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GHC.Read | Portability | non-portable (GHC Extensions) | Stability | internal | Maintainer | cvs-ghc@haskell.org |
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Description |
The Read class and instances for basic data types.
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Synopsis |
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Documentation |
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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 | | :: 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 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.
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| | | 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.
| | | Proposed replacement for readsPrec using new-style parsers (GHC only).
| | | 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.
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| | Instances | |
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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).
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equivalent to readsPrec with a precedence of 0.
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The read function reads input from a string, which must be
completely consumed by the input process.
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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
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Read a string representation of a character, using Haskell
source-language escape conventions. For example:
lexLitChar "\\nHello" = [("\\n", "Hello")]
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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")]
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Reads a non-empty string of decimal digits.
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Parse a single lexeme
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(paren p) parses "(P0)"
where p parses "P0" in precedence context zero
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(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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(list p) parses a list of things parsed by p,
using the usual square-bracket syntax.
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Parse the specified lexeme and continue as specified.
Esp useful for nullary constructors; e.g.
choose [("A", return A), ("B", return B)]
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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.
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A possible replacement definition for the readListPrec method,
defined using readPrec (GHC only).
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readParen True p parses what p parses, but surrounded with
parentheses.
readParen False p parses what p parses, but optionally
surrounded with parentheses.
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