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/Haskell 7

Text.Read

Copyright (c) The University of Glasgow 2001
License BSD-style (see the file libraries/base/LICENSE)
Maintainer [email protected]
Stability provisional
Portability non-portable (uses Text.ParserCombinators.ReadP)
Safe Haskell Trustworthy
Language Haskell2010

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.

The Read class

class Read a where Source

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 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 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 = 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

Minimal complete definition

readsPrec | readPrec

Methods

readsPrec Source

Arguments

:: Int

the 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] Source

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 Source

Proposed replacement for readsPrec using new-style parsers (GHC only).

readListPrec :: ReadPrec [a] Source

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 Bool
Read Char
Read Double
Read Float
Read Int
Read Int8
Read Int16
Read Int32
Read Int64
Read Integer
Read Ordering
Read Word
Read Word8
Read Word16
Read Word32
Read Word64
Read ()
Read Lexeme
Read GeneralCategory
Read Associativity
Read Fixity
Read Arity
Read Any
Read All
Read CUIntMax
Read CIntMax
Read CUIntPtr
Read CIntPtr
Read CSUSeconds
Read CUSeconds
Read CTime
Read CClock
Read CSigAtomic
Read CWchar
Read CSize
Read CPtrdiff
Read CDouble
Read CFloat
Read CULLong
Read CLLong
Read CULong
Read CLong
Read CUInt
Read CInt
Read CUShort
Read CShort
Read CUChar
Read CSChar
Read CChar
Read IntPtr
Read WordPtr
Read SeekMode
Read NewlineMode
Read Newline
Read BufferMode
Read ExitCode
Read Fd
Read CRLim
Read CTcflag
Read CSpeed
Read CCc
Read CUid
Read CNlink
Read CGid
Read CSsize
Read CPid
Read COff
Read CMode
Read CIno
Read CDev
Read IOMode
Read GCStats
Read Version
Read Natural
Read SomeSymbol
Read SomeNat
Read Void

Reading a Void value is always a parse error, considering Void as a data type with no constructors.

Read a => Read [a]
(Integral a, Read a) => Read (Ratio a)
Read (U1 p)
Read p => Read (Par1 p)
Read a => Read (Maybe a)
Read a => Read (Down a)
Read a => Read (Last a)
Read a => Read (First a)
Read a => Read (Product a)
Read a => Read (Sum a)
Read a => Read (Dual a)
Read a => Read (ZipList a)
Read a => Read (Complex a)
HasResolution a => Read (Fixed a)
Read a => Read (Identity a)

This instance would be equivalent to the derived instances of the Identity newtype if the runIdentity field were removed

(Read a, Read b) => Read (Either a b)
Read (f p) => Read (Rec1 f p)
(Read a, Read b) => Read (a, b)
Read (Proxy k s)
Read a => Read (Const a b)
Read c => Read (K1 i c p)
(Read (f p), Read (g p)) => Read ((:+:) f g p)
(Read (f p), Read (g p)) => Read ((:*:) f g p)
Read (f (g p)) => Read ((:.:) f g p)
(Read a, Read b, Read c) => Read (a, b, c)
(~) k a b => Read ((:~:) k a b)
Coercible k a b => Read (Coercion k a b)
Read (f a) => Read (Alt k f a)
Read (f p) => Read (M1 i c f p)
(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)

type ReadS a = String -> [(a, String)] Source

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 2010 functions

reads :: Read a => ReadS a Source

equivalent to readsPrec with a precedence of 0.

read :: Read a => String -> a Source

The read function reads input from a string, which must be completely consumed by the input process.

readParen :: Bool -> ReadS a -> ReadS a Source

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 Source

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 Source

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. foo, Baz

Symbol String

Haskell symbol, e.g. >>, :%

Number Number

Since: 4.6.0.0

EOF

lexP :: ReadPrec Lexeme Source

Parse a single lexeme

parens :: ReadPrec a -> ReadPrec a Source

(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] Source

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] Source

A possible replacement definition for the readListPrec method, defined using readPrec (GHC only).

readEither :: Read a => String -> Either String a Source

Parse a string using the Read instance. Succeeds if there is exactly one valid result. A Left value indicates a parse error.

Since: 4.6.0.0

readMaybe :: Read a => String -> Maybe a Source

Parse a string using the Read instance. Succeeds if there is exactly one valid result.

Since: 4.6.0.0

© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/7.10.3/docs/html/libraries/base-4.8.2.0/Text-Read.html