| 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.
TheText.Read library is the canonical library to import forRead-class facilities. For GHC only, it offers an extended and much improvedRead class, which constitutes a proposed alternative to the Haskell 2010Read. In particular, writing parsers is easier, and the parsers are much more efficient.
Read classParsing ofStrings, producing values.
Derived instances ofRead make the following assumptions, which derived instances ofShow obey:
Read instance will parse only infix applications of the constructor (not the prefix form).Read will parse only the record-syntax form, and furthermore, the fields must be given in the same order as the original declaration.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 ofRead 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 bothreadsPrec andreadPrec exist, and why does GHC opt to implementreadPrec in derivedRead instances instead ofreadsPrec? The reason is thatreadsPrec is based on theReadS type, and althoughReadS 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 efficientReadPrec datatype (a.k.a "new-style parsers"), but its definition relies on the use of theRankNTypes language extension. Therefore,readPrec (and its cousin,readListPrec) are marked as GHC-only. Nevertheless, it is recommended to usereadPrec instead ofreadsPrec whenever possible for the efficiency improvements it brings.
As mentioned above, derivedRead instances in GHC will implementreadPrec instead ofreadsPrec. The default implementations ofreadsPrec (and its cousin,readList) will simply usereadPrec under the hood. If you are writing aRead 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 ofRead andShow satisfy the following:
That is,readsPrec parses the string produced byshowsPrec, and delivers the value thatshowsPrec started with.
The methodreadList is provided to allow the programmer to give a specialised way of parsing lists of values. For example, this is used by the predefinedRead instance of theChar type, where values of typeString should be are expected to use double quotes, rather than square brackets.
Proposed replacement forreadsPrec using new-style parsers (GHC only).
readListPrec ::ReadPrec [a]Source#
Proposed replacement forreadList using new-style parsers (GHC only). The default definition usesreadList. Instances that definereadPrec should also definereadListPrec asreadListPrecDefault.
| ReadBoolSource# | Since: 2.1 |
| ReadCharSource# | Since: 2.1 |
| ReadDoubleSource# | Since: 2.1 |
| ReadFloatSource# | Since: 2.1 |
| ReadIntSource# | Since: 2.1 |
| ReadInt8Source# | Since: 2.1 |
| ReadInt16Source# | Since: 2.1 |
| ReadInt32Source# | Since: 2.1 |
| ReadInt64Source# | Since: 2.1 |
| ReadIntegerSource# | Since: 2.1 |
| ReadNaturalSource# | Since: 4.8.0.0 |
| ReadOrderingSource# | Since: 2.1 |
| ReadWordSource# | Since: 4.5.0.0 |
| ReadWord8Source# | Since: 2.1 |
| ReadWord16Source# | Since: 2.1 |
| ReadWord32Source# | Since: 2.1 |
| ReadWord64Source# | Since: 2.1 |
| Read ()Source# | Since: 2.1 |
| ReadGeneralCategorySource# | Since: 2.1 |
Instance detailsDefined inGHC.Read | |
| ReadLexemeSource# | Since: 2.1 |
| ReadIOModeSource# | Since: 4.2.0.0 |
| ReadIntPtrSource# | |
| ReadWordPtrSource# | |
| ReadCUIntMaxSource# | |
| ReadCIntMaxSource# | |
| ReadCUIntPtrSource# | |
| ReadCIntPtrSource# | |
| ReadCSUSecondsSource# | |
Instance detailsDefined inForeign.C.Types | |
| ReadCUSecondsSource# | |
| ReadCTimeSource# | |
| ReadCClockSource# | |
| ReadCSigAtomicSource# | |
Instance detailsDefined inForeign.C.Types | |
| ReadCWcharSource# | |
| ReadCSizeSource# | |
| ReadCPtrdiffSource# | |
| ReadCDoubleSource# | |
| ReadCFloatSource# | |
| ReadCBoolSource# | |
| ReadCULLongSource# | |
| ReadCLLongSource# | |
| ReadCULongSource# | |
| ReadCLongSource# | |
| ReadCUIntSource# | |
| ReadCIntSource# | |
| ReadCUShortSource# | |
| ReadCShortSource# | |
| ReadCUCharSource# | |
| ReadCSCharSource# | |
| ReadCCharSource# | |
| ReadSomeNatSource# | Since: 4.7.0.0 |
| ReadSomeSymbolSource# | Since: 4.7.0.0 |
Instance detailsDefined inGHC.TypeLits | |
| ReadDecidedStrictnessSource# | Since: 4.9.0.0 |
Instance detailsDefined inGHC.Generics | |
| ReadSourceStrictnessSource# | Since: 4.9.0.0 |
Instance detailsDefined inGHC.Generics | |
| ReadSourceUnpackednessSource# | Since: 4.9.0.0 |
Instance detailsDefined inGHC.Generics | |
| ReadAssociativitySource# | Since: 4.6.0.0 |
Instance detailsDefined inGHC.Generics | |
| ReadFixitySource# | Since: 4.6.0.0 |
| ReadAnySource# | Since: 2.1 |
| ReadAllSource# | Since: 2.1 |
| ReadSeekModeSource# | Since: 4.2.0.0 |
| ReadNewlineModeSource# | Since: 4.3.0.0 |
Instance detailsDefined inGHC.IO.Handle.Types | |
| ReadNewlineSource# | Since: 4.3.0.0 |
| ReadBufferModeSource# | Since: 4.2.0.0 |
Instance detailsDefined inGHC.IO.Handle.Types | |
| ReadExitCodeSource# | |
| ReadFdSource# | |
| ReadCKeySource# | |
| ReadCIdSource# | |
| ReadCFsFilCntSource# | |
| ReadCFsBlkCntSource# | |
| ReadCClockIdSource# | |
| ReadCBlkCntSource# | |
| ReadCBlkSizeSource# | |
| ReadCRLimSource# | |
| ReadCTcflagSource# | |
| ReadCSpeedSource# | |
| ReadCCcSource# | |
| ReadCUidSource# | |
| ReadCNlinkSource# | |
| ReadCGidSource# | |
| ReadCSsizeSource# | |
| ReadCPidSource# | |
| ReadCOffSource# | |
| ReadCModeSource# | |
| ReadCInoSource# | |
| ReadCDevSource# | |
| ReadVersionSource# | Since: 2.1 |
| ReadByteOrderSource# | Since: 4.11.0.0 |
| ReadGCDetailsSource# | Since: 4.10.0.0 |
| ReadRTSStatsSource# | Since: 4.10.0.0 |
| ReadVoidSource# | Reading a Since: 4.8.0.0 |
| Read a =>Read [a]Source# | Since: 2.1 |
| Read a =>Read (Maybe a)Source# | Since: 2.1 |
| (Integral a,Read a) =>Read (Ratio a)Source# | Since: 2.1 |
| Read p =>Read (Par1 p)Source# | Since: 4.7.0.0 |
| Read a =>Read (NonEmpty a)Source# | Since: 4.11.0.0 |
| Read a =>Read (Down a)Source# | Since: 4.7.0.0 |
| Read a =>Read (Product a)Source# | Since: 2.1 |
| Read a =>Read (Sum a)Source# | Since: 2.1 |
| Read a =>Read (Dual a)Source# | Since: 2.1 |
| Read a =>Read (Last a)Source# | Since: 2.1 |
| Read a =>Read (First a)Source# | Since: 2.1 |
| Read a =>Read (Identity a)Source# | This instance would be equivalent to the derived instances of the Since: 4.8.0.0 |
| Read a =>Read (ZipList a)Source# | Since: 4.7.0.0 |
| Read a =>Read (Option a)Source# | Since: 4.9.0.0 |
| Read m =>Read (WrappedMonoid m)Source# | Since: 4.9.0.0 |
Instance detailsDefined inData.Semigroup | |
| Read a =>Read (Last a)Source# | Since: 4.9.0.0 |
| Read a =>Read (First a)Source# | Since: 4.9.0.0 |
| Read a =>Read (Max a)Source# | Since: 4.9.0.0 |
| Read a =>Read (Min a)Source# | Since: 4.9.0.0 |
| HasResolution a =>Read (Fixed a)Source# | Since: 4.3.0.0 |
| Read a =>Read (Complex a)Source# | Since: 2.1 |
| (Read a,Read b) =>Read (Either a b)Source# | Since: 3.0 |
| Read (V1 p)Source# | Since: 4.9.0.0 |
| Read (U1 p)Source# | Since: 4.9.0.0 |
| (Read a,Read b) =>Read (a, b)Source# | Since: 2.1 |
| Read (Proxy t)Source# | Since: 4.7.0.0 |
| (Read a,Read b) =>Read (Arg a b)Source# | Since: 4.9.0.0 |
| Read (f p) =>Read (Rec1 f p)Source# | Since: 4.7.0.0 |
| (Read a,Read b,Read c) =>Read (a, b, c)Source# | Since: 2.1 |
| a ~ b =>Read (a:~: b)Source# | Since: 4.7.0.0 |
| Coercible a b =>Read (Coercion a b)Source# | Since: 4.7.0.0 |
| Read (f a) =>Read (Alt f a)Source# | Since: 4.8.0.0 |
| Read (f a) =>Read (Ap f a)Source# | Since: 4.12.0.0 |
| Read a =>Read (Const a b)Source# | This instance would be equivalent to the derived instances of the Since: 4.8.0.0 |
| Read c =>Read (K1 i c p)Source# | Since: 4.7.0.0 |
| (Read (f p),Read (g p)) =>Read ((f:+: g) p)Source# | Since: 4.7.0.0 |
| (Read (f p),Read (g p)) =>Read ((f:*: g) p)Source# | Since: 4.7.0.0 |
| (Read a,Read b,Read c,Read d) =>Read (a, b, c, d)Source# | Since: 2.1 |
| a~~ b =>Read (a:~~: b)Source# | Since: 4.10.0.0 |
| (Read1 f,Read1 g,Read a) =>Read (Sum f g a)Source# | Since: 4.9.0.0 |
| (Read1 f,Read1 g,Read a) =>Read (Product f g a)Source# | Since: 4.9.0.0 |
| Read (f p) =>Read (M1 i c f p)Source# | Since: 4.7.0.0 |
| Read (f (g p)) =>Read ((f:.: g) p)Source# | Since: 4.7.0.0 |
| (Read a,Read b,Read c,Read d,Read e) =>Read (a, b, c, d, e)Source# | Since: 2.1 |
| (Read1 f,Read1 g,Read a) =>Read (Compose f g a)Source# | Since: 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: 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: 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: 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: 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: 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: 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: 2.1 |
Instance detailsDefined inGHC.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: 2.1 |
Instance detailsDefined inGHC.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: 2.1 |
Instance detailsDefined inGHC.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 o) =>Read (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o)Source# | Since: 2.1 |
Instance detailsDefined inGHC.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# | |
read ::Read a =>String -> aSource#
Theread function reads input from a string, which must be completely consumed by the input process.read fails with anerror if the parse is unsuccessful, and it is therefore discouraged from being used in real applications. UsereadMaybe orreadEither for safe alternatives.
>>>read "123" :: Int123
>>>read "hello" :: Int*** Exception: Prelude.read: no parse
Thelex 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. (Thuslex "" = [("","")]lex fails (i.e. returns[]).
This lexer is not completely faithful to the Haskell lexical syntax in the following respects:
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: 4.6.0.0 |
| EOF |
parens ::ReadPrec a ->ReadPrec aSource#
(parens p) parses "P", "(P0)", "((P0))", etc, wherep 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 thereadList method (GHC only). This is only needed for GHC, and even then only forRead instances wherereadListPrec isn't defined asreadListPrecDefault.
readListPrecDefault ::Read a =>ReadPrec [a]Source#
A possible replacement definition for thereadListPrec method, defined usingreadPrec (GHC only).
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