| 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 | Safe |
| Language | Haskell2010 |
Text.Read
Description
Converting strings to values.
The Text.Read module is the canonical place 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 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 Void | Reading a @since base-4.8.0.0 |
| Read ByteOrder | @since base-4.11.0.0 |
| Read All | @since base-2.01 |
| Read Any | @since base-2.01 |
| Read Version | @since base-2.01 |
| Read CBool | |
| Read CChar | |
| Read CClock | |
| Read CDouble | |
| Read CFloat | |
| Read CInt | |
| Read CIntMax | |
| Read CIntPtr | |
| Read CLLong | |
| Read CLong | |
| Read CPtrdiff | |
| Read CSChar | |
| Read CSUSeconds | |
Defined in GHC.Internal.Foreign.C.Types | |
| Read CShort | |
| Read CSigAtomic | |
Defined in GHC.Internal.Foreign.C.Types | |
| Read CSize | |
| Read CTime | |
| Read CUChar | |
| Read CUInt | |
| Read CUIntMax | |
| Read CUIntPtr | |
| Read CULLong | |
| Read CULong | |
| Read CUSeconds | |
| Read CUShort | |
| Read CWchar | |
| Read IntPtr | |
| Read WordPtr | |
| Read Associativity | @since base-4.6.0.0 |
Defined in GHC.Internal.Generics | |
| Read DecidedStrictness | @since base-4.9.0.0 |
Defined in GHC.Internal.Generics | |
| Read Fixity | @since base-4.6.0.0 |
| Read SourceStrictness | @since base-4.9.0.0 |
Defined in GHC.Internal.Generics | |
| Read SourceUnpackedness | @since base-4.9.0.0 |
Defined in GHC.Internal.Generics | |
| Read SeekMode | @since base-4.2.0.0 |
| Read ExitCode | |
| Read BufferMode | @since base-4.2.0.0 |
Defined in GHC.Internal.IO.Handle.Types | |
| Read Newline | @since base-4.3.0.0 |
| Read NewlineMode | @since base-4.3.0.0 |
Defined in GHC.Internal.IO.Handle.Types | |
| Read IOMode | @since base-4.2.0.0 |
| Read Int16 | @since base-2.01 |
| Read Int32 | @since base-2.01 |
| Read Int64 | @since base-2.01 |
| Read Int8 | @since base-2.01 |
| Read GCDetails | @since base-4.10.0.0 |
| Read RTSStats | @since base-4.10.0.0 |
| Read CBlkCnt | |
| Read CBlkSize | |
| Read CCc | |
| Read CClockId | |
| Read CDev | |
| Read CFsBlkCnt | |
| Read CFsFilCnt | |
| Read CGid | |
| Read CId | |
| Read CIno | |
| Read CKey | |
| Read CMode | |
| Read CNfds | |
| Read CNlink | |
| Read COff | |
| Read CPid | |
| Read CRLim | |
| Read CSocklen | |
| Read CSpeed | |
| Read CSsize | |
| Read CTcflag | |
| Read CUid | |
| Read Fd | |
| Read Lexeme | @since base-2.01 |
| Read SomeChar | |
| Read SomeSymbol | @since base-4.7.0.0 |
Defined in GHC.Internal.TypeLits | |
| Read SomeNat | @since base-4.7.0.0 |
| Read GeneralCategory | @since base-2.01 |
Defined in GHC.Internal.Read | |
| Read Word16 | @since base-2.01 |
| Read Word32 | @since base-2.01 |
| Read Word64 | @since base-2.01 |
| Read Word8 | @since base-2.01 |
| Read Ordering | @since base-2.01 |
| Read Integer | @since base-2.01 |
| Read Natural | @since base-4.8.0.0 |
| Read () | @since base-2.01 |
| Read Bool | @since base-2.01 |
| Read Char | @since base-2.01 |
| Read Double | @since base-2.01 |
| Read Float | @since base-2.01 |
| Read Int | @since base-2.01 |
| Read Word | @since base-4.5.0.0 |
| Read a => Read (Complex a) Source # | Since: base-2.1 |
| Read a => Read (First a) Source # | Since: base-4.9.0.0 |
| Read a => Read (Last a) Source # | Since: base-4.9.0.0 |
| Read a => Read (Max a) Source # | Since: base-4.9.0.0 |
| Read a => Read (Min a) Source # | Since: base-4.9.0.0 |
| Read m => Read (WrappedMonoid m) Source # | Since: base-4.9.0.0 |
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 (NonEmpty a) | @since base-4.11.0.0 |
| Read a => Read (And a) | @since base-4.16 |
| Read a => Read (Iff a) | @since base-4.16 |
| Read a => Read (Ior a) | @since base-4.16 |
| Read a => Read (Xor a) | @since base-4.16 |
| Read a => Read (Identity a) | This instance would be equivalent to the derived instances of the
@since base-4.8.0.0 |
| Read a => Read (First a) | @since base-2.01 |
| Read a => Read (Last a) | @since base-2.01 |
| Read a => Read (Down a) | This instance would be equivalent to the derived instances of the
@since base-4.7.0.0 |
| Read a => Read (Dual a) | @since base-2.01 |
| Read a => Read (Product a) | @since base-2.01 |
| Read a => Read (Sum a) | @since base-2.01 |
| Read a => Read (ZipList a) | @since base-4.7.0.0 |
| Read p => Read (Par1 p) | @since base-4.7.0.0 |
| (Integral a, Read a) => Read (Ratio a) | @since base-2.01 |
| Read a => Read (Maybe a) | @since base-2.01 |
| Read a => Read (Solo a) | @since base-4.15 |
| Read a => Read [a] | @since base-2.01 |
| HasResolution a => Read (Fixed a) Source # | Since: base-4.3.0.0 |
| (Read a, Read b) => Read (Arg a b) Source # | Since: base-4.9.0.0 |
| (Ix a, Read a, Read b) => Read (Array a b) | @since base-2.01 |
| (Read a, Read b) => Read (Either a b) | @since base-3.0 |
| Read (Proxy t) | @since base-4.7.0.0 |
| Read (U1 p) | @since base-4.9.0.0 |
| Read (V1 p) | @since base-4.9.0.0 |
| (Read a, Read b) => Read (a, b) | @since base-2.01 |
| Read a => Read (Const a b) | This instance would be equivalent to the derived instances of the
@since base-4.8.0.0 |
| Read (f a) => Read (Ap f a) | @since base-4.12.0.0 |
| Read (f a) => Read (Alt f a) | @since base-4.8.0.0 |
| Coercible a b => Read (Coercion a b) | @since base-4.7.0.0 |
| a ~ b => Read (a :~: b) | @since base-4.7.0.0 |
| Read (f p) => Read (Rec1 f p) | @since base-4.7.0.0 |
| (Read a, Read b, Read c) => Read (a, b, c) | @since base-2.01 |
| (Read (f a), Read (g a)) => Read (Product f g a) Source # | Since: base-4.18.0.0 |
| (Read (f a), Read (g a)) => Read (Sum f g a) Source # | Since: base-4.18.0.0 |
| a ~~ b => Read (a :~~: b) | @since base-4.10.0.0 |
| (Read (f p), Read (g p)) => Read ((f :*: g) p) | @since base-4.7.0.0 |
| (Read (f p), Read (g p)) => Read ((f :+: g) p) | @since base-4.7.0.0 |
| Read c => Read (K1 i c p) | @since base-4.7.0.0 |
| (Read a, Read b, Read c, Read d) => Read (a, b, c, d) | @since base-2.01 |
| Read (f (g a)) => Read (Compose f g a) Source # | Since: base-4.18.0.0 |
| Read (f (g p)) => Read ((f :.: g) p) | @since base-4.7.0.0 |
| Read (f p) => Read (M1 i c f p) | @since base-4.7.0.0 |
| (Read a, Read b, Read c, Read d, Read e) => Read (a, b, c, d, e) | @since base-2.01 |
| (Read a, Read b, Read c, Read d, Read e, Read f) => Read (a, b, c, d, e, f) | @since base-2.01 |
| (Read a, Read b, Read c, Read d, Read e, Read f, Read g) => Read (a, b, c, d, e, f, g) | @since base-2.01 |
| (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) | @since base-2.01 |
| (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) | @since base-2.01 |
| (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) | @since base-2.01 |
| (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) | @since base-2.01 |
| (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) | @since base-2.01 |
Defined in GHC.Internal.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) | @since base-2.01 |
Defined in GHC.Internal.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) | @since base-2.01 |
Defined in GHC.Internal.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) | @since base-2.01 |
Defined in GHC.Internal.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).