| 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
- classRead a where
- typeReadS a = String -> [(a, String)]
- reads :: Read a => ReadS a
- read :: Read a => String -> a
- readParen :: Bool -> ReadS a -> ReadS a
- lex :: ReadSString
- module Text.ParserCombinators.ReadPrec
- dataLexeme
- lexP :: ReadPrecLexeme
- parens :: ReadPrec a -> ReadPrec a
- readListDefault :: Read a => ReadS [a]
- readListPrecDefault :: Read a => ReadPrec [a]
- readEither :: Read a => String -> EitherString 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 = 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
| :: 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.
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
| ReadAllSource# | Since: base-2.1 |
| ReadAnySource# | Since: base-2.1 |
| ReadVersionSource# | Since: base-2.1 |
| ReadVoidSource# | Reading a Since: base-4.8.0.0 |
| ReadCBoolSource# | |
| ReadCCharSource# | |
| ReadCClockSource# | |
| ReadCDoubleSource# | |
| ReadCFloatSource# | |
| ReadCIntSource# | |
| ReadCIntMaxSource# | |
| ReadCIntPtrSource# | |
| ReadCLLongSource# | |
| ReadCLongSource# | |
| ReadCPtrdiffSource# | |
| ReadCSCharSource# | |
| ReadCSUSecondsSource# | |
Defined in Foreign.C.Types | |
| ReadCShortSource# | |
| ReadCSigAtomicSource# | |
Defined in Foreign.C.Types | |
| ReadCSizeSource# | |
| ReadCTimeSource# | |
| ReadCUCharSource# | |
| ReadCUIntSource# | |
| ReadCUIntMaxSource# | |
| ReadCUIntPtrSource# | |
| ReadCULLongSource# | |
| ReadCULongSource# | |
| ReadCUSecondsSource# | |
| ReadCUShortSource# | |
| ReadCWcharSource# | |
| ReadIntPtrSource# | |
| ReadWordPtrSource# | |
| ReadByteOrderSource# | Since: base-4.11.0.0 |
| ReadAssociativitySource# | Since: base-4.6.0.0 |
Defined in GHC.Generics | |
| ReadDecidedStrictnessSource# | Since: base-4.9.0.0 |
Defined in GHC.Generics | |
| ReadFixitySource# | Since: base-4.6.0.0 |
| ReadSourceStrictnessSource# | Since: base-4.9.0.0 |
Defined in GHC.Generics | |
| ReadSourceUnpackednessSource# | Since: base-4.9.0.0 |
Defined in GHC.Generics | |
| ReadSeekModeSource# | Since: base-4.2.0.0 |
| ReadExitCodeSource# | |
| ReadBufferModeSource# | Since: base-4.2.0.0 |
Defined in GHC.IO.Handle.Types | |
| ReadNewlineSource# | Since: base-4.3.0.0 |
| ReadNewlineModeSource# | Since: base-4.3.0.0 |
Defined in GHC.IO.Handle.Types | |
| ReadIOModeSource# | Since: base-4.2.0.0 |
| ReadInt16Source# | Since: base-2.1 |
| ReadInt32Source# | Since: base-2.1 |
| ReadInt64Source# | Since: base-2.1 |
| ReadInt8Source# | Since: base-2.1 |
| ReadGCDetailsSource# | Since: base-4.10.0.0 |
| ReadRTSStatsSource# | Since: base-4.10.0.0 |
| ReadSomeCharSource# | |
| ReadSomeSymbolSource# | Since: base-4.7.0.0 |
Defined in GHC.TypeLits | |
| ReadSomeNatSource# | Since: base-4.7.0.0 |
| ReadGeneralCategorySource# | Since: base-2.1 |
Defined in GHC.Read | |
| ReadWord16Source# | Since: base-2.1 |
| ReadWord32Source# | Since: base-2.1 |
| ReadWord64Source# | Since: base-2.1 |
| ReadWord8Source# | Since: base-2.1 |
| ReadCBlkCntSource# | |
| ReadCBlkSizeSource# | |
| ReadCCcSource# | |
| ReadCClockIdSource# | |
| ReadCDevSource# | |
| ReadCFsBlkCntSource# | |
| ReadCFsFilCntSource# | |
| ReadCGidSource# | |
| ReadCIdSource# | |
| ReadCInoSource# | |
| ReadCKeySource# | |
| ReadCModeSource# | |
| ReadCNfdsSource# | |
| ReadCNlinkSource# | |
| ReadCOffSource# | |
| ReadCPidSource# | |
| ReadCRLimSource# | |
| ReadCSocklenSource# | |
| ReadCSpeedSource# | |
| ReadCSsizeSource# | |
| ReadCTcflagSource# | |
| ReadCUidSource# | |
| ReadFdSource# | |
| ReadLexemeSource# | Since: base-2.1 |
| ReadOrderingSource# | Since: base-2.1 |
| ReadIntegerSource# | Since: base-2.1 |
| ReadNaturalSource# | Since: base-4.8.0.0 |
| Read ()Source# | Since: base-2.1 |
| ReadBoolSource# | Since: base-2.1 |
| ReadCharSource# | Since: base-2.1 |
| ReadDoubleSource# | Since: base-2.1 |
| ReadFloatSource# | Since: base-2.1 |
| ReadIntSource# | Since: base-2.1 |
| ReadWordSource# | Since: base-4.5.0.0 |
| Read a => Read (ZipList a)Source# | Since: base-4.7.0.0 |
| Read a => Read (And a)Source# | Since: base-4.16 |
| Read a => Read (Iff a)Source# | Since: base-4.16 |
| Read a => Read (Ior a)Source# | Since: base-4.16 |
| Read a => Read (Xor a)Source# | Since: base-4.16 |
| Read a => Read (Complex a)Source# | Since: base-2.1 |
| 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 (First a)Source# | Since: base-2.1 |
| Read a => Read (Last a)Source# | Since: base-2.1 |
| Read a => Read (Down a)Source# | This instance would be equivalent to the derived instances of the Since: base-4.7.0.0 |
| 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 (Dual a)Source# | Since: base-2.1 |
| Read a => Read (Product a)Source# | Since: base-2.1 |
| Read a => Read (Sum a)Source# | Since: base-2.1 |
| Read p => Read (Par1 p)Source# | Since: base-4.7.0.0 |
| (Integral a, Read a) => Read (Ratio a)Source# | Since: base-2.1 |
| Read a => Read (NonEmpty a)Source# | Since: base-4.11.0.0 |
| Read a => Read (Maybe a)Source# | Since: base-2.1 |
| Read a => Read (a)Source# | Since: base-4.15 |
| Read a => Read [a]Source# | Since: base-2.1 |
| (Read a, Read b) => Read (Either a b)Source# | Since: base-3.0 |
| HasResolution a => Read (Fixed a)Source# | Since: base-4.3.0.0 |
| Read (Proxy t)Source# | Since: base-4.7.0.0 |
| (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)Source# | Since: base-2.1 |
| Read (U1 p)Source# | Since: base-4.9.0.0 |
| Read (V1 p)Source# | Since: base-4.9.0.0 |
| (Read a, Read b) => Read (a, b)Source# | Since: base-2.1 |
| 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 (f a) => Read (Ap f a)Source# | Since: base-4.12.0.0 |
| Read (f a) => Read (Alt f a)Source# | Since: base-4.8.0.0 |
| Coercible a b => Read (Coercion a b)Source# | Since: base-4.7.0.0 |
| a ~ b => Read (a :~: b)Source# | Since: base-4.7.0.0 |
| Read (f p) => Read (Rec1 f p)Source# | Since: base-4.7.0.0 |
| (Read a, Read b, Read c) => Read (a, b, c)Source# | Since: base-2.1 |
| (Read1 f, Read1 g, Read a) => Read (Product f g a)Source# | Since: base-4.9.0.0 |
| (Read1 f, Read1 g, Read a) => Read (Sum f g a)Source# | Since: base-4.9.0.0 |
| a ~~ b => Read (a :~~: b)Source# | Since: base-4.10.0.0 |
| (Read (f p), Read (g p)) => Read ((f :*: g) p)Source# | Since: base-4.7.0.0 |
| (Read (f p), Read (g p)) => Read ((f :+: g) p)Source# | Since: base-4.7.0.0 |
| Read c => Read (K1 i c p)Source# | Since: base-4.7.0.0 |
| (Read a, Read b, Read c, Read d) => Read (a, b, c, d)Source# | Since: base-2.1 |
| (Read1 f, Read1 g, Read a) => Read (Compose f g a)Source# | Since: base-4.9.0.0 |
| Read (f (g p)) => Read ((f :.: g) p)Source# | Since: base-4.7.0.0 |
| Read (f p) => Read (M1 i c f p)Source# | Since: base-4.7.0.0 |
| (Read a, Read b, Read c, Read d, Read e) => Read (a, b, c, d, e)Source# | Since: base-2.1 |
| (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 |
| (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
| CharChar | Character literal |
| StringString | String literal, with escapes interpreted |
| PuncString | Punctuation or reserved symbol, e.g. |
| IdentString | Haskell identifier, e.g. |
| SymbolString | Haskell symbol, e.g. |
| NumberNumber | 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).
