| Copyright | (c) The University of Glasgow 1994-2002 |
|---|---|
| License | see libraries/base/LICENSE |
| Maintainer | cvs-ghc@haskell.org |
| Stability | internal |
| Portability | non-portable (GHC Extensions) |
| Safe Haskell | Trustworthy |
| Language | Haskell2010 |
GHC.Read
Description
The Read class and instances for basic data types.
Synopsis
- class Read a where
- type ReadS a = String -> [(a, String)]
- lex :: ReadS String
- lexLitChar :: ReadS String
- readLitChar :: ReadS Char
- lexDigits :: ReadS String
- lexP :: ReadPrec Lexeme
- expectP :: Lexeme -> ReadPrec ()
- 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]
- readNumber :: Num a => (Lexeme -> ReadPrec a) -> ReadPrec a
- readField :: String -> ReadPrec a -> ReadPrec a
- readFieldHash :: String -> ReadPrec a -> ReadPrec a
- readSymField :: String -> ReadPrec a -> ReadPrec a
- readParen :: Bool -> ReadS a -> ReadS a
Documentation
Parsing of String s, producing values.
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
Readinstance 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
Readwill parse only the record-syntax form, and furthermore, the fields must be given in the same order as the original declaration. - The derived
Readinstance 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 2010 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 = 5Note 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
Why do both readsPrec and readPrec exist, and why does GHC opt to
implement readPrec in derived Read instances instead of readsPrec ?
The reason is that readsPrec is based on the ReadS type, and although
ReadS is mentioned in the Haskell 2010 Report, it is not a very efficient
parser data structure.
readPrec , on the other hand, is based on a much more efficient ReadPrec
datatype (a.k.a "new-style parsers"), but its definition relies on the use
of the RankNTypes language extension. Therefore, readPrec (and its
cousin, readListPrec ) are marked as GHC-only. Nevertheless, it is
recommended to use readPrec instead of readsPrec whenever possible
for the efficiency improvements it brings.
As mentioned above, derived Read instances in GHC will implement
readPrec instead of readsPrec . The default implementations of
readsPrec (and its cousin, readList ) will simply use readPrec under
the hood. If you are writing a Read instance by hand, it is recommended
to write it like so:
instanceReadT wherereadPrec= ...readListPrec=readListPrecDefault
Methods
Arguments
the operator precedence of the enclosing
context (a number from 0 to 11).
Function application has precedence 10.
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] 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 a Source #
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
Instances details
Instance details
Defined in Foreign.C.Types
Instance details
Defined in Foreign.C.Types
Instance details
Defined in GHC.TypeLits
Instance details
Defined in GHC.Generics
Instance details
Defined in GHC.Generics
Instance details
Defined in GHC.Generics
Instance details
Defined in GHC.Generics
Instance details
Defined in GHC.IO.Handle.Types
Instance details
Defined in GHC.IO.Handle.Types
Reading a Void value is always a parse error, considering
Void as a data type with no constructors.
Since: base-4.8.0.0
This instance would be equivalent to the derived instances of the
Down newtype if the getDown field were removed
Since: base-4.7.0.0
This instance would be equivalent to the derived instances of the
Identity newtype if the runIdentity field were removed
Since: base-4.8.0.0
Instance details
Defined in Data.Semigroup
Methods
readsPrec :: Int -> ReadS (WrappedMonoid m) Source #
readList :: ReadS [WrappedMonoid m] Source #
readPrec :: ReadPrec (WrappedMonoid m) Source #
readListPrec :: ReadPrec [WrappedMonoid m] Source #
This instance would be equivalent to the derived instances of the
Const newtype if the getConst field were removed
Since: base-4.8.0.0
Since: base-2.1
Since: base-2.1
Since: base-2.1
Since: base-2.1
Since: base-2.1
Instance details
Defined in GHC.Read
Since: base-2.1
Instance details
Defined in GHC.Read
Since: base-2.1
Instance details
Defined in GHC.Read
Since: base-2.1
Instance details
Defined in GHC.Read
Methods
readsPrec :: Int -> ReadS (a, b, c, d, e, f, g, h, i, j, k, l, m, n) Source #
readList :: ReadS [(a, b, c, d, e, f, g, h, i, j, k, l, m, n)] Source #
readPrec :: ReadPrec (a, b, c, d, e, f, g, h, i, j, k, l, m, n) Source #
readListPrec :: ReadPrec [(a, b, c, d, e, f, g, h, i, j, k, l, m, n)] Source #
Since: base-2.1
Instance details
Defined in GHC.Read
Methods
readsPrec :: Int -> ReadS (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o) Source #
readList :: ReadS [(a, b, c, d, e, f, g, h, i, j, k, l, m, n, o)] Source #
readPrec :: ReadPrec (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o) Source #
readListPrec :: ReadPrec [(a, b, c, d, e, f, g, h, i, j, k, l, m, n, o)] 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 "" = [("","")]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 String Source #
Read a string representation of a character, using Haskell source-language escape conventions. For example:
lexLitChar "\\nHello" = [("\\n", "Hello")]readLitChar :: ReadS Char Source #
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")]paren :: ReadPrec a -> ReadPrec a Source #
(paren p) parses "(P0)"
where p parses "P0" in precedence context zero
parens :: ReadPrec a -> ReadPrec a Source #
(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 a Source #
Parse the specified lexeme and continue as specified.
Esp useful for nullary constructors; e.g.
choose [("A", return A), ("B", return B)]
We match both Ident and Symbol because the constructor
might be an operator eg (:~:)
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).
readField :: String -> ReadPrec a -> ReadPrec a Source #
Read parser for a record field, of the form fieldName=value. The
fieldName must be an alphanumeric identifier; for symbols (operator-style)
field names, e.g. (#), use readSymField ). The second argument is a
parser for the field value.