Haskell Hierarchical Libraries (base package)ContentsIndex
Text.Read
Portabilitynon-portable (uses Text.ParserCombinators.ReadP)
Stabilityprovisional
Maintainerlibraries@haskell.org
Contents
The Read class
Haskell 98 functions
New parsing functions
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 98 Read. In particular, writing parsers is easier, and the parsers are much more efficient.

Synopsis
class Read a where
readsPrec :: Int -> ReadS a
readList :: ReadS [a]
readPrec :: ReadPrec a
readListPrec :: ReadPrec [a]
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
= Char Char
| String String
| Punc String
| Ident String
| Symbol String
| Int Integer
| Rat Rational
| EOF
lexP :: ReadPrec Lexeme
parens :: ReadPrec a -> ReadPrec a
readListDefault :: Read a => ReadS [a]
readListPrecDefault :: Read a => ReadPrec [a]
The Read class
class Read a where

Parsing of Strings, producing values.

Minimal complete definition: readsPrec (or, for GHC only, readPrec)

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 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
Methods
readsPrec
:: Intthe operator precedence of the enclosing context (a number from 0 to 11). Function application has precedence 10.
-> 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.

readList :: 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.
readPrec :: ReadPrec a
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.
show/hide Instances
Read All
Read Any
Read Bool
Read BufferMode
Read CCc
Read CChar
Read CClock
Read CDev
Read CDouble
Read CFloat
Read CGid
Read CIno
Read CInt
Read CLDouble
Read CLLong
Read CLong
Read CMode
Read CNlink
Read COff
Read CPid
Read CPtrdiff
Read CRLim
Read CSChar
Read CShort
Read CSigAtomic
Read CSize
Read CSpeed
Read CSsize
Read CTcflag
Read CTime
Read CUChar
Read CUInt
Read CULLong
Read CULong
Read CUShort
Read CUid
Read CWchar
Read CalendarTime
Read Char
Read Day
Read Double
Read ExitCode
Read Fd
Read Float
Read GeneralCategory
Read IOMode
Read Int
Read Int16
Read Int32
Read Int64
Read Int8
Read IntSet
Read Integer
Read Lexeme
Read Month
Read Ordering
Read Permissions
Read SeekMode
Read StdGen
Read TimeDiff
Read Version
Read Word
Read Word16
Read Word32
Read Word64
Read Word8
Read ()
(Read a, Read b) => Read (a, b)
(Read a, Read b, Read c) => Read (a, b, c)
(Read a, Read b, Read c, Read d) => Read (a, b, c, d)
(Read a, Read b, Read c, Read d, Read e) => Read (a, b, c, d, e)
(Read a, Read b, Read c, Read d, Read e, Read f) => Read (a, b, c, d, e, f)
(Read a, Read b, Read c, Read d, Read e, Read f, Read g) => Read (a, b, c, d, e, f, g)
(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)
(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)
(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)
(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)
(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)
(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)
(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)
(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)
(RealFloat a, Read a) => Read (Complex a)
Read e => Read (IntMap e)
Read a => Read (Maybe a)
Read a => Read (Product a)
(Integral a, Read a) => Read (Ratio a)
Read a => Read (Seq a)
(Read a, Ord a) => Read (Set a)
Read a => Read (Sum a)
Read a => Read (Tree a)
Read a => Read (ViewL a)
Read a => Read (ViewR a)
Read a => Read [a]
(Ix a, Read a, Read b) => Read (Array a b)
(Read a, Read b) => Read (Either a b)
(Ord k, Read k, Read e) => Read (Map k e)
type ReadS a = String -> [(a, String)]

A parser for a type a, represented as a function that takes a String and returns a list of possible parses as (a,String) pairs.

Note that this kind of backtracking parser is very inefficient; reading a large structure may be quite slow (cf ReadP).

Haskell 98 functions
reads :: Read a => ReadS a
equivalent to readsPrec with a precedence of 0.
read :: Read a => String -> a
The read function reads input from a string, which must be completely consumed by the input process.
readParen :: Bool -> ReadS a -> ReadS a

readParen True p parses what p parses, but surrounded with parentheses.

readParen False p parses what p parses, but optionally surrounded with parentheses.

lex :: ReadS String

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
New parsing functions
module Text.ParserCombinators.ReadPrec
data Lexeme
Haskell lexemes.
Constructors
Char CharCharacter literal
String StringString literal, with escapes interpreted
Punc StringPunctuation or reserved symbol, e.g. (, ::
Ident StringHaskell identifier, e.g. foo, Baz
Symbol StringHaskell symbol, e.g. >>, :%
Int IntegerInteger literal
Rat RationalFloating point literal
EOF
show/hide Instances
lexP :: ReadPrec Lexeme
Parse a single lexeme
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).
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