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Showing content from http://hackage.haskell.org/packages/archive/base/4.6.0.0/doc/html/Text-Read.html below:

Text.Read

class Read a whereSource

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 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:

(x,"") is an element of (readsPrec d (showsPrec d x "")).

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 aSource

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 IOMode Read SeekMode 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 GeneralCategory Read Associativity Read Fixity Read Arity Read IntPtr Read WordPtr Read Any Read All 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 GCStats Read Version Read a => Read [a] (Integral a, Read a) => Read (Ratio a) Read a => Read (Maybe 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 (Complex a) HasResolution a => Read (Fixed a) (Read a, Read b) => Read (Either a b) (Read a, Read b) => Read (a, b) (Ix a, Read a, Read b) => Read (Array a b) (SingRep k a rep, Read rep, Eq rep) => Read (Sing k a) (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)

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).

read :: Read a => String -> aSource

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

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

parens :: ReadPrec a -> ReadPrec aSource

(parens p) parses "P", "(P0)", "((P0))", etc, where p parses "P" in the current precedence context and parses "P0" in precedence context zero

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