| Copyright | (c) The University of Glasgow 2001 | 
|---|---|
| License | BSD-style (see the file libraries/base/LICENSE) | 
| Maintainer | libraries@haskell.org | 
| Stability | provisional | 
| Portability | non-portable (uses Text.ParserCombinators.ReadP) | 
| Safe Haskell | Trustworthy | 
| Language | Haskell2010 | 
Text.Read
Description
Converting strings to values.
The Text.Read library is the canonical library to import for
 Read-class facilities.  For GHC only, it offers an extended and much
 improved Read class, which constitutes a proposed alternative to the
 Haskell 2010 Read.  In particular, writing parsers is easier, and
 the parsers are much more efficient.
Synopsis
- class Read a where
- type ReadS a = String -> [(a, String)]
- reads :: Read a => ReadS a
- read :: Read a => String -> a
- readParen :: Bool -> ReadS a -> ReadS a
- lex :: ReadS String
- module Text.ParserCombinators.ReadPrec
- data Lexeme
- lexP :: ReadPrec Lexeme
- parens :: ReadPrec a -> ReadPrec a
- readListDefault :: Read a => ReadS [a]
- readListPrecDefault :: Read a => ReadPrec [a]
- readEither :: Read a => String -> Either String a
- readMaybe :: Read a => String -> Maybe a
The Read class
Parsing of Strings, producing values.
Derived instances of Read make the following assumptions, which
 derived instances of Show obey:
- If the constructor is defined to be an infix operator, then the
   derived Readinstance 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 Readwill parse only the record-syntax form, and furthermore, the fields must be given in the same order as the original declaration.
- The derived Readinstance 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 2010 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 = 5Note 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 = readListPrecDefaultWhy do both readsPrec and readPrec exist, and why does GHC opt to
 implement readPrec in derived Read instances instead of readsPrec?
 The reason is that readsPrec is based on the ReadS type, and although
 ReadS is mentioned in the Haskell 2010 Report, it is not a very efficient
 parser data structure.
readPrec, on the other hand, is based on a much more efficient ReadPrec
 datatype (a.k.a "new-style parsers"), but its definition relies on the use
 of the RankNTypes language extension. Therefore, readPrec (and its
 cousin, readListPrec) are marked as GHC-only. Nevertheless, it is
 recommended to use readPrec instead of readsPrec whenever possible
 for the efficiency improvements it brings.
As mentioned above, derived Read instances in GHC will implement
 readPrec instead of readsPrec. The default implementations of
 readsPrec (and its cousin, readList) will simply use readPrec under
 the hood. If you are writing a Read instance by hand, it is recommended
 to write it like so:
instanceReadT wherereadPrec= ...readListPrec=readListPrecDefault
Methods
Arguments
| :: Int | the operator precedence of the enclosing
 context (a number from  | 
| -> 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 Show satisfy the following:
That is, readsPrec parses the string produced by
 showsPrec, and delivers the value that
 showsPrec started with.
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.
Proposed replacement for readsPrec using new-style parsers (GHC only).
readListPrec :: 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.
Instances
| Read All # | Since: base-2.1 | 
| Read Any # | Since: base-2.1 | 
| Read Version # | Since: base-2.1 | 
| Read Void # | Reading a  Since: base-4.8.0.0 | 
| Read CBool # | |
| Read CChar # | |
| Read CClock # | |
| Read CDouble # | |
| Read CFloat # | |
| Read CInt # | |
| Read CIntMax # | |
| Read CIntPtr # | |
| Read CLLong # | |
| Read CLong # | |
| Read CPtrdiff # | |
| Read CSChar # | |
| Read CSUSeconds # | |
| Defined in Foreign.C.Types Methods readsPrec :: Int -> ReadS CSUSeconds # readList :: ReadS [CSUSeconds] # readPrec :: ReadPrec CSUSeconds # readListPrec :: ReadPrec [CSUSeconds] # | |
| Read CShort # | |
| Read CSigAtomic # | |
| Defined in Foreign.C.Types Methods readsPrec :: Int -> ReadS CSigAtomic # readList :: ReadS [CSigAtomic] # readPrec :: ReadPrec CSigAtomic # readListPrec :: ReadPrec [CSigAtomic] # | |
| Read CSize # | |
| Read CTime # | |
| Read CUChar # | |
| Read CUInt # | |
| Read CUIntMax # | |
| Read CUIntPtr # | |
| Read CULLong # | |
| Read CULong # | |
| Read CUSeconds # | |
| Read CUShort # | |
| Read CWchar # | |
| Read IntPtr # | |
| Read WordPtr # | |
| Read ByteOrder # | Since: base-4.11.0.0 | 
| Read Associativity # | Since: base-4.6.0.0 | 
| Defined in GHC.Generics Methods readsPrec :: Int -> ReadS Associativity # readList :: ReadS [Associativity] # | |
| Read DecidedStrictness # | Since: base-4.9.0.0 | 
| Defined in GHC.Generics Methods readsPrec :: Int -> ReadS DecidedStrictness # readList :: ReadS [DecidedStrictness] # | |
| Read Fixity # | Since: base-4.6.0.0 | 
| Read SourceStrictness # | Since: base-4.9.0.0 | 
| Defined in GHC.Generics Methods readsPrec :: Int -> ReadS SourceStrictness # readList :: ReadS [SourceStrictness] # | |
| Read SourceUnpackedness # | Since: base-4.9.0.0 | 
| Defined in GHC.Generics Methods readsPrec :: Int -> ReadS SourceUnpackedness # readList :: ReadS [SourceUnpackedness] # | |
| Read SeekMode # | Since: base-4.2.0.0 | 
| Read ExitCode # | |
| Read BufferMode # | Since: base-4.2.0.0 | 
| Defined in GHC.IO.Handle.Types Methods readsPrec :: Int -> ReadS BufferMode # readList :: ReadS [BufferMode] # readPrec :: ReadPrec BufferMode # readListPrec :: ReadPrec [BufferMode] # | |
| Read Newline # | Since: base-4.3.0.0 | 
| Read NewlineMode # | Since: base-4.3.0.0 | 
| Defined in GHC.IO.Handle.Types Methods readsPrec :: Int -> ReadS NewlineMode # readList :: ReadS [NewlineMode] # readPrec :: ReadPrec NewlineMode # readListPrec :: ReadPrec [NewlineMode] # | |
| Read IOMode # | Since: base-4.2.0.0 | 
| Read Int16 # | Since: base-2.1 | 
| Read Int32 # | Since: base-2.1 | 
| Read Int64 # | Since: base-2.1 | 
| Read Int8 # | Since: base-2.1 | 
| Read GCDetails # | Since: base-4.10.0.0 | 
| Read RTSStats # | Since: base-4.10.0.0 | 
| Read SomeSymbol # | Since: base-4.7.0.0 | 
| Defined in GHC.TypeLits Methods readsPrec :: Int -> ReadS SomeSymbol # readList :: ReadS [SomeSymbol] # readPrec :: ReadPrec SomeSymbol # readListPrec :: ReadPrec [SomeSymbol] # | |
| Read SomeNat # | Since: base-4.7.0.0 | 
| Read GeneralCategory # | Since: base-2.1 | 
| Defined in GHC.Read Methods readsPrec :: Int -> ReadS GeneralCategory # readList :: ReadS [GeneralCategory] # | |
| Read Word16 # | Since: base-2.1 | 
| Read Word32 # | Since: base-2.1 | 
| Read Word64 # | Since: base-2.1 | 
| Read CBlkCnt # | |
| Read CBlkSize # | |
| Read CCc # | |
| Read CClockId # | |
| Read CDev # | |
| Read CFsBlkCnt # | |
| Read CFsFilCnt # | |
| Read CGid # | |
| Read CId # | |
| Read CIno # | |
| Read CKey # | |
| Read CMode # | |
| Read CNfds # | |
| Read CNlink # | |
| Read COff # | |
| Read CPid # | |
| Read CRLim # | |
| Read CSocklen # | |
| Read CSpeed # | |
| Read CSsize # | |
| Read CTcflag # | |
| Read CUid # | |
| Read Fd # | |
| Read Lexeme # | Since: base-2.1 | 
| Read Ordering # | Since: base-2.1 | 
| Read Word8 # | Since: base-2.1 | 
| Read Integer # | Since: base-2.1 | 
| Read Natural # | Since: base-4.8.0.0 | 
| Read () # | Since: base-2.1 | 
| Read Bool # | Since: base-2.1 | 
| Read Char # | Since: base-2.1 | 
| Read Double # | Since: base-2.1 | 
| Read Float # | Since: base-2.1 | 
| Read Int # | Since: base-2.1 | 
| Read Word # | Since: base-4.5.0.0 | 
| Read a => Read (ZipList a) # | Since: base-4.7.0.0 | 
| Read a => Read (Complex a) # | Since: base-2.1 | 
| Read a => Read (Identity a) # | This instance would be equivalent to the derived instances of the
  Since: base-4.8.0.0 | 
| Read a => Read (First a) # | Since: base-2.1 | 
| Read a => Read (Last a) # | Since: base-2.1 | 
| Read a => Read (Down a) # | This instance would be equivalent to the derived instances of the
  Since: base-4.7.0.0 | 
| Read a => Read (First a) # | Since: base-4.9.0.0 | 
| Read a => Read (Last a) # | Since: base-4.9.0.0 | 
| Read a => Read (Max a) # | Since: base-4.9.0.0 | 
| Read a => Read (Min a) # | Since: base-4.9.0.0 | 
| Read a => Read (Option a) # | Since: base-4.9.0.0 | 
| Read m => Read (WrappedMonoid m) # | Since: base-4.9.0.0 | 
| Defined in Data.Semigroup Methods readsPrec :: Int -> ReadS (WrappedMonoid m) # readList :: ReadS [WrappedMonoid m] # readPrec :: ReadPrec (WrappedMonoid m) # readListPrec :: ReadPrec [WrappedMonoid m] # | |
| Read a => Read (Dual a) # | Since: base-2.1 | 
| Read a => Read (Product a) # | Since: base-2.1 | 
| Read a => Read (Sum a) # | Since: base-2.1 | 
| Read a => Read (NonEmpty a) # | Since: base-4.11.0.0 | 
| Read p => Read (Par1 p) # | Since: base-4.7.0.0 | 
| (Integral a, Read a) => Read (Ratio a) # | Since: base-2.1 | 
| Read a => Read (Maybe a) # | Since: base-2.1 | 
| Read a => Read (a) # | Since: base-4.15 | 
| Read a => Read [a] # | Since: base-2.1 | 
| (Read a, Read b) => Read (Either a b) # | Since: base-3.0 | 
| HasResolution a => Read (Fixed a) # | Since: base-4.3.0.0 | 
| Read (Proxy t) # | Since: base-4.7.0.0 | 
| (Read a, Read b) => Read (Arg a b) # | Since: base-4.9.0.0 | 
| (Ix a, Read a, Read b) => Read (Array a b) # | Since: base-2.1 | 
| Read (U1 p) # | Since: base-4.9.0.0 | 
| Read (V1 p) # | Since: base-4.9.0.0 | 
| (Read a, Read b) => Read (a, b) # | Since: base-2.1 | 
| Read a => Read (Const a b) # | This instance would be equivalent to the derived instances of the
  Since: base-4.8.0.0 | 
| Read (f a) => Read (Ap f a) # | Since: base-4.12.0.0 | 
| Read (f a) => Read (Alt f a) # | Since: base-4.8.0.0 | 
| Coercible a b => Read (Coercion a b) # | Since: base-4.7.0.0 | 
| a ~ b => Read (a :~: b) # | Since: base-4.7.0.0 | 
| Read (f p) => Read (Rec1 f p) # | Since: base-4.7.0.0 | 
| (Read a, Read b, Read c) => Read (a, b, c) # | Since: base-2.1 | 
| (Read1 f, Read1 g, Read a) => Read (Product f g a) # | Since: base-4.9.0.0 | 
| (Read1 f, Read1 g, Read a) => Read (Sum f g a) # | Since: base-4.9.0.0 | 
| a ~~ b => Read (a :~~: b) # | Since: base-4.10.0.0 | 
| (Read (f p), Read (g p)) => Read ((f :*: g) p) # | Since: base-4.7.0.0 | 
| (Read (f p), Read (g p)) => Read ((f :+: g) p) # | Since: base-4.7.0.0 | 
| Read c => Read (K1 i c p) # | Since: base-4.7.0.0 | 
| (Read a, Read b, Read c, Read d) => Read (a, b, c, d) # | Since: base-2.1 | 
| (Read1 f, Read1 g, Read a) => Read (Compose f g a) # | Since: base-4.9.0.0 | 
| Read (f (g p)) => Read ((f :.: g) p) # | Since: base-4.7.0.0 | 
| Read (f p) => Read (M1 i c f p) # | Since: base-4.7.0.0 | 
| (Read a, Read b, Read c, Read d, Read e) => Read (a, b, c, d, e) # | Since: base-2.1 | 
| (Read a, Read b, Read c, Read d, Read e, Read f) => Read (a, b, c, d, e, f) # | Since: base-2.1 | 
| (Read a, Read b, Read c, Read d, Read e, Read f, Read g) => Read (a, b, c, d, e, f, g) # | Since: base-2.1 | 
| (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) # | Since: base-2.1 | 
| (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) # | Since: base-2.1 | 
| (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) # | Since: base-2.1 | 
| (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) # | Since: base-2.1 | 
| (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) # | Since: base-2.1 | 
| (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) # | Since: base-2.1 | 
| (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) # | Since: base-2.1 | 
| (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) # | Since: base-2.1 | 
| Defined in GHC.Read | |
Haskell 2010 functions
read :: Read a => String -> a #
The read function reads input from a string, which must be
 completely consumed by the input process. read fails with an error if the
 parse is unsuccessful, and it is therefore discouraged from being used in
 real applications. Use readMaybe or readEither for safe alternatives.
>>>read "123" :: Int123
>>>read "hello" :: Int*** Exception: Prelude.read: no parse
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 "" = [("","")]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
New parsing functions
Constructors
| Char Char | Character literal | 
| String String | String literal, with escapes interpreted | 
| Punc String | Punctuation or reserved symbol, e.g.  | 
| Ident String | Haskell identifier, e.g.  | 
| Symbol String | Haskell symbol, e.g.  | 
| Number Number | Since: base-4.6.0.0 | 
| EOF | 
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
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.
readListPrecDefault :: Read a => ReadPrec [a] #
A possible replacement definition for the readListPrec method,
   defined using readPrec (GHC only).