base-3.0.1.0: Basic librariesSource codeContentsIndex
GHC.Read
Portabilitynon-portable (GHC Extensions)
Stabilityinternal
Maintainercvs-ghc@haskell.org
Description
The Read class and instances for basic data types.
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
readp :: Read a => ReadP a
readEither :: Read a => String -> Either String a
read :: Read a => String -> a
lex :: ReadS String
lexLitChar :: ReadS String
readLitChar :: ReadS Char
lexDigits :: ReadS String
lexP :: ReadPrec Lexeme
paren :: ReadPrec a -> ReadPrec a
parens :: ReadPrec a -> ReadPrec a
list :: ReadPrec a -> ReadPrec [a]
choose :: [(String, ReadPrec a)] -> ReadPrec a
readListDefault :: Read a => ReadS [a]
readListPrecDefault :: Read a => ReadPrec [a]
readParen :: Bool -> ReadS a -> ReadS a
Documentation
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 Text.Show.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
readsPrecSource
:: 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 Text.Show.Show satisfy the following:

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

That is, readsPrec parses the string produced by Text.Show.showsPrec, and delivers the value that Text.Show.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.
show/hide Instances
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).

reads :: Read a => ReadS aSource
equivalent to readsPrec with a precedence of 0.
readp :: Read a => ReadP aSource
readEither :: Read a => String -> Either String aSource
read :: Read a => String -> aSource
The read function reads input from a string, which must be completely consumed by the input process.
lex :: ReadS StringSource

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
lexLitChar :: ReadS StringSource

Read a string representation of a character, using Haskell source-language escape conventions. For example:

 lexLitChar  "\\nHello"  =  [("\\n", "Hello")]
readLitChar :: ReadS CharSource

Read a string representation of a character, using Haskell source-language escape conventions, and convert it to the character that it encodes. For example:

 readLitChar "\\nHello"  =  [('\n', "Hello")]
lexDigits :: ReadS StringSource
Reads a non-empty string of decimal digits.
lexP :: ReadPrec LexemeSource
Parse a single lexeme
paren :: ReadPrec a -> ReadPrec aSource
(paren p) parses "(P0)" where p parses "P0" in precedence context zero
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
list :: ReadPrec a -> ReadPrec [a]Source
(list p) parses a list of things parsed by p, using the usual square-bracket syntax.
choose :: [(String, ReadPrec a)] -> ReadPrec aSource
Parse the specified lexeme and continue as specified. Esp useful for nullary constructors; e.g. choose [("A", return A), ("B", return B)]
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).
readParen :: Bool -> ReadS a -> ReadS aSource

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

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

Produced by Haddock version 2.0.0.0