\begin{code}
-{-# OPTIONS -fno-implicit-prelude #-}
+{-# OPTIONS_GHC -fno-implicit-prelude #-}
-----------------------------------------------------------------------------
-- |
-- Module : GHC.Read
--- Copyright : (c) The FFI Task Force, 1994-2002
+-- Copyright : (c) The University of Glasgow, 1994-2002
-- License : see libraries/base/LICENSE
--
-- Maintainer : cvs-ghc@haskell.org
--
-----------------------------------------------------------------------------
+-- #hide
module GHC.Read
( Read(..) -- class
, readListDefault, readListPrecDefault
-- Temporary
- , readList__
, readParen
)
where
import Text.ParserCombinators.ReadP
( ReadP
+ , ReadS
, readP_to_S
- , readS_to_P
)
import qualified Text.Read.Lex as L
import Data.Either
import {-# SOURCE #-} GHC.Err ( error )
+#ifndef __HADDOCK__
+import {-# SOURCE #-} GHC.Unicode ( isDigit )
+#endif
import GHC.Num
import GHC.Real
import GHC.Float
import GHC.List
-import GHC.Show -- isAlpha etc
+import GHC.Show
import GHC.Base
import GHC.Arr
-
-ratioPrec = 7 -- Precedence of ':%' constructor
-appPrec = 10 -- Precedence of applictaion
\end{code}
--------------------------------------------------------
- TEMPORARY UNTIL I DO DERIVED READ
+
\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
(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{The @Read@ class and @ReadS@ type}
+\subsection{The @Read@ class}
%* *
%*********************************************************
\begin{code}
------------------------------------------------------------------------
--- ReadS
-
--- | A parser for a type @a@, represented as a function that takes a
--- 'String' and returns a list of possible parses @(a,'String')@ pairs.
-type ReadS a = String -> [(a,String)]
-
-------------------------------------------------------------------------
-- 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' is equivalent to
+--
+-- > instance (Read a) => Read (Tree a) where
+-- >
+-- > readsPrec d 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
+-- >
+-- > ++ readParen (d > app_prec)
+-- > (\r -> [(Leaf m,t) |
+-- > ("Leaf",s) <- lex r,
+-- > (m,t) <- readsPrec (app_prec+1) s]) r
+-- >
+-- > where up_prec = 5
+-- > app_prec = 10
+--
+-- Note that right-associativity of @:^:@ is unused.
+
class Read a where
- readsPrec :: Int -> ReadS a
+ -- | 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).
readListPrec :: ReadPrec [a]
-- default definitions
readListPrec = readS_to_Prec (\_ -> readList)
readListDefault :: Read a => ReadS [a]
--- ^ Use this to define the 'readList' method, if you
--- don't want a special case
+-- ^ Use this to define the 'readList' method, if you don't want a special
+-- case (GHC only; for other systems the default suffices).
readListDefault = readPrec_to_S readListPrec 0
readListPrecDefault :: Read a => ReadPrec [a]
-- ^ Use this to define the 'readListPrec' method, if you
--- don't want a special case
+-- don't want a special case (GHC only).
readListPrecDefault = list readPrec
------------------------------------------------------------------------
-- utility functions
+-- | equivalent to 'readsPrec' with a precedence of 0.
reads :: Read a => ReadS a
reads = readsPrec minPrec
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 { P.skipSpaces ;
- (s, L.Char _) <- P.gather L.lex ;
+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 (do { L.Char c <- L.lex ;
- return c })
+readLitChar = readP_to_S L.lexChar
+-- | Reads a non-empty string of decimal digits.
lexDigits :: ReadS String
lexDigits = readP_to_S (P.munch1 isDigit)
lexP = lift L.lex
paren :: ReadPrec a -> ReadPrec a
--- ^ @(paren p)@ parses "(P0)"
--- where @p@ parses "P0" in precedence context zero
+-- ^ @(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
--- parses "P0" in precedence context zero
+-- ^ @(parens p)@ parses \"P\", \"(P0)\", \"((P0))\", etc,
+-- where @p@ parses \"P\" in the current precedence context
+-- parses \"P0\" in precedence context zero
parens p = optional
where
optional = p +++ mandatory
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 [(\"A\", return A), (\"B\", return B)]@
choose sps = foldr ((+++) . try_one) pfail sps
where
try_one (s,p) = do { L.Ident s' <- lexP ;
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 (maybe
-it should be called 'appPrec' instead). 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.
+'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
instance Read a => Read (Maybe a) where
readPrec =
parens
- ( prec appPrec
- ( do L.Ident "Nothing" <- lexP
- return Nothing
- +++
- do L.Ident "Just" <- lexP
- x <- step readPrec
- return (Just x)
- )
+ (do L.Ident "Nothing" <- lexP
+ return Nothing
+ +++
+ prec appPrec (
+ do L.Ident "Just" <- lexP
+ x <- step readPrec
+ return (Just x))
)
readListPrec = readListPrecDefault
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