    \begin{code}
    {-# OPTIONS_GHC -fno-implicit-prelude #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  GHC.Read
    -- Copyright   :  (c) The University of Glasgow, 1994-2002
    -- License     :  see libraries/base/LICENSE
    --
    -- Maintainer  :  cvs-ghc@haskell.org
    -- Stability   :  internal
    -- Portability :  non-portable (GHC Extensions)
    --
    -- The 'Read' class and instances for basic data types.
    --
    -----------------------------------------------------------------------------

    -- #hide
    module GHC.Read
      ( Read(..)   -- class

      -- ReadS type
      , ReadS      -- :: *; = String -> [(a,String)]

      -- utility functions
      , reads      -- :: Read a => ReadS a
      , readp      -- :: Read a => ReadP a
      , readEither -- :: Read a => String -> Either String a
      , read       -- :: Read a => String -> a

      -- H98 compatibility
      , lex         -- :: ReadS String
      , lexLitChar  -- :: ReadS String
      , readLitChar -- :: ReadS Char
      , lexDigits   -- :: ReadS String

      -- defining readers
      , lexP       -- :: ReadPrec Lexeme
      , paren      -- :: ReadPrec a -> ReadPrec a
      , parens     -- :: ReadPrec a -> ReadPrec a
      , list       -- :: ReadPrec a -> ReadPrec [a]
      , choose     -- :: [(String, ReadPrec a)] -> ReadPrec a
      , readListDefault, readListPrecDefault

      -- Temporary
      , readParen
      )
     where

    import qualified Text.ParserCombinators.ReadP as P

    import Text.ParserCombinators.ReadP
      ( ReadP
      , ReadS
      , readP_to_S
      )

    import qualified Text.Read.Lex as L
    -- Lex exports 'lex', which is also defined here,
    -- hence the qualified import.
    -- We can't import *anything* unqualified, because that
    -- confuses Haddock.

    import Text.ParserCombinators.ReadPrec

    import Data.Maybe
    import Data.Either

    #ifndef __HADDOCK__
    import {-# SOURCE #-} GHC.Unicode       ( isDigit )
    #endif
    import GHC.Num
    import GHC.Real
    import GHC.Float
    import GHC.Show
    import GHC.Base
    import GHC.Arr
    \end{code}

    \begin{code}
    -- | @'readParen' 'True' p@ parses what @p@ parses, but surrounded with
    -- parentheses.
    --
    -- @'readParen' 'False' p@ parses what @p@ parses, but optionally
    -- surrounded with parentheses.
    readParen       :: Bool -> ReadS a -> ReadS a
    -- A Haskell 98 function
    readParen b g   =  if b then mandatory else optional
                       where optional r  = g r ++ mandatory r
                             mandatory r = do
                                    ("(",s) <- lex r
                                    (x,t)   <- optional s
                                    (")",u) <- lex t
                                    return (x,u)
    \end{code}

    %*********************************************************
    %*                                                      *
    \subsection{The @Read@ class}
    %*                                                      *
    %*********************************************************

    \begin{code}
    ------------------------------------------------------------------------
    -- class Read

    -- | Parsing of 'String's, producing values.
    --
    -- Minimal complete definition: 'readsPrec' (or, for GHC only, 'readPrec')
    --
    -- Derived instances of 'Read' make the following assumptions, which
    -- derived instances of 'Text.Show.Show' obey:
    --
    -- * If the constructor is defined to be an infix operator, then the
    --   derived 'Read' instance will parse only infix applications of
    --   the constructor (not the prefix form).
    --
    -- * Associativity is not used to reduce the occurrence of parentheses,
    --   although precedence may be.
    --
    -- * If the constructor is defined using record syntax, the derived 'Read'
    --   will parse only the record-syntax form, and furthermore, the fields
    --   must be given in the same order as the original declaration.
    --
    -- * The derived 'Read' instance allows arbitrary Haskell whitespace
    --   between tokens of the input string.  Extra parentheses are also
    --   allowed.
    --
    -- For example, given the declarations
    --
    -- > infixr 5 :^:
    -- > data Tree a =  Leaf a  |  Tree a :^: Tree a
    --
    -- the derived instance of 'Read' in Haskell 98 is equivalent to
    --
    -- > instance (Read a) => Read (Tree a) where
    -- >
    -- >         readsPrec d r =  readParen (d > app_prec)
    -- >                          (\r -> [(Leaf m,t) |
    -- >                                  ("Leaf",s) <- lex r,
    -- >                                  (m,t) <- readsPrec (app_prec+1) s]) r
    -- >
    -- >                       ++ readParen (d > up_prec)
    -- >                          (\r -> [(u:^:v,w) |
    -- >                                  (u,s) <- readsPrec (up_prec+1) r,
    -- >                                  (":^:",t) <- lex s,
    -- >                                  (v,w) <- readsPrec (up_prec+1) t]) r
    -- >
    -- >           where app_prec = 10
    -- >                 up_prec = 5
    --
    -- Note that right-associativity of @:^:@ is unused.
    --
    -- The derived instance in GHC is equivalent to
    --
    -- > instance (Read a) => Read (Tree a) where
    -- >
    -- >         readPrec = parens $ (prec app_prec $ do
    -- >                                  Ident "Leaf" <- lexP
    -- >                                  m <- step readPrec
    -- >                                  return (Leaf m))
    -- >
    -- >                      +++ (prec up_prec $ do
    -- >                                  u <- step readPrec
    -- >                                  Symbol ":^:" <- lexP
    -- >                                  v <- step readPrec
    -- >                                  return (u :^: v))
    -- >
    -- >           where app_prec = 10
    -- >                 up_prec = 5
    -- >
    -- >         readListPrec = readListPrecDefault

    class Read a where
      -- | attempts to parse a value from the front of the string, returning
      -- a list of (parsed value, remaining string) pairs.  If there is no
      -- successful parse, the returned list is empty.
      --
      -- Derived instances of 'Read' and 'Text.Show.Show' satisfy the following:
      --
      -- * @(x,\"\")@ is an element of
      --   @('readsPrec' d ('Text.Show.showsPrec' d x \"\"))@.
      --
      -- That is, 'readsPrec' parses the string produced by
      -- 'Text.Show.showsPrec', and delivers the value that
      -- 'Text.Show.showsPrec' started with.

      readsPrec    :: Int   -- ^ the operator precedence of the enclosing
                            -- context (a number from @0@ to @11@).
                            -- Function application has precedence @10@.
                    -> ReadS a

      -- | The method 'readList' is provided to allow the programmer to
      -- give a specialised way of parsing lists of values.
      -- For example, this is used by the predefined 'Read' instance of
      -- the 'Char' type, where values of type 'String' should be are
      -- expected to use double quotes, rather than square brackets.
      readList     :: ReadS [a]

      -- | Proposed replacement for 'readsPrec' using new-style parsers (GHC only).
      readPrec     :: ReadPrec a

      -- | Proposed replacement for 'readList' using new-style parsers (GHC only).
      -- The default definition uses 'readList'.  Instances that define 'readPrec'
      -- should also define 'readListPrec' as 'readListPrecDefault'.
      readListPrec :: ReadPrec [a]

      -- default definitions
      readsPrec    = readPrec_to_S readPrec
      readList     = readPrec_to_S (list readPrec) 0
      readPrec     = readS_to_Prec readsPrec
      readListPrec = readS_to_Prec (\_ -> readList)

    readListDefault :: Read a => ReadS [a]
    -- ^ A possible replacement definition for the 'readList' method (GHC only).
    --   This is only needed for GHC, and even then only for 'Read' instances
    --   where 'readListPrec' isn't defined as 'readListPrecDefault'.
    readListDefault = readPrec_to_S readListPrec 0

    readListPrecDefault :: Read a => ReadPrec [a]
    -- ^ A possible replacement definition for the 'readListPrec' method,
    --   defined using 'readPrec' (GHC only).
    readListPrecDefault = list readPrec

    ------------------------------------------------------------------------
    -- utility functions

    -- | equivalent to 'readsPrec' with a precedence of 0.
    reads :: Read a => ReadS a
    reads = readsPrec minPrec

    readp :: Read a => ReadP a
    readp = readPrec_to_P readPrec minPrec

    readEither :: Read a => String -> Either String a
    readEither s =
      case [ x | (x,"") <- readPrec_to_S read' minPrec s ] of
        [x] -> Right x
        []  -> Left "Prelude.read: no parse"
        _   -> Left "Prelude.read: ambiguous parse"
     where
      read' =
        do x <- readPrec
           lift P.skipSpaces
           return x

    -- | The 'read' function reads input from a string, which must be
    -- completely consumed by the input process.
    read :: Read a => String -> a
    read s = either error id (readEither s)

    ------------------------------------------------------------------------
    -- H98 compatibility

    -- | The 'lex' function reads a single lexeme from the input, discarding
    -- initial white space, and returning the characters that constitute the
    -- lexeme.  If the input string contains only white space, 'lex' returns a
    -- single successful \`lexeme\' consisting of the empty string.  (Thus
    -- @'lex' \"\" = [(\"\",\"\")]@.)  If there is no legal lexeme at the
    -- beginning of the input string, 'lex' fails (i.e. returns @[]@).
    --
    -- This lexer is not completely faithful to the Haskell lexical syntax
    -- in the following respects:
    --
    -- * Qualified names are not handled properly
    --
    -- * Octal and hexadecimal numerics are not recognized as a single token
    --
    -- * Comments are not treated properly
    lex :: ReadS String             -- As defined by H98
    lex s  = readP_to_S L.hsLex s

    -- | Read a string representation of a character, using Haskell
    -- source-language escape conventions.  For example:
    --
    -- > lexLitChar  "\\nHello"  =  [("\\n", "Hello")]
    --
    lexLitChar :: ReadS String      -- As defined by H98
    lexLitChar = readP_to_S (do { (s, _) <- P.gather L.lexChar ;
                                  return s })
            -- There was a skipSpaces before the P.gather L.lexChar,
            -- but that seems inconsistent with readLitChar

    -- | Read a string representation of a character, using Haskell
    -- source-language escape conventions, and convert it to the character
    -- that it encodes.  For example:
    --
    -- > readLitChar "\\nHello"  =  [('\n', "Hello")]
    --
    readLitChar :: ReadS Char       -- As defined by H98
    readLitChar = readP_to_S L.lexChar

    -- | Reads a non-empty string of decimal digits.
    lexDigits :: ReadS String
    lexDigits = readP_to_S (P.munch1 isDigit)

    ------------------------------------------------------------------------
    -- utility parsers

    lexP :: ReadPrec L.Lexeme
    -- ^ Parse a single lexeme
    lexP = lift L.lex

    paren :: ReadPrec a -> ReadPrec a
    -- ^ @(paren p)@ parses \"(P0)\"
    --      where @p@ parses \"P0\" in precedence context zero
    paren p = do L.Punc "(" <- lexP
                 x          <- reset p
                 L.Punc ")" <- lexP
                 return x

    parens :: ReadPrec a -> ReadPrec a
    -- ^ @(parens p)@ parses \"P\", \"(P0)\", \"((P0))\", etc,
    --      where @p@ parses \"P\"  in the current precedence context
    --          and parses \"P0\" in precedence context zero
    parens p = optional
     where
      optional  = p +++ mandatory
      mandatory = paren optional

    list :: ReadPrec a -> ReadPrec [a]
    -- ^ @(list p)@ parses a list of things parsed by @p@,
    -- using the usual square-bracket syntax.
    list readx =
      parens
      ( do L.Punc "[" <- lexP
           (listRest False +++ listNext)
      )
     where
      listRest started =
        do L.Punc c <- lexP
           case c of
             "]"           -> return []
             "," | started -> listNext
             _             -> pfail

      listNext =
        do x  <- reset readx
           xs <- listRest True
           return (x:xs)

    choose :: [(String, ReadPrec a)] -> ReadPrec a
    -- ^ Parse the specified lexeme and continue as specified.
    -- Esp useful for nullary constructors; e.g.
    --    @choose [(\"A\", return A), (\"B\", return B)]@
    choose sps = foldr ((+++) . try_one) pfail sps
               where
                 try_one (s,p) = do { L.Ident s' <- lexP ;
                                      if s == s' then p else pfail }
    \end{code}

    %*********************************************************
    %*                                                      *
    \subsection{Simple instances of Read}
    %*                                                      *
    %*********************************************************

    \begin{code}
    instance Read Char where
      readPrec =
        parens
        ( do L.Char c <- lexP
             return c
        )

      readListPrec =
        parens
        ( do L.String s <- lexP     -- Looks for "foo"
             return s
         +++
          readListPrecDefault       -- Looks for ['f','o','o']
        )                           -- (more generous than H98 spec)

      readList = readListDefault

    instance Read Bool where
      readPrec =
        parens
        ( do L.Ident s <- lexP
             case s of
               "True"  -> return True
               "False" -> return False
               _       -> pfail
        )

      readListPrec = readListPrecDefault
      readList     = readListDefault

    instance Read Ordering where
      readPrec =
        parens
        ( do L.Ident s <- lexP
             case s of
               "LT" -> return LT
               "EQ" -> return EQ
               "GT" -> return GT
               _    -> pfail
        )

      readListPrec = readListPrecDefault
      readList     = readListDefault
    \end{code}

    %*********************************************************
    %*                                                      *
    \subsection{Structure instances of Read: Maybe, List etc}
    %*                                                      *
    %*********************************************************

    For structured instances of Read we start using the precedences.  The
    idea is then that 'parens (prec k p)' will fail immediately when trying
    to parse it in a context with a higher precedence level than k. But if
    there is one parenthesis parsed, then the required precedence level
    drops to 0 again, and parsing inside p may succeed.

    'appPrec' is just the precedence level of function application.  So,
    if we are parsing function application, we'd better require the
    precedence level to be at least 'appPrec'. Otherwise, we have to put
    parentheses around it.

    'step' is used to increase the precedence levels inside a
    parser, and can be used to express left- or right- associativity. For
    example, % is defined to be left associative, so we only increase
    precedence on the right hand side.

    Note how step is used in for example the Maybe parser to increase the
    precedence beyond appPrec, so that basically only literals and
    parenthesis-like objects such as (...) and [...] can be an argument to
    'Just'.

    \begin{code}
    instance Read a => Read (Maybe a) where
      readPrec =
        parens
        (do L.Ident "Nothing" <- lexP
            return Nothing
         +++
         prec appPrec (
            do L.Ident "Just" <- lexP
               x              <- step readPrec
               return (Just x))
        )

      readListPrec = readListPrecDefault
      readList     = readListDefault

    instance (Read a, Read b) => Read (Either a b) where
      readPrec =
        parens
        ( prec appPrec
          ( do L.Ident "Left" <- lexP
               x            <- step readPrec
               return (Left x)
           +++
            do L.Ident "Right" <- lexP
               y             <- step readPrec
               return (Right y)
          )
        )

      readListPrec = readListPrecDefault
      readList     = readListDefault

    instance Read a => Read [a] where
      readPrec     = readListPrec
      readListPrec = readListPrecDefault
      readList     = readListDefault

    instance  (Ix a, Read a, Read b) => Read (Array a b)  where
        readPrec = parens $ prec appPrec $
                   do L.Ident "array" <- lexP
                      bounds <- step readPrec
                      vals   <- step readPrec
                      return (array bounds vals)

        readListPrec = readListPrecDefault
        readList     = readListDefault

    instance Read L.Lexeme where
      readPrec     = lexP
      readListPrec = readListPrecDefault
      readList     = readListDefault
    \end{code}

    %*********************************************************
    %*                                                      *
    \subsection{Numeric instances of Read}
    %*                                                      *
    %*********************************************************

    \begin{code}
    readNumber :: Num a => (L.Lexeme -> Maybe a) -> ReadPrec a
    -- Read a signed number
    readNumber convert =
      parens
      ( do x <- lexP
           case x of
             L.Symbol "-" -> do n <- readNumber convert
                                return (negate n)

             _   -> case convert x of
                       Just n  -> return n
                       Nothing -> pfail
      )

    convertInt :: Num a => L.Lexeme -> Maybe a
    convertInt (L.Int i) = Just (fromInteger i)
    convertInt _         = Nothing

    convertFrac :: Fractional a => L.Lexeme -> Maybe a
    convertFrac (L.Int i) = Just (fromInteger i)
    convertFrac (L.Rat r) = Just (fromRational r)
    convertFrac _         = Nothing

    instance Read Int where
      readPrec     = readNumber convertInt
      readListPrec = readListPrecDefault
      readList     = readListDefault

    instance Read Integer where
      readPrec     = readNumber convertInt
      readListPrec = readListPrecDefault
      readList     = readListDefault

    instance Read Float where
      readPrec     = readNumber convertFrac
      readListPrec = readListPrecDefault
      readList     = readListDefault

    instance Read Double where
      readPrec     = readNumber convertFrac
      readListPrec = readListPrecDefault
      readList     = readListDefault

    instance (Integral a, Read a) => Read (Ratio a) where
      readPrec =
        parens
        ( prec ratioPrec
          ( do x            <- step readPrec
               L.Symbol "%" <- lexP
               y            <- step readPrec
               return (x % y)
          )
        )

      readListPrec = readListPrecDefault
      readList     = readListDefault
    \end{code}

    %*********************************************************
    %*                                                      *
            Tuple instances of Read, up to size 15
    %*                                                      *
    %*********************************************************

    \begin{code}
    instance Read () where
      readPrec =
        parens
        ( paren
          ( return ()
          )
        )

      readListPrec = readListPrecDefault
      readList     = readListDefault

    instance (Read a, Read b) => Read (a,b) where
      readPrec = wrap_tup read_tup2
      readListPrec = readListPrecDefault
      readList     = readListDefault

    wrap_tup :: ReadPrec a -> ReadPrec a
    wrap_tup p = parens (paren p)

    read_comma :: ReadPrec ()
    read_comma = do { L.Punc "," <- lexP; return () }

    read_tup2 :: (Read a, Read b) => ReadPrec (a,b)
    -- Reads "a , b"  no parens!
    read_tup2 = do x <- readPrec
                   read_comma
                   y <- readPrec
                   return (x,y)

    read_tup4 :: (Read a, Read b, Read c, Read d) => ReadPrec (a,b,c,d)
    read_tup4 = do  (a,b) <- read_tup2
                    read_comma
                    (c,d) <- read_tup2
                    return (a,b,c,d)

    read_tup8 :: (Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h)
              => ReadPrec (a,b,c,d,e,f,g,h)
    read_tup8 = do  (a,b,c,d) <- read_tup4
                    read_comma
                    (e,f,g,h) <- read_tup4
                    return (a,b,c,d,e,f,g,h)

    instance (Read a, Read b, Read c) => Read (a, b, c) where
      readPrec = wrap_tup (do { (a,b) <- read_tup2; read_comma
                              ; c <- readPrec
                              ; return (a,b,c) })
      readListPrec = readListPrecDefault
      readList     = readListDefault

    instance (Read a, Read b, Read c, Read d) => Read (a, b, c, d) where
      readPrec = wrap_tup read_tup4
      readListPrec = readListPrecDefault
      readList     = readListDefault

    instance (Read a, Read b, Read c, Read d, Read e) => Read (a, b, c, d, e) where
      readPrec = wrap_tup (do { (a,b,c,d) <- read_tup4; read_comma
                              ; e <- readPrec
                              ; return (a,b,c,d,e) })
      readListPrec = readListPrecDefault
      readList     = readListDefault

    instance (Read a, Read b, Read c, Read d, Read e, Read f)
            => Read (a, b, c, d, e, f) where
      readPrec = wrap_tup (do { (a,b,c,d) <- read_tup4; read_comma
                              ; (e,f) <- read_tup2
                              ; return (a,b,c,d,e,f) })
      readListPrec = readListPrecDefault
      readList     = readListDefault

    instance (Read a, Read b, Read c, Read d, Read e, Read f, Read g)
            => Read (a, b, c, d, e, f, g) where
      readPrec = wrap_tup (do { (a,b,c,d) <- read_tup4; read_comma
                              ; (e,f) <- read_tup2; read_comma
                              ; g <- readPrec
                              ; return (a,b,c,d,e,f,g) })
      readListPrec = readListPrecDefault
      readList     = readListDefault

    instance (Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h)
            => Read (a, b, c, d, e, f, g, h) where
      readPrec     = wrap_tup read_tup8
      readListPrec = readListPrecDefault
      readList     = readListDefault

    instance (Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h,
              Read i)
            => Read (a, b, c, d, e, f, g, h, i) where
      readPrec = wrap_tup (do { (a,b,c,d,e,f,g,h) <- read_tup8; read_comma
                              ; i <- readPrec
                              ; return (a,b,c,d,e,f,g,h,i) })
      readListPrec = readListPrecDefault
      readList     = readListDefault

    instance (Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h,
              Read i, Read j)
            => Read (a, b, c, d, e, f, g, h, i, j) where
      readPrec = wrap_tup (do { (a,b,c,d,e,f,g,h) <- read_tup8; read_comma
                              ; (i,j) <- read_tup2
                              ; return (a,b,c,d,e,f,g,h,i,j) })
      readListPrec = readListPrecDefault
      readList     = readListDefault

    instance (Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h,
              Read i, Read j, Read k)
            => Read (a, b, c, d, e, f, g, h, i, j, k) where
      readPrec = wrap_tup (do { (a,b,c,d,e,f,g,h) <- read_tup8; read_comma
                              ; (i,j) <- read_tup2; read_comma
                              ; k <- readPrec
                              ; return (a,b,c,d,e,f,g,h,i,j,k) })
      readListPrec = readListPrecDefault
      readList     = readListDefault

    instance (Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h,
              Read i, Read j, Read k, Read l)
            => Read (a, b, c, d, e, f, g, h, i, j, k, l) where
      readPrec = wrap_tup (do { (a,b,c,d,e,f,g,h) <- read_tup8; read_comma
                              ; (i,j,k,l) <- read_tup4
                              ; return (a,b,c,d,e,f,g,h,i,j,k,l) })
      readListPrec = readListPrecDefault
      readList     = readListDefault

    instance (Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h,
              Read i, Read j, Read k, Read l, Read m)
            => Read (a, b, c, d, e, f, g, h, i, j, k, l, m) where
      readPrec = wrap_tup (do { (a,b,c,d,e,f,g,h) <- read_tup8; read_comma
                              ; (i,j,k,l) <- read_tup4; read_comma
                              ; m <- readPrec
                              ; return (a,b,c,d,e,f,g,h,i,j,k,l,m) })
      readListPrec = readListPrecDefault
      readList     = readListDefault

    instance (Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h,
              Read i, Read j, Read k, Read l, Read m, Read n)
            => Read (a, b, c, d, e, f, g, h, i, j, k, l, m, n) where
      readPrec = wrap_tup (do { (a,b,c,d,e,f,g,h) <- read_tup8; read_comma
                              ; (i,j,k,l) <- read_tup4; read_comma
                              ; (m,n) <- read_tup2
                              ; return (a,b,c,d,e,f,g,h,i,j,k,l,m,n) })
      readListPrec = readListPrecDefault
      readList     = readListDefault

    instance (Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h,
              Read i, Read j, Read k, Read l, Read m, Read n, Read o)
            => Read (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o) where
      readPrec = wrap_tup (do { (a,b,c,d,e,f,g,h) <- read_tup8; read_comma
                              ; (i,j,k,l) <- read_tup4; read_comma
                              ; (m,n) <- read_tup2; read_comma
                              ; o <- readPrec
                              ; return (a,b,c,d,e,f,g,h,i,j,k,l,m,n,o) })
      readListPrec = readListPrecDefault
      readList     = readListDefault
    \end{code}