| Copyright | (c) The University of Glasgow 2001 |
|---|---|
| License | BSD-style (see the file libraries/base/LICENSE) |
| Maintainer | libraries@haskell.org |
| Stability | provisional |
| Portability | non-portable (uses Text.ParserCombinators.ReadP) |
| Safe Haskell | Trustworthy |
| Language | Haskell2010 |
Text.Read
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 2010 Read. In particular, writing parsers is easier, and
the parsers are much more efficient.
Synopsis
- class Read a where
- 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
- lexP :: ReadPrec Lexeme
- parens :: ReadPrec a -> ReadPrec a
- readListDefault :: Read a => ReadS [a]
- readListPrecDefault :: Read a => ReadPrec [a]
- readEither :: Read a => String -> Either String a
- readMaybe :: Read a => String -> Maybe a
The Read class
Parsing of Strings, 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 = readListPrecDefaultWhy 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
| :: Int | the operator precedence of the enclosing
context (a number from |
| -> 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] 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
| Read Bool Source # | Since: base-2.1 |
| Read Char Source # | Since: base-2.1 |
| Read Double Source # | Since: base-2.1 |
| Read Float Source # | Since: base-2.1 |
| Read Int Source # | Since: base-2.1 |
| Read Int8 Source # | Since: base-2.1 |
| Read Int16 Source # | Since: base-2.1 |
| Read Int32 Source # | Since: base-2.1 |
| Read Int64 Source # | Since: base-2.1 |
| Read Integer Source # | Since: base-2.1 |
| Read Natural Source # | Since: base-4.8.0.0 |
| Read Ordering Source # | Since: base-2.1 |
| Read Word Source # | Since: base-4.5.0.0 |
| Read Word8 Source # | Since: base-2.1 |
| Read Word16 Source # | Since: base-2.1 |
| Read Word32 Source # | Since: base-2.1 |
| Read Word64 Source # | Since: base-2.1 |
| Read () Source # | Since: base-2.1 |
| Read GeneralCategory Source # | |
| Read Lexeme Source # | Since: base-2.1 |
| Read IOMode Source # | |
| Read IntPtr Source # | |
| Read WordPtr Source # | |
| Read CUIntMax Source # | |
| Read CIntMax Source # | |
| Read CUIntPtr Source # | |
| Read CIntPtr Source # | |
| Read CSUSeconds Source # | |
Defined in Foreign.C.Types | |
| Read CUSeconds Source # | |
| Read CTime Source # | |
| Read CClock Source # | |
| Read CSigAtomic Source # | |
Defined in Foreign.C.Types | |
| Read CWchar Source # | |
| Read CSize Source # | |
| Read CPtrdiff Source # | |
| Read CDouble Source # | |
| Read CFloat Source # | |
| Read CBool Source # | |
| Read CULLong Source # | |
| Read CLLong Source # | |
| Read CULong Source # | |
| Read CLong Source # | |
| Read CUInt Source # | |
| Read CInt Source # | |
| Read CUShort Source # | |
| Read CShort Source # | |
| Read CUChar Source # | |
| Read CSChar Source # | |
| Read CChar Source # | |
| Read SomeNat Source # | Since: base-4.7.0.0 |
| Read SomeSymbol Source # | Since: base-4.7.0.0 |
Defined in GHC.TypeLits | |
| Read DecidedStrictness Source # | |
Defined in GHC.Generics | |
| Read SourceStrictness Source # | |
Defined in GHC.Generics | |
| Read SourceUnpackedness Source # | |
Defined in GHC.Generics | |
| Read Associativity Source # | |
Defined in GHC.Generics | |
| Read Fixity Source # | |
| Read Any Source # | |
| Read All Source # | |
| Read SeekMode Source # | |
| Read NewlineMode Source # | |
Defined in GHC.IO.Handle.Types | |
| Read Newline Source # | |
| Read BufferMode Source # | |
Defined in GHC.IO.Handle.Types | |
| Read ExitCode Source # | |
| Read Fd Source # | |
| Read CKey Source # | |
| Read CId Source # | |
| Read CFsFilCnt Source # | |
| Read CFsBlkCnt Source # | |
| Read CClockId Source # | |
| Read CBlkCnt Source # | |
| Read CBlkSize Source # | |
| Read CRLim Source # | |
| Read CTcflag Source # | |
| Read CSpeed Source # | |
| Read CCc Source # | |
| Read CUid Source # | |
| Read CNlink Source # | |
| Read CGid Source # | |
| Read CSsize Source # | |
| Read CPid Source # | |
| Read COff Source # | |
| Read CMode Source # | |
| Read CIno Source # | |
| Read CDev Source # | |
| Read Version Source # | |
| Read ByteOrder Source # | |
| Read GCDetails Source # | |
| Read RTSStats Source # | |
| Read Void Source # | Reading a Since: base-4.8.0.0 |
| Read a => Read [a] Source # | Since: base-2.1 |
| Read a => Read (Maybe a) Source # | Since: base-2.1 |
| (Integral a, Read a) => Read (Ratio a) Source # | Since: base-2.1 |
| Read p => Read (Par1 p) Source # | |
| Read a => Read (NonEmpty a) Source # | |
| Read a => Read (Down a) Source # | Since: base-4.7.0.0 |
| Read a => Read (Product a) Source # | |
| Read a => Read (Sum a) Source # | |
| Read a => Read (Dual a) Source # | |
| Read a => Read (Last a) Source # | |
| Read a => Read (First a) Source # | |
| Read a => Read (Identity a) Source # | This instance would be equivalent to the derived instances of the
Since: base-4.8.0.0 |
| Read a => Read (ZipList a) Source # | |
| Read a => Read (Option a) Source # | |
| Read m => Read (WrappedMonoid m) Source # | |
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 # | |
| Read a => Read (Last a) Source # | |
| Read a => Read (First a) Source # | |
| Read a => Read (Max a) Source # | |
| Read a => Read (Min a) Source # | |
| HasResolution a => Read (Fixed a) Source # | Since: base-4.3.0.0 |
| Read a => Read (Complex a) Source # | |
| (Read a, Read b) => Read (Either a b) Source # | |
| Read (V1 p) Source # | Since: base-4.9.0.0 |
| Read (U1 p) Source # | Since: base-4.9.0.0 |
| (Read a, Read b) => Read (a, b) Source # | Since: base-2.1 |
| Read (Proxy t) Source # | Since: base-4.7.0.0 |
| (Read a, Read b) => Read (Arg a b) Source # | |
| Read (f p) => Read (Rec1 f p) Source # | |
| (Read a, Read b, Read c) => Read (a, b, c) Source # | Since: base-2.1 |
| a ~ b => Read (a :~: b) Source # | Since: base-4.7.0.0 |
| Coercible a b => Read (Coercion a b) Source # | Since: base-4.7.0.0 |
| Read (f a) => Read (Alt f a) Source # | |
| Read a => Read (Const a b) Source # | This instance would be equivalent to the derived instances of the
Since: base-4.8.0.0 |
| Read c => Read (K1 i c p) Source # | |
| (Read (f p), Read (g p)) => Read ((f :+: g) p) Source # | |
| (Read (f p), Read (g p)) => Read ((f :*: g) p) Source # | |
| (Read a, Read b, Read c, Read d) => Read (a, b, c, d) Source # | Since: base-2.1 |
| a ~~ b => Read (a :~~: b) Source # | Since: base-4.10.0.0 |
| (Read1 f, Read1 g, Read a) => Read (Sum f g a) Source # | Since: base-4.9.0.0 |
| (Read1 f, Read1 g, Read a) => Read (Product f g a) Source # | Since: base-4.9.0.0 |
| Read (f p) => Read (M1 i c f p) Source # | |
| Read (f (g p)) => Read ((f :.: g) p) Source # | |
| (Read a, Read b, Read c, Read d, Read e) => Read (a, b, c, d, e) Source # | Since: base-2.1 |
| (Read1 f, Read1 g, Read a) => Read (Compose f g a) Source # | Since: base-4.9.0.0 |
| (Read a, Read b, Read c, Read d, Read e, Read f) => Read (a, b, c, d, e, f) Source # | Since: base-2.1 |
| (Read a, Read b, Read c, Read d, Read e, Read f, Read g) => Read (a, b, c, d, e, f, g) Source # | Since: base-2.1 |
| (Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h) => Read (a, b, c, d, e, f, g, h) Source # | Since: base-2.1 |
| (Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h, Read i) => Read (a, b, c, d, e, f, g, h, i) Source # | Since: base-2.1 |
| (Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h, Read i, Read j) => Read (a, b, c, d, e, f, g, h, i, j) Source # | Since: base-2.1 |
| (Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h, Read i, Read j, Read k) => Read (a, b, c, d, e, f, g, h, i, j, k) Source # | Since: base-2.1 |
| (Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h, Read i, Read j, Read k, Read l) => Read (a, b, c, d, e, f, g, h, i, j, k, l) Source # | Since: base-2.1 |
Defined in GHC.Read | |
| (Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h, Read i, Read j, Read k, Read l, Read m) => Read (a, b, c, d, e, f, g, h, i, j, k, l, m) Source # | Since: base-2.1 |
Defined in GHC.Read | |
| (Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h, Read i, Read j, Read k, Read l, Read m, Read n) => Read (a, b, c, d, e, f, g, h, i, j, k, l, m, n) Source # | Since: base-2.1 |
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 # | |
| (Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h, Read i, Read j, Read k, Read l, Read m, Read n, Read o) => Read (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o) Source # | Since: base-2.1 |
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 # | |
Haskell 2010 functions
read :: Read a => String -> a Source #
The read function reads input from a string, which must be
completely consumed by the input process. read fails with an error if the
parse is unsuccessful, and it is therefore discouraged from being used in
real applications. Use readMaybe or readEither for safe alternatives.
>>>read "123" :: Int123
>>>read "hello" :: Int*** Exception: Prelude.read: no parse
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
New parsing functions
Constructors
| 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. |
| Symbol String | Haskell symbol, e.g. |
| Number Number | Since: base-4.6.0.0 |
| EOF |
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
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).