Constr] -> DataType
            mkIntType,      -- :: String -> DataType
            mkFloatType,    -- :: String -> DataType
            mkStringType,   -- :: String -> DataType
            mkNorepType,    -- :: String -> DataType
            -- ** Observers
            dataTypeName,   -- :: DataType -> String
            DataRep(..),    -- instance of: Eq, Show
            dataTypeRep,    -- :: DataType -> DataRep
            -- ** Convenience functions
            repConstr,      -- :: DataType -> ConstrRep -> Constr
            isAlgType,      -- :: DataType -> Bool
            dataTypeConstrs,-- :: DataType -> [Constr]
            indexConstr,    -- :: DataType -> ConIndex -> Constr
            maxConstrIndex, -- :: DataType -> ConIndex
            isNorepType,    -- :: DataType -> Bool

            -- * Data constructor representations
            Constr,         -- abstract, instance of: Eq, Show
            ConIndex,       -- alias for Int, start at 1
            Fixity(..),     -- instance of: Eq, Show
            -- ** Constructors
            mkConstr,       -- :: DataType -> String -> Fixity -> Constr
            mkIntConstr,    -- :: DataType -> Integer -> Constr
            mkFloatConstr,  -- :: DataType -> Double  -> Constr
            mkStringConstr, -- :: DataType -> String  -> Constr
            -- ** Observers
            constrType,     -- :: Constr   -> DataType
            ConstrRep(..),  -- instance of: Eq, Show
            constrRep,      -- :: Constr   -> ConstrRep
            constrFields,   -- :: Constr   -> [String]
            constrFixity,   -- :: Constr   -> Fixity
            -- ** Convenience function: algebraic data types
            constrIndex,    -- :: Constr   -> ConIndex
            -- ** From strings to constructors and vice versa: all data types
            showConstr,     -- :: Constr   -> String
            readConstr,     -- :: DataType -> String -> Maybe Constr

            -- * Convenience functions: take type constructors apart
            tyconUQname,    -- :: String -> String
            tyconModule,    -- :: String -> String

            -- * Generic operations defined in terms of 'gunfold'
            fromConstr,     -- :: Constr -> a
            fromConstrB,    -- :: ... -> Constr -> a
            fromConstrM     -- :: Monad m => ... -> Constr -> m a

      ) where

    ------------------------------------------------------------------------------

    import Prelude -- necessary to get dependencies right

    import Data.Typeable
    import Data.Maybe
    import Control.Monad

    ------------------------------------------------------------------------------
    --
    --      The Data class
    --
    ------------------------------------------------------------------------------

    {- |
    The 'Data' class comprehends a fundamental primitive 'gfoldl' for
    folding over constructor applications, say terms. This primitive can
    be instantiated in several ways to map over the immediate subterms
    of a term; see the @gmap@ combinators later in this class.  Indeed, a
    generic programmer does not necessarily need to use the ingenious gfoldl
    primitive but rather the intuitive @gmap@ combinators.  The 'gfoldl'
    primitive is completed by means to query top-level constructors, to
    turn constructor representations into proper terms, and to list all
    possible datatype constructors.  This completion allows us to serve
    generic programming scenarios like read, show, equality, term generation.

    The combinators 'gmapT', 'gmapQ', 'gmapM', etc are all provided with
    default definitions in terms of 'gfoldl', leaving open the opportunity
    to provide datatype-specific definitions.
    (The inclusion of the @gmap@ combinators as members of class 'Data'
    allows the programmer or the compiler to derive specialised, and maybe
    more efficient code per datatype.  /Note/: 'gfoldl' is more higher-order
    than the @gmap@ combinators.  This is subject to ongoing benchmarking
    experiments.  It might turn out that the @gmap@ combinators will be
    moved out of the class 'Data'.)

    Conceptually, the definition of the @gmap@ combinators in terms of the
    primitive 'gfoldl' requires the identification of the 'gfoldl' function
    arguments.  Technically, we also need to identify the type constructor
    @c@ for the construction of the result type from the folded term type.

    In the definition of @gmapQ@/x/ combinators, we use phantom type
    constructors for the @c@ in the type of 'gfoldl' because the result type
    of a query does not involve the (polymorphic) type of the term argument.
    In the definition of 'gmapQl' we simply use the plain constant type
    constructor because 'gfoldl' is left-associative anyway and so it is
    readily suited to fold a left-associative binary operation over the
    immediate subterms.  In the definition of gmapQr, extra effort is
    needed. We use a higher-order accumulation trick to mediate between
    left-associative constructor application vs. right-associative binary
    operation (e.g., @(:)@).  When the query is meant to compute a value
    of type @r@, then the result type withing generic folding is @r -> r@.
    So the result of folding is a function to which we finally pass the
    right unit.

    With the @-fglasgow-exts@ option, GHC can generate instances of the
    'Data' class automatically.  For example, given the declaration

    > data T a b = C1 a b | C2 deriving (Typeable, Data)

    GHC will generate an instance that is equivalent to

    > instance (Data a, Data b) => Data (T a b) where
    >     gfoldl k z (C1 a b) = z C1 `k` a `k` b
    >     gfoldl k z C2       = z C2
    >
    >     gunfold k z c = case constrIndex c of
    >                         1 -> k (k (z C1))
    >                         2 -> z C2
    >
    >     toConstr (C1 _ _) = con_C1
    >     toConstr C2       = con_C2
    >
    >     dataTypeOf _ = ty_T
    >
    > con_C1 = mkConstr ty_T "C1" [] Prefix
    > con_C2 = mkConstr ty_T "C2" [] Prefix
    > ty_T   = mkDataType "Module.T" [con_C1, con_C2]

    This is suitable for datatypes that are exported transparently.

    -}

    class Typeable a => Data a where

      -- | Left-associative fold operation for constructor applications.
      --
      -- The type of 'gfoldl' is a headache, but operationally it is a simple
      -- generalisation of a list fold.
      --
      -- The default definition for 'gfoldl' is @'const' 'id'@, which is
      -- suitable for abstract datatypes with no substructures.
      gfoldl  :: (forall a b. Data a => c (a -> b) -> a -> c b)
                    -- ^ defines how nonempty constructor applications are
                    -- folded.  It takes the folded tail of the constructor
                    -- application and its head, i.e., an immediate subterm,
                    -- and combines them in some way.
              -> (forall g. g -> c g)
                    -- ^ defines how the empty constructor application is
                    -- folded, like the neutral \/ start element for list
                    -- folding.
              -> a
                    -- ^ structure to be folded.
              -> c a
                    -- ^ result, with a type defined in terms of @a@, but
                    -- variability is achieved by means of type constructor
                    -- @c@ for the construction of the actual result type.

      -- See the 'Data' instances in this file for an illustration of 'gfoldl'.

      gfoldl _ z = z

      -- | Unfolding constructor applications
      gunfold :: (forall b r. Data b => c (b -> r) -> c r)
              -> (forall r. r -> c r)
              -> Constr
              -> c a

      -- | Obtaining the constructor from a given datum.
      -- For proper terms, this is meant to be the top-level constructor.
      -- Primitive datatypes are here viewed as potentially infinite sets of
      -- values (i.e., constructors).
      toConstr   :: a -> Constr

      -- | The outer type constructor of the type
      dataTypeOf  :: a -> DataType

    ------------------------------------------------------------------------------
    --
    -- Mediate types and type constructors
    --
    ------------------------------------------------------------------------------

      -- | Mediate types and unary type constructors.
      -- In 'Data' instances of the form @T a@, 'dataCast1' should be defined
      -- as 'gcast1'.
      --
      -- The default definition is @'const' 'Nothing'@, which is appropriate
      -- for non-unary type constructors.
      dataCast1 :: Typeable1 t
                => (forall a. Data a => c (t a))
                -> Maybe (c a)
      dataCast1 _ = Nothing

      -- | Mediate types and binary type constructors.
      -- In 'Data' instances of the form @T a b@, 'dataCast2' should be
      -- defined as 'gcast2'.
      --
      -- The default definition is @'const' 'Nothing'@, which is appropriate
      -- for non-binary type constructors.
      dataCast2 :: Typeable2 t
                => (forall a b. (Data a, Data b) => c (t a b))
                -> Maybe (c a)
      dataCast2 _ = Nothing

    ------------------------------------------------------------------------------
    --
    --      Typical generic maps defined in terms of gfoldl
    --
    ------------------------------------------------------------------------------

      -- | A generic transformation that maps over the immediate subterms
      --
      -- The default definition instantiates the type constructor @c@ in the
      -- type of 'gfoldl' to an identity datatype constructor, using the
      -- isomorphism pair as injection and projection.
      gmapT :: (forall b. Data b => b -> b) -> a -> a

      -- Use an identity datatype constructor ID (see below)
      -- to instantiate the type constructor c in the type of gfoldl,
      -- and perform injections ID and projections unID accordingly.
      --
      gmapT f x = unID (gfoldl k ID x)
        where
          k (ID c) x = ID (c (f x))

      -- | A generic query with a left-associative binary operator
      gmapQl :: (r -> r' -> r) -> r -> (forall a. Data a => a -> r') -> a -> r
      gmapQl o r f = unCONST . gfoldl k z
        where
          k c x = CONST $ (unCONST c) `o` f x
          z _   = CONST r

      -- | A generic query with a right-associative binary operator
      gmapQr :: (r' -> r -> r) -> r -> (forall a. Data a => a -> r') -> a -> r
      gmapQr o r f x = unQr (gfoldl k (const (Qr id)) x) r
        where
          k (Qr c) x = Qr (\r -> c (f x `o` r))

      -- | A generic query that processes the immediate subterms and returns a list
      -- of results.  The list is given in the same order as originally specified
      -- in the declaratoin of the data constructors.
      gmapQ :: (forall a. Data a => a -> u) -> a -> [u]
      gmapQ f = gmapQr (:) [] f

      -- | A generic query that processes one child by index (zero-based)
      gmapQi :: Int -> (forall a. Data a => a -> u) -> a -> u
      gmapQi i f x = case gfoldl k z x of { Qi _ q -> fromJust q }
        where
          k (Qi i' q) a = Qi (i'+1) (if i==i' then Just (f a) else q)
          z f           = Qi 0 Nothing

      -- | A generic monadic transformation that maps over the immediate subterms
      --
      -- The default definition instantiates the type constructor @c@ in
      -- the type of 'gfoldl' to the monad datatype constructor, defining
      -- injection and projection using 'return' and '>>='.
      gmapM   :: Monad m => (forall a. Data a => a -> m a) -> a -> m a

      -- Use immediately the monad datatype constructor
      -- to instantiate the type constructor c in the type of gfoldl,
      -- so injection and projection is done by return and >>=.
      --
      gmapM f = gfoldl k return
        where
          k c x = do c' <- c
                     x' <- f x
                     return (c' x')

      -- | Transformation of at least one immediate subterm does not fail
      gmapMp :: MonadPlus m => (forall a. Data a => a -> m a) -> a -> m a

    {-

    The type constructor that we use here simply keeps track of the fact
    if we already succeeded for an immediate subterm; see Mp below. To
    this end, we couple the monadic computation with a Boolean.

    -}

      gmapMp f x = unMp (gfoldl k z x) >>= \(x',b) ->
                    if b then return x' else mzero
        where
          z g = Mp (return (g,False))
          k (Mp c) x
            = Mp ( c >>= \(h,b) ->
                     (f x >>= \x' -> return (h x',True))
                     `mplus` return (h x,b)
                 )

      -- | Transformation of one immediate subterm with success
      gmapMo :: MonadPlus m => (forall a. Data a => a -> m a) -> a -> m a

    {-

    We use the same pairing trick as for gmapMp,
    i.e., we use an extra Bool component to keep track of the
    fact whether an immediate subterm was processed successfully.
    However, we cut of mapping over subterms once a first subterm
    was transformed successfully.

    -}

      gmapMo f x = unMp (gfoldl k z x) >>= \(x',b) ->
                    if b then return x' else mzero
        where
          z g = Mp (return (g,False))
          k (Mp c) x
            = Mp ( c >>= \(h,b) -> if b
                            then return (h x,b)
                            else (f x >>= \x' -> return (h x',True))
                                 `mplus` return (h x,b)
                 )

    -- | The identity type constructor needed for the definition of gmapT
    newtype ID x = ID { unID :: x }

    -- | The constant type constructor needed for the definition of gmapQl
    newtype CONST c a = CONST { unCONST :: c }

    -- | Type constructor for adding counters to queries
    data Qi q a = Qi Int (Maybe q)

    -- | The type constructor used in definition of gmapQr
    newtype Qr r a = Qr { unQr  :: r -> r }

    -- | The type constructor used in definition of gmapMp
    newtype Mp m x = Mp { unMp :: m (x, Bool) }

    ------------------------------------------------------------------------------
    --
    --      Generic unfolding
    --
    ------------------------------------------------------------------------------

    -- | Build a term skeleton
    fromConstr :: Data a => Constr -> a
    fromConstr = fromConstrB undefined

    -- | Build a term and use a generic function for subterms
    fromConstrB :: Data a
                => (forall a. Data a => a)
                -> Constr
                -> a
    fromConstrB f = unID . gunfold k z
     where
      k c = ID (unID c f)
      z = ID

    -- | Monadic variation on 'fromConstrB'
    fromConstrM :: (Monad m, Data a)
                => (forall a. Data a => m a)
                -> Constr
                -> m a
    fromConstrM f = gunfold k z
     where
      k c = do { c' <- c; b <- f; return (c' b) }
      z = return

    ------------------------------------------------------------------------------
    --
    --      Datatype and constructor representations
    --
    ------------------------------------------------------------------------------

    --
    -- | Representation of datatypes.
    -- A package of constructor representations with names of type and module.
    --
    data DataType = DataType
                            { tycon   :: String
                            , datarep :: DataRep
                            }

                  deriving Show

    -- | Representation of constructors
    data Constr = Constr
                            { conrep    :: ConstrRep
                            , constring :: String
                            , confields :: [String] -- for AlgRep only
                            , confixity :: Fixity   -- for AlgRep only
                            , datatype  :: DataType
                            }

    instance Show Constr where
     show = constring

    -- | Equality of constructors
    instance Eq Constr where
      c == c' = constrRep c == constrRep c'

    -- | Public representation of datatypes
    data DataRep = AlgRep [Constr]
                 | IntRep
                 | FloatRep
                 | StringRep
                 | NoRep

                deriving (Eq,Show)
    -- The list of constructors could be an array, a balanced tree, or others.

    -- | Public representation of constructors
    data ConstrRep = AlgConstr    ConIndex
                   | IntConstr    Integer
                   | FloatConstr  Double
                   | StringConstr String

                   deriving (Eq,Show)

    -- | Unique index for datatype constructors,
    -- counting from 1 in the order they are given in the program text.
    type ConIndex = Int

    -- | Fixity of constructors
    data Fixity = Prefix
                | Infix     -- Later: add associativity and precedence

                deriving (Eq,Show)

    ------------------------------------------------------------------------------
    --
    --      Observers for datatype representations
    --
    ------------------------------------------------------------------------------

    -- | Gets the type constructor including the module
    dataTypeName :: DataType -> String
    dataTypeName = tycon

    -- | Gets the public presentation of a datatype
    dataTypeRep :: DataType -> DataRep
    dataTypeRep = datarep

    -- | Gets the datatype of a constructor
    constrType :: Constr -> DataType
    constrType = datatype

    -- | Gets the public presentation of constructors
    constrRep :: Constr -> ConstrRep
    constrRep = conrep

    -- | Look up a constructor by its representation
    repConstr :: DataType -> ConstrRep -> Constr
    repConstr dt cr =
          case (dataTypeRep dt, cr) of
            (AlgRep cs, AlgConstr i)      -> cs !! (i-1)
            (IntRep,    IntConstr i)      -> mkIntConstr dt i
            (FloatRep,  FloatConstr f)    -> mkFloatConstr dt f
            (StringRep, StringConstr str) -> mkStringConstr dt str
            _ -> error "repConstr"

    ------------------------------------------------------------------------------
    --
    --      Representations of algebraic data types
    --
    ------------------------------------------------------------------------------

    -- | Constructs an algebraic datatype
    mkDataType :: String -> [Constr] -> DataType
    mkDataType str cs = DataType
                            { tycon   = str
                            , datarep = AlgRep cs
                            }

    -- | Constructs a constructor
    mkConstr :: DataType -> String -> [String] -> Fixity -> Constr
    mkConstr dt str fields fix =
            Constr
                    { conrep    = AlgConstr idx
                    , constring = str
                    , confields = fields
                    , confixity = fix
                    , datatype  = dt
                    }
      where
        idx = head [ i | (c,i) <- dataTypeConstrs dt `zip` [1..],
                         showConstr c == str ]

    -- | Gets the constructors of an algebraic datatype
    dataTypeConstrs :: DataType -> [Constr]
    dataTypeConstrs dt = case datarep dt of
                            (AlgRep cons) -> cons
                            _ -> error "dataTypeConstrs"

    -- | Gets the field labels of a constructor.  The list of labels
    -- is returned in the same order as they were given in the original
    -- constructor declaration.
    constrFields :: Constr -> [String]
    constrFields = confields

    -- | Gets the fixity of a constructor
    constrFixity :: Constr -> Fixity
    constrFixity = confixity

    ------------------------------------------------------------------------------
    --
    --      From strings to constr's and vice versa: all data types
    --
    ------------------------------------------------------------------------------

    -- | Gets the string for a constructor
    showConstr :: Constr -> String
    showConstr = constring

    -- | Lookup a constructor via a string
    readConstr :: DataType -> String -> Maybe Constr
    readConstr dt str =
          case dataTypeRep dt of
            AlgRep cons -> idx cons
            IntRep      -> mkReadCon (\i -> (mkPrimCon dt str (IntConstr i)))
            FloatRep    -> mkReadCon (\f -> (mkPrimCon dt str (FloatConstr f)))
            StringRep   -> Just (mkStringConstr dt str)
            NoRep       -> Nothing
      where

        -- Read a value and build a constructor
        mkReadCon :: Read t => (t -> Constr) -> Maybe Constr
        mkReadCon f = case (reads str) of
                        [(t,"")] -> Just (f t)
                        _ -> Nothing

        -- Traverse list of algebraic datatype constructors
        idx :: [Constr] -> Maybe Constr
        idx cons = let fit = filter ((==) str . showConstr) cons
                    in if fit == []
                         then Nothing
                         else Just (head fit)

    ------------------------------------------------------------------------------
    --
    --      Convenience funtions: algebraic data types
    --
    ------------------------------------------------------------------------------

    -- | Test for an algebraic type
    isAlgType :: DataType -> Bool
    isAlgType dt = case datarep dt of
                     (AlgRep _) -> True
                     _ -> False

    -- | Gets the constructor for an index (algebraic datatypes only)
    indexConstr :: DataType -> ConIndex -> Constr
    indexConstr dt idx = case datarep dt of
                            (AlgRep cs) -> cs !! (idx-1)
                            _           -> error "indexConstr"

    -- | Gets the index of a constructor (algebraic datatypes only)
    constrIndex :: Constr -> ConIndex
    constrIndex con = case constrRep con of
                        (AlgConstr idx) -> idx
                        _ -> error "constrIndex"

    -- | Gets the maximum constructor index of an algebraic datatype
    maxConstrIndex :: DataType -> ConIndex
    maxConstrIndex dt = case dataTypeRep dt of
                            AlgRep cs -> length cs
                            _            -> error "maxConstrIndex"

    ------------------------------------------------------------------------------
    --
    --      Representation of primitive types
    --
    ------------------------------------------------------------------------------

    -- | Constructs the 'Int' type
    mkIntType :: String -> DataType
    mkIntType = mkPrimType IntRep

    -- | Constructs the 'Float' type
    mkFloatType :: String -> DataType
    mkFloatType = mkPrimType FloatRep

    -- | Constructs the 'String' type
    mkStringType :: String -> DataType
    mkStringType = mkPrimType StringRep

    -- | Helper for 'mkIntType', 'mkFloatType', 'mkStringType'
    mkPrimType :: DataRep -> String -> DataType
    mkPrimType dr str = DataType
                            { tycon   = str
                            , datarep = dr
                            }

    -- Makes a constructor for primitive types
    mkPrimCon :: DataType -> String -> ConstrRep -> Constr
    mkPrimCon dt str cr = Constr
                            { datatype  = dt
                            , conrep    = cr
                            , constring = str
                            , confields = error "constrFields"
                            , confixity = error "constrFixity"
                            }

    mkIntConstr :: DataType -> Integer -> Constr
    mkIntConstr dt i = case datarep dt of
                      IntRep -> mkPrimCon dt (show i) (IntConstr i)
                      _ -> error "mkIntConstr"

    mkFloatConstr :: DataType -> Double -> Constr
    mkFloatConstr dt f = case datarep dt of
                        FloatRep -> mkPrimCon dt (show f) (FloatConstr f)
                        _ -> error "mkFloatConstr"

    mkStringConstr :: DataType -> String -> Constr
    mkStringConstr dt str = case datarep dt of
                           StringRep -> mkPrimCon dt str (StringConstr str)
                           _ -> error "mkStringConstr"

    ------------------------------------------------------------------------------
    --
    --      Non-representations for non-presentable types
    --
    ------------------------------------------------------------------------------

    -- | Constructs a non-representation for a non-presentable type
    mkNorepType :: String -> DataType
    mkNorepType str = DataType
                            { tycon   = str
                            , datarep = NoRep
                            }

    -- | Test for a non-representable type
    isNorepType :: DataType -> Bool
    isNorepType dt = case datarep dt of
                       NoRep -> True
                       _ -> False

    ------------------------------------------------------------------------------
    --
    --      Convenience for qualified type constructors
    --
    ------------------------------------------------------------------------------

    -- | Gets the unqualified type constructor:
    -- drop *.*.*... before name
    --
    tyconUQname :: String -> String
    tyconUQname x = let x' = dropWhile (not . (==) '.') x
                     in if x' == [] then x else tyconUQname (tail x')

    -- | Gets the module of a type constructor:
    -- take *.*.*... before name
    tyconModule :: String -> String
    tyconModule x = let (a,b) = break ((==) '.') x
                     in if b == ""
                          then b
                          else a ++ tyconModule' (tail b)
      where
        tyconModule' x = let x' = tyconModule x
                          in if x' == "" then "" else ('.':x')