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
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 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.

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).
show/hide Instances
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
readListDefault :: Read a => ReadS [a]
Use this to define the readList method, if you don't want a special case (GHC only; for other systems the default suffices).
readListPrecDefault :: Read a => ReadPrec [a]
Use this to define the readListPrec method, if you don't want a special case (GHC only).
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