+++ /dev/null
-% ------------------------------------------------------------------------------
-% $Id: PrelRead.lhs,v 1.22 2001/11/23 16:20:08 simonpj Exp $
-%
-% (c) The University of Glasgow, 1994-2000
-%
-
-\section[PrelRead]{Module @PrelRead@}
-
-Instances of the Read class.
-
-\begin{code}
-{-# OPTIONS -fno-implicit-prelude #-}
-
-module PrelRead where
-
-import {-# SOURCE #-} PrelErr ( error )
-import PrelEnum ( Enum(..), maxBound )
-import PrelNum
-import PrelReal
-import PrelFloat
-import PrelList
-import PrelMaybe
-import PrelShow -- isAlpha etc
-import PrelBase
-\end{code}
-
-%*********************************************************
-%* *
-\subsection{The @Read@ class}
-%* *
-%*********************************************************
-
-Note: if you compile this with -DNEW_READS_REP, you'll get
-a (simpler) ReadS representation that only allow one valid
-parse of a string of characters, instead of a list of
-possible ones.
-
-[changing the ReadS rep has implications for the deriving
-machinery for Read, a change that hasn't been made, so you
-probably won't want to compile in this new rep. except
-when in an experimental mood.]
-
-\begin{code}
-
-#ifndef NEW_READS_REP
-type ReadS a = String -> [(a,String)]
-#else
-type ReadS a = String -> Maybe (a,String)
-#endif
-
-class Read a where
- readsPrec :: Int -> ReadS a
-
- readList :: ReadS [a]
- readList = readList__ reads
-\end{code}
-
-In this module we treat [(a,String)] as a monad in MonadPlus
-But MonadPlus isn't defined yet, so we simply give local
-declarations for mzero and guard suitable for this particular
-type. It would also be reasonably to move MonadPlus to PrelBase
-along with Monad and Functor, but that seems overkill for one
-example
-
-\begin{code}
-mzero :: [a]
-mzero = []
-
-guard :: Bool -> [()]
-guard True = [()]
-guard False = []
-\end{code}
-
-%*********************************************************
-%* *
-\subsection{Utility functions}
-%* *
-%*********************************************************
-
-\begin{code}
-reads :: (Read a) => ReadS a
-reads = readsPrec 0
-
-read :: (Read a) => String -> a
-read s =
- case read_s s of
-#ifndef NEW_READS_REP
- [x] -> x
- [] -> error "Prelude.read: no parse"
- _ -> error "Prelude.read: ambiguous parse"
-#else
- Just x -> x
- Nothing -> error "Prelude.read: no parse"
-#endif
- where
- read_s str = do
- (x,str1) <- reads str
- ("","") <- lex str1
- return x
-\end{code}
-
-\begin{code}
-readParen :: Bool -> ReadS a -> ReadS a
-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)
-
-
-readList__ :: ReadS a -> ReadS [a]
-
-readList__ readx
- = readParen False (\r -> do
- ("[",s) <- lex r
- readl s)
- where readl s =
- (do { ("]",t) <- lex s ; return ([],t) }) ++
- (do { (x,t) <- readx s ; (xs,u) <- readl2 t ; return (x:xs,u) })
-
- readl2 s =
- (do { ("]",t) <- lex s ; return ([],t) }) ++
- (do { (",",t) <- lex s ; (x,u) <- readx t ; (xs,v) <- readl2 u ; return (x:xs,v) })
-
-\end{code}
-
-
-%*********************************************************
-%* *
-\subsection{Lexical analysis}
-%* *
-%*********************************************************
-
-This lexer is not completely faithful to the Haskell lexical syntax.
-Current limitations:
- Qualified names are not handled properly
- A `--' does not terminate a symbol
- Octal and hexidecimal numerics are not recognized as a single token
-
-\begin{code}
-lex :: ReadS String
-
-lex "" = return ("","")
-lex (c:s) | isSpace c = lex (dropWhile isSpace s)
-lex ('\'':s) = do
- (ch, '\'':t) <- lexLitChar s
- guard (ch /= "'")
- return ('\'':ch++"'", t)
-lex ('"':s) = do
- (str,t) <- lexString s
- return ('"':str, t)
-
- where
- lexString ('"':s) = return ("\"",s)
- lexString s = do
- (ch,t) <- lexStrItem s
- (str,u) <- lexString t
- return (ch++str, u)
-
-
- lexStrItem ('\\':'&':s) = return ("\\&",s)
- lexStrItem ('\\':c:s) | isSpace c = do
- ('\\':t) <- return (dropWhile isSpace s)
- return ("\\&",t)
- lexStrItem s = lexLitChar s
-
-lex (c:s) | isSingle c = return ([c],s)
- | isSym c = do
- (sym,t) <- return (span isSym s)
- return (c:sym,t)
- | isAlpha c = do
- (nam,t) <- return (span isIdChar s)
- return (c:nam, t)
- | isDigit c = do
-{- Removed, 13/03/2000 by SDM.
- Doesn't work, and not required by Haskell report.
- let
- (pred, s', isDec) =
- case s of
- ('o':rs) -> (isOctDigit, rs, False)
- ('O':rs) -> (isOctDigit, rs, False)
- ('x':rs) -> (isHexDigit, rs, False)
- ('X':rs) -> (isHexDigit, rs, False)
- _ -> (isDigit, s, True)
--}
- (ds,s) <- return (span isDigit s)
- (fe,t) <- lexFracExp s
- return (c:ds++fe,t)
- | otherwise = mzero -- bad character
- where
- isSingle c = c `elem` ",;()[]{}_`"
- isSym c = c `elem` "!@#$%&*+./<=>?\\^|:-~"
- isIdChar c = isAlphaNum c || c `elem` "_'"
-
- lexFracExp ('.':c:cs) | isDigit c = do
- (ds,t) <- lex0Digits cs
- (e,u) <- lexExp t
- return ('.':c:ds++e,u)
- lexFracExp s = return ("",s)
-
- lexExp (e:s) | e `elem` "eE" =
- (do
- (c:t) <- return s
- guard (c `elem` "+-")
- (ds,u) <- lexDecDigits t
- return (e:c:ds,u)) ++
- (do
- (ds,t) <- lexDecDigits s
- return (e:ds,t))
-
- lexExp s = return ("",s)
-
-lexDigits :: ReadS String
-lexDigits = lexDecDigits
-
-lexDecDigits :: ReadS String
-lexDecDigits = nonnull isDigit
-
-lexOctDigits :: ReadS String
-lexOctDigits = nonnull isOctDigit
-
-lexHexDigits :: ReadS String
-lexHexDigits = nonnull isHexDigit
-
--- 0 or more digits
-lex0Digits :: ReadS String
-lex0Digits s = return (span isDigit s)
-
-nonnull :: (Char -> Bool) -> ReadS String
-nonnull p s = do
- (cs@(_:_),t) <- return (span p s)
- return (cs,t)
-
-lexLitChar :: ReadS String
-lexLitChar ('\\':s) = do
- (esc,t) <- lexEsc s
- return ('\\':esc, t)
- where
- lexEsc (c:s) | c `elem` escChars = return ([c],s)
- lexEsc s@(d:_) | isDigit d = checkSize 10 lexDecDigits s
- lexEsc ('o':d:s) | isOctDigit d = checkSize 8 lexOctDigits (d:s)
- lexEsc ('O':d:s) | isOctDigit d = checkSize 8 lexOctDigits (d:s)
- lexEsc ('x':d:s) | isHexDigit d = checkSize 16 lexHexDigits (d:s)
- lexEsc ('X':d:s) | isHexDigit d = checkSize 16 lexHexDigits (d:s)
- lexEsc ('^':c:s) | c >= '@' && c <= '_' = [(['^',c],s)] -- cf. cntrl in 2.6 of H. report.
- lexEsc s@(c:_) | isUpper c = fromAsciiLab s
- lexEsc _ = mzero
-
- escChars = "abfnrtv\\\"'"
-
- fromAsciiLab (x:y:z:ls) | isUpper y && (isUpper z || isDigit z) &&
- [x,y,z] `elem` asciiEscTab = return ([x,y,z], ls)
- fromAsciiLab (x:y:ls) | isUpper y &&
- [x,y] `elem` asciiEscTab = return ([x,y], ls)
- fromAsciiLab _ = mzero
-
- asciiEscTab = "DEL" : asciiTab
-
- {-
- Check that the numerically escaped char literals are
- within accepted boundaries.
-
- Note: this allows char lits with leading zeros, i.e.,
- \0000000000000000000000000000001.
- -}
- checkSize base f str = do
- (num, res) <- f str
- if toAnInteger base num > toInteger (ord maxBound) then
- mzero
- else
- case base of
- 8 -> return ('o':num, res)
- 16 -> return ('x':num, res)
- _ -> return (num, res)
-
- toAnInteger base = foldl (\ acc n -> acc*base + toInteger (digitToInt n)) 0
-
-
-lexLitChar (c:s) = return ([c],s)
-lexLitChar "" = mzero
-
-digitToInt :: Char -> Int
-digitToInt c
- | isDigit c = fromEnum c - fromEnum '0'
- | c >= 'a' && c <= 'f' = fromEnum c - fromEnum 'a' + 10
- | c >= 'A' && c <= 'F' = fromEnum c - fromEnum 'A' + 10
- | otherwise = error ("Char.digitToInt: not a digit " ++ show c) -- sigh
-\end{code}
-
-%*********************************************************
-%* *
-\subsection{Instances of @Read@}
-%* *
-%*********************************************************
-
-\begin{code}
-instance Read Char where
- readsPrec _ = readParen False
- (\r -> do
- ('\'':s,t) <- lex r
- (c,"\'") <- readLitChar s
- return (c,t))
-
- readList = readParen False (\r -> do
- ('"':s,t) <- lex r
- (l,_) <- readl s
- return (l,t))
- where readl ('"':s) = return ("",s)
- readl ('\\':'&':s) = readl s
- readl s = do
- (c,t) <- readLitChar s
- (cs,u) <- readl t
- return (c:cs,u)
-
-instance Read Bool where
- readsPrec _ = readParen False
- (\r ->
- lex r >>= \ lr ->
- (do { ("True", rest) <- return lr ; return (True, rest) }) ++
- (do { ("False", rest) <- return lr ; return (False, rest) }))
-
-
-instance Read Ordering where
- readsPrec _ = readParen False
- (\r ->
- lex r >>= \ lr ->
- (do { ("LT", rest) <- return lr ; return (LT, rest) }) ++
- (do { ("EQ", rest) <- return lr ; return (EQ, rest) }) ++
- (do { ("GT", rest) <- return lr ; return (GT, rest) }))
-
-instance Read a => Read (Maybe a) where
- readsPrec _ = readParen False
- (\r ->
- lex r >>= \ lr ->
- (do { ("Nothing", rest) <- return lr ; return (Nothing, rest)}) ++
- (do
- ("Just", rest1) <- return lr
- (x, rest2) <- reads rest1
- return (Just x, rest2)))
-
-instance (Read a, Read b) => Read (Either a b) where
- readsPrec _ = readParen False
- (\r ->
- lex r >>= \ lr ->
- (do
- ("Left", rest1) <- return lr
- (x, rest2) <- reads rest1
- return (Left x, rest2)) ++
- (do
- ("Right", rest1) <- return lr
- (x, rest2) <- reads rest1
- return (Right x, rest2)))
-
-instance Read Int where
- readsPrec _ x = readSigned readDec x
-
-instance Read Integer where
- readsPrec _ x = readSigned readDec x
-
-instance Read Float where
- readsPrec _ x = readSigned readFloat x
-
-instance Read Double where
- readsPrec _ x = readSigned readFloat x
-
-instance (Integral a, Read a) => Read (Ratio a) where
- readsPrec p = readParen (p > ratio_prec)
- (\r -> do
- (x,s) <- reads r
- ("%",t) <- lex s
- (y,u) <- reads t
- return (x % y,u))
-
-instance (Read a) => Read [a] where
- readsPrec _ = readList
-
-instance Read () where
- readsPrec _ = readParen False
- (\r -> do
- ("(",s) <- lex r
- (")",t) <- lex s
- return ((),t))
-
-instance (Read a, Read b) => Read (a,b) where
- readsPrec _ = readParen False
- (\r -> do
- ("(",s) <- lex r
- (x,t) <- readsPrec 0 s
- (",",u) <- lex t
- (y,v) <- readsPrec 0 u
- (")",w) <- lex v
- return ((x,y), w))
-
-instance (Read a, Read b, Read c) => Read (a, b, c) where
- readsPrec _ = readParen False
- (\a -> do
- ("(",b) <- lex a
- (x,c) <- readsPrec 0 b
- (",",d) <- lex c
- (y,e) <- readsPrec 0 d
- (",",f) <- lex e
- (z,g) <- readsPrec 0 f
- (")",h) <- lex g
- return ((x,y,z), h))
-
-instance (Read a, Read b, Read c, Read d) => Read (a, b, c, d) where
- readsPrec _ = readParen False
- (\a -> do
- ("(",b) <- lex a
- (w,c) <- readsPrec 0 b
- (",",d) <- lex c
- (x,e) <- readsPrec 0 d
- (",",f) <- lex e
- (y,g) <- readsPrec 0 f
- (",",h) <- lex g
- (z,h) <- readsPrec 0 h
- (")",i) <- lex h
- return ((w,x,y,z), i))
-
-instance (Read a, Read b, Read c, Read d, Read e) => Read (a, b, c, d, e) where
- readsPrec _ = readParen False
- (\a -> do
- ("(",b) <- lex a
- (v,c) <- readsPrec 0 b
- (",",d) <- lex c
- (w,e) <- readsPrec 0 d
- (",",f) <- lex e
- (x,g) <- readsPrec 0 f
- (",",h) <- lex g
- (y,i) <- readsPrec 0 h
- (",",j) <- lex i
- (z,k) <- readsPrec 0 j
- (")",l) <- lex k
- return ((v,w,x,y,z), l))
-\end{code}
-
-
-%*********************************************************
-%* *
-\subsection{Reading characters}
-%* *
-%*********************************************************
-
-\begin{code}
-readLitChar :: ReadS Char
-
-readLitChar [] = mzero
-readLitChar ('\\':s) = readEsc s
- where
- readEsc ('a':s) = return ('\a',s)
- readEsc ('b':s) = return ('\b',s)
- readEsc ('f':s) = return ('\f',s)
- readEsc ('n':s) = return ('\n',s)
- readEsc ('r':s) = return ('\r',s)
- readEsc ('t':s) = return ('\t',s)
- readEsc ('v':s) = return ('\v',s)
- readEsc ('\\':s) = return ('\\',s)
- readEsc ('"':s) = return ('"',s)
- readEsc ('\'':s) = return ('\'',s)
- readEsc ('^':c:s) | c >= '@' && c <= '_'
- = return (chr (ord c - ord '@'), s)
- readEsc s@(d:_) | isDigit d
- = do
- (n,t) <- readDec s
- return (chr n,t)
- readEsc ('o':s) = do
- (n,t) <- readOct s
- return (chr n,t)
- readEsc ('x':s) = do
- (n,t) <- readHex s
- return (chr n,t)
-
- readEsc s@(c:_) | isUpper c
- = let table = ('\DEL', "DEL") : zip ['\NUL'..] asciiTab
- in case [(c,s') | (c, mne) <- table,
- ([],s') <- [match mne s]]
- of (pr:_) -> return pr
- [] -> mzero
- readEsc _ = mzero
-
-readLitChar (c:s) = return (c,s)
-
-match :: (Eq a) => [a] -> [a] -> ([a],[a])
-match (x:xs) (y:ys) | x == y = match xs ys
-match xs ys = (xs,ys)
-
-\end{code}
-
-
-%*********************************************************
-%* *
-\subsection{Reading numbers}
-%* *
-%*********************************************************
-
-Note: reading numbers at bases different than 10, does not
-include lexing common prefixes such as '0x' or '0o' etc.
-
-\begin{code}
-{-# SPECIALISE readDec ::
- ReadS Int,
- ReadS Integer #-}
-readDec :: (Integral a) => ReadS a
-readDec = readInt 10 isDigit (\d -> ord d - ord '0')
-
-{-# SPECIALISE readOct ::
- ReadS Int,
- ReadS Integer #-}
-readOct :: (Integral a) => ReadS a
-readOct = readInt 8 isOctDigit (\d -> ord d - ord '0')
-
-{-# SPECIALISE readHex ::
- ReadS Int,
- ReadS Integer #-}
-readHex :: (Integral a) => ReadS a
-readHex = readInt 16 isHexDigit hex
- where hex d = ord d - (if isDigit d then ord '0'
- else ord (if isUpper d then 'A' else 'a') - 10)
-
-readInt :: (Integral a) => a -> (Char -> Bool) -> (Char -> Int) -> ReadS a
-readInt radix isDig digToInt s = do
- (ds,r) <- nonnull isDig s
- return (foldl1 (\n d -> n * radix + d)
- (map (fromInteger . toInteger . digToInt) ds), r)
-
-{-# SPECIALISE readSigned ::
- ReadS Int -> ReadS Int,
- ReadS Integer -> ReadS Integer,
- ReadS Double -> ReadS Double #-}
-readSigned :: (Real a) => ReadS a -> ReadS a
-readSigned readPos = readParen False read'
- where read' r = read'' r ++
- (do
- ("-",s) <- lex r
- (x,t) <- read'' s
- return (-x,t))
- read'' r = do
- (str,s) <- lex r
- (n,"") <- readPos str
- return (n,s)
-\end{code}
-
-The functions readFloat below uses rational arithmetic
-to ensure correct conversion between the floating-point radix and
-decimal. It is often possible to use a higher-precision floating-
-point type to obtain the same results.
-
-\begin{code}
-{-# SPECIALISE readFloat ::
- ReadS Double,
- ReadS Float #-}
-readFloat :: (RealFloat a) => ReadS a
-readFloat r =
- (do
- (x,t) <- readRational r
- return (fromRational x,t) ) ++
- (do
- ("NaN",t) <- lex r
- return (0/0,t) ) ++
- (do
- ("Infinity",t) <- lex r
- return (1/0,t) )
-
-readRational :: ReadS Rational -- NB: doesn't handle leading "-"
-readRational r = do
- (n,d,s) <- readFix r
- (k,t) <- readExp s
- return ((n%1)*10^^(k-d), t)
- where
- readFix r = do
- (ds,s) <- lexDecDigits r
- (ds',t) <- lexDotDigits s
- return (read (ds++ds'), length ds', t)
-
- readExp (e:s) | e `elem` "eE" = readExp' s
- readExp s = return (0,s)
-
- readExp' ('+':s) = readDec s
- readExp' ('-':s) = do
- (k,t) <- readDec s
- return (-k,t)
- readExp' s = readDec s
-
- lexDotDigits ('.':s) = lex0Digits s
- lexDotDigits s = return ("",s)
-
-readRational__ :: String -> Rational -- we export this one (non-std)
- -- NB: *does* handle a leading "-"
-readRational__ top_s
- = case top_s of
- '-' : xs -> - (read_me xs)
- xs -> read_me xs
- where
- read_me s
- = case (do { (x,t) <- readRational s ; ("","") <- lex t ; return x }) of
-#ifndef NEW_READS_REP
- [x] -> x
- [] -> error ("readRational__: no parse:" ++ top_s)
- _ -> error ("readRational__: ambiguous parse:" ++ top_s)
-#else
- Just x -> x
- Nothing -> error ("readRational__: no parse:" ++ top_s)
-#endif
-
-\end{code}
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