|Portability||non-portable (uses Text.ParserCombinators.ReadP)|
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.
|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
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.
|:: 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]|
|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).
|(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)|
|(RealFloat a, Read a) => Read (Complex a)|
|Read a => Read (Maybe a)|
|(Integral a, Read a) => Read (Ratio a)|
|Read a => Read (Tree a)|
|Read a => Read [a]|
|(Ix a, Read a, Read b) => Read (Array a b)|
|(Read a, Read b) => Read (Either a b)|
|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
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
|data Lexeme |
|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. >>, :%
|Int Integer||Integer literal
|Rat Rational||Floating point literal
|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).
|Produced by Haddock version 0.7|