content: \begin{code}
  {-# LANGUAGE Trustworthy #-}
  {-# LANGUAGE CPP, NoImplicitPrelude, StandaloneDeriving, PatternGuards,
               ScopedTypeVariables #-}
  {-# OPTIONS_HADDOCK hide #-}
  
  -----------------------------------------------------------------------------
  -- |
  -- 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)]
  
    -- 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
  
    -- XXX Can this be removed?
    , readp
    )
   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
  
  #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.Err
  import GHC.Arr
  -- For defining instances for the generic deriving mechanism
  import GHC.Generics (Arity(..), Associativity(..), Fixity(..))
  \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
  
  ------------------------------------------------------------------------
  -- 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
  
  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)]@
  -- We match both Ident and Symbol because the constructor
  -- might be an operator eg (:=:)
  choose sps = foldr ((+++) . try_one) pfail sps
             where
               try_one (s,p) = do { token <- lexP ;
                                    case token of
                                      L.Ident s'  | s==s' -> p
                                      L.Symbol s' | s==s' -> p
                                      _other              -> 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 [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
                    theBounds <- step readPrec
                    vals   <- step readPrec
                    return (array theBounds 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' -> ReadPrec a) -> ReadPrec a
  -- Read a signed number
  readNumber convert =
    parens
    ( do x <- lexP'
         case x of
           L.Symbol' "-" -> do y <- lexP'
                               n <- convert y
                               return (negate n)
  
           _   -> convert x
    )
  
  
  convertInt :: Num a => L.Lexeme' -> ReadPrec a
  convertInt (L.Number n)
   | Just i <- L.numberToInteger n = return (fromInteger i)
  convertInt _ = pfail
  
  convertFrac :: forall a . RealFloat a => L.Lexeme' -> ReadPrec a
  convertFrac (L.Ident' "NaN")      = return (0 / 0)
  convertFrac (L.Ident' "Infinity") = return (1 / 0)
  convertFrac (L.Number n) = let resRange = floatRange (undefined :: a)
                             in case L.numberToRangedRational resRange n of
                                Nothing -> return (1 / 0)
                                Just rat -> return $ fromRational rat
  convertFrac _            = pfail
  
  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}
  
  \begin{code}
  -- XXX Can this be removed?
  
  readp :: Read a => ReadP a
  readp = readPrec_to_P readPrec minPrec
  \end{code}
  
  Instances for types of the generic deriving mechanism.
  
  \begin{code}
  deriving instance Read Arity
  deriving instance Read Associativity
  deriving instance Read Fixity
  \end{code}
  