base-4.20.0.0: Core data structures and operations
Copyright(c) The University of Glasgow 2001
LicenseBSD-style (see the file libraries/base/LICENSE)
Maintainerlibraries@haskell.org
Stabilityprovisional
Portabilitynon-portable (uses Text.ParserCombinators.ReadP)
Safe HaskellSafe
LanguageHaskell2010

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

The Read class

class Read a where Source #

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 Read instance 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 Read will parse only the record-syntax form, and furthermore, the fields must be given in the same order as the original declaration.
  • The derived 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 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 = 5

Note 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:

instance Read T where
  readPrec     = ...
  readListPrec = readListPrecDefault

Minimal complete definition

readsPrec | readPrec

Methods

readsPrec Source #

Arguments

:: Int

the operator precedence of the enclosing context (a number from 0 to 11). Function application has precedence 10.

-> 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

Instances details
Read Void

Reading a Void value is always a parse error, considering Void as a data type with no constructors.

@since base-4.8.0.0

Instance details

Defined in GHC.Internal.Read

Read ByteOrder

@since base-4.11.0.0

Instance details

Defined in GHC.Internal.ByteOrder

Read All

@since base-2.01

Instance details

Defined in GHC.Internal.Data.Semigroup.Internal

Read Any

@since base-2.01

Instance details

Defined in GHC.Internal.Data.Semigroup.Internal

Read Version

@since base-2.01

Instance details

Defined in GHC.Internal.Data.Version

Read CBool 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CChar 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CClock 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CDouble 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CFloat 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CInt 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CIntMax 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CIntPtr 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CLLong 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CLong 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CPtrdiff 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CSChar 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CSUSeconds 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CShort 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CSigAtomic 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CSize 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CTime 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CUChar 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CUInt 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CUIntMax 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CUIntPtr 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CULLong 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CULong 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CUSeconds 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CUShort 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read CWchar 
Instance details

Defined in GHC.Internal.Foreign.C.Types

Read IntPtr 
Instance details

Defined in GHC.Internal.Foreign.Ptr

Read WordPtr 
Instance details

Defined in GHC.Internal.Foreign.Ptr

Read Associativity

@since base-4.6.0.0

Instance details

Defined in GHC.Internal.Generics

Read DecidedStrictness

@since base-4.9.0.0

Instance details

Defined in GHC.Internal.Generics

Read Fixity

@since base-4.6.0.0

Instance details

Defined in GHC.Internal.Generics

Read SourceStrictness

@since base-4.9.0.0

Instance details

Defined in GHC.Internal.Generics

Read SourceUnpackedness

@since base-4.9.0.0

Instance details

Defined in GHC.Internal.Generics

Read SeekMode

@since base-4.2.0.0

Instance details

Defined in GHC.Internal.IO.Device

Read ExitCode 
Instance details

Defined in GHC.Internal.IO.Exception

Read BufferMode

@since base-4.2.0.0

Instance details

Defined in GHC.Internal.IO.Handle.Types

Read Newline

@since base-4.3.0.0

Instance details

Defined in GHC.Internal.IO.Handle.Types

Read NewlineMode

@since base-4.3.0.0

Instance details

Defined in GHC.Internal.IO.Handle.Types

Read IOMode

@since base-4.2.0.0

Instance details

Defined in GHC.Internal.IO.IOMode

Read Int16

@since base-2.01

Instance details

Defined in GHC.Internal.Int

Read Int32

@since base-2.01

Instance details

Defined in GHC.Internal.Int

Read Int64

@since base-2.01

Instance details

Defined in GHC.Internal.Int

Read Int8

@since base-2.01

Instance details

Defined in GHC.Internal.Int

Read GCDetails

@since base-4.10.0.0

Instance details

Defined in GHC.Internal.Stats

Read RTSStats

@since base-4.10.0.0

Instance details

Defined in GHC.Internal.Stats

Read CBlkCnt 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read CBlkSize 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read CCc 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read CClockId 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read CDev 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read CFsBlkCnt 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read CFsFilCnt 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read CGid 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read CId 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read CIno 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read CKey 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read CMode 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read CNfds 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read CNlink 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read COff 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read CPid 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read CRLim 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read CSocklen 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read CSpeed 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read CSsize 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read CTcflag 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read CUid 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read Fd 
Instance details

Defined in GHC.Internal.System.Posix.Types

Read Lexeme

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Read SomeChar 
Instance details

Defined in GHC.Internal.TypeLits

Read SomeSymbol

@since base-4.7.0.0

Instance details

Defined in GHC.Internal.TypeLits

Read SomeNat

@since base-4.7.0.0

Instance details

Defined in GHC.Internal.TypeNats

Read GeneralCategory

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Read Word16

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Read Word32

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Read Word64

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Read Word8

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Read Ordering

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Read Integer

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Read Natural

@since base-4.8.0.0

Instance details

Defined in GHC.Internal.Read

Read ()

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Read Bool

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Read Char

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Read Double

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Read Float

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Read Int

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Read Word

@since base-4.5.0.0

Instance details

Defined in GHC.Internal.Read

Read a => Read (Complex a) Source #

Since: base-2.1

Instance details

Defined in Data.Complex

Read a => Read (First a) Source #

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

Read a => Read (Last a) Source #

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

Read a => Read (Max a) Source #

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

Read a => Read (Min a) Source #

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

Read m => Read (WrappedMonoid m) Source #

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

Read a => Read (NonEmpty a)

@since base-4.11.0.0

Instance details

Defined in GHC.Internal.Read

Read a => Read (And a)

@since base-4.16

Instance details

Defined in GHC.Internal.Data.Bits

Read a => Read (Iff a)

@since base-4.16

Instance details

Defined in GHC.Internal.Data.Bits

Read a => Read (Ior a)

@since base-4.16

Instance details

Defined in GHC.Internal.Data.Bits

Read a => Read (Xor a)

@since base-4.16

Instance details

Defined in GHC.Internal.Data.Bits

Read a => Read (Identity a)

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 GHC.Internal.Data.Functor.Identity

Read a => Read (First a)

@since base-2.01

Instance details

Defined in GHC.Internal.Data.Monoid

Read a => Read (Last a)

@since base-2.01

Instance details

Defined in GHC.Internal.Data.Monoid

Read a => Read (Down a)

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

Instance details

Defined in GHC.Internal.Data.Ord

Read a => Read (Dual a)

@since base-2.01

Instance details

Defined in GHC.Internal.Data.Semigroup.Internal

Read a => Read (Product a)

@since base-2.01

Instance details

Defined in GHC.Internal.Data.Semigroup.Internal

Read a => Read (Sum a)

@since base-2.01

Instance details

Defined in GHC.Internal.Data.Semigroup.Internal

Read a => Read (ZipList a)

@since base-4.7.0.0

Instance details

Defined in GHC.Internal.Functor.ZipList

Read p => Read (Par1 p)

@since base-4.7.0.0

Instance details

Defined in GHC.Internal.Generics

(Integral a, Read a) => Read (Ratio a)

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Read a => Read (Maybe a)

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Read a => Read (Solo a)

@since base-4.15

Instance details

Defined in GHC.Internal.Read

Read a => Read [a]

@since base-2.01

Instance details

Defined in GHC.Internal.Read

HasResolution a => Read (Fixed a) Source #

Since: base-4.3.0.0

Instance details

Defined in Data.Fixed

(Read a, Read b) => Read (Arg a b) Source #

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

(Ix a, Read a, Read b) => Read (Array a b)

@since base-2.01

Instance details

Defined in GHC.Internal.Read

(Read a, Read b) => Read (Either a b)

@since base-3.0

Instance details

Defined in GHC.Internal.Data.Either

Read (Proxy t)

@since base-4.7.0.0

Instance details

Defined in GHC.Internal.Data.Proxy

Read (U1 p)

@since base-4.9.0.0

Instance details

Defined in GHC.Internal.Generics

Read (V1 p)

@since base-4.9.0.0

Instance details

Defined in GHC.Internal.Generics

(Read a, Read b) => Read (a, b)

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Methods

readsPrec :: Int -> ReadS (a, b) Source #

readList :: ReadS [(a, b)] Source #

readPrec :: ReadPrec (a, b) Source #

readListPrec :: ReadPrec [(a, b)] Source #

Read a => Read (Const a b)

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

Instance details

Defined in GHC.Internal.Data.Functor.Const

Read (f a) => Read (Ap f a)

@since base-4.12.0.0

Instance details

Defined in GHC.Internal.Data.Monoid

Read (f a) => Read (Alt f a)

@since base-4.8.0.0

Instance details

Defined in GHC.Internal.Data.Semigroup.Internal

Coercible a b => Read (Coercion a b)

@since base-4.7.0.0

Instance details

Defined in GHC.Internal.Data.Type.Coercion

a ~ b => Read (a :~: b)

@since base-4.7.0.0

Instance details

Defined in GHC.Internal.Data.Type.Equality

Read (f p) => Read (Rec1 f p)

@since base-4.7.0.0

Instance details

Defined in GHC.Internal.Generics

(Read a, Read b, Read c) => Read (a, b, c)

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Methods

readsPrec :: Int -> ReadS (a, b, c) Source #

readList :: ReadS [(a, b, c)] Source #

readPrec :: ReadPrec (a, b, c) Source #

readListPrec :: ReadPrec [(a, b, c)] Source #

(Read (f a), Read (g a)) => Read (Product f g a) Source #

Since: base-4.18.0.0

Instance details

Defined in Data.Functor.Product

(Read (f a), Read (g a)) => Read (Sum f g a) Source #

Since: base-4.18.0.0

Instance details

Defined in Data.Functor.Sum

Methods

readsPrec :: Int -> ReadS (Sum f g a) Source #

readList :: ReadS [Sum f g a] Source #

readPrec :: ReadPrec (Sum f g a) Source #

readListPrec :: ReadPrec [Sum f g a] Source #

a ~~ b => Read (a :~~: b)

@since base-4.10.0.0

Instance details

Defined in GHC.Internal.Data.Type.Equality

(Read (f p), Read (g p)) => Read ((f :*: g) p)

@since base-4.7.0.0

Instance details

Defined in GHC.Internal.Generics

Methods

readsPrec :: Int -> ReadS ((f :*: g) p) Source #

readList :: ReadS [(f :*: g) p] Source #

readPrec :: ReadPrec ((f :*: g) p) Source #

readListPrec :: ReadPrec [(f :*: g) p] Source #

(Read (f p), Read (g p)) => Read ((f :+: g) p)

@since base-4.7.0.0

Instance details

Defined in GHC.Internal.Generics

Methods

readsPrec :: Int -> ReadS ((f :+: g) p) Source #

readList :: ReadS [(f :+: g) p] Source #

readPrec :: ReadPrec ((f :+: g) p) Source #

readListPrec :: ReadPrec [(f :+: g) p] Source #

Read c => Read (K1 i c p)

@since base-4.7.0.0

Instance details

Defined in GHC.Internal.Generics

Methods

readsPrec :: Int -> ReadS (K1 i c p) Source #

readList :: ReadS [K1 i c p] Source #

readPrec :: ReadPrec (K1 i c p) Source #

readListPrec :: ReadPrec [K1 i c p] Source #

(Read a, Read b, Read c, Read d) => Read (a, b, c, d)

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Methods

readsPrec :: Int -> ReadS (a, b, c, d) Source #

readList :: ReadS [(a, b, c, d)] Source #

readPrec :: ReadPrec (a, b, c, d) Source #

readListPrec :: ReadPrec [(a, b, c, d)] Source #

Read (f (g a)) => Read (Compose f g a) Source #

Since: base-4.18.0.0

Instance details

Defined in Data.Functor.Compose

Read (f (g p)) => Read ((f :.: g) p)

@since base-4.7.0.0

Instance details

Defined in GHC.Internal.Generics

Methods

readsPrec :: Int -> ReadS ((f :.: g) p) Source #

readList :: ReadS [(f :.: g) p] Source #

readPrec :: ReadPrec ((f :.: g) p) Source #

readListPrec :: ReadPrec [(f :.: g) p] Source #

Read (f p) => Read (M1 i c f p)

@since base-4.7.0.0

Instance details

Defined in GHC.Internal.Generics

Methods

readsPrec :: Int -> ReadS (M1 i c f p) Source #

readList :: ReadS [M1 i c f p] Source #

readPrec :: ReadPrec (M1 i c f p) Source #

readListPrec :: ReadPrec [M1 i c f p] Source #

(Read a, Read b, Read c, Read d, Read e) => Read (a, b, c, d, e)

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Methods

readsPrec :: Int -> ReadS (a, b, c, d, e) Source #

readList :: ReadS [(a, b, c, d, e)] Source #

readPrec :: ReadPrec (a, b, c, d, e) Source #

readListPrec :: ReadPrec [(a, b, c, d, e)] Source #

(Read a, Read b, Read c, Read d, Read e, Read f) => Read (a, b, c, d, e, f)

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Methods

readsPrec :: Int -> ReadS (a, b, c, d, e, f) Source #

readList :: ReadS [(a, b, c, d, e, f)] Source #

readPrec :: ReadPrec (a, b, c, d, e, f) Source #

readListPrec :: ReadPrec [(a, b, c, d, e, f)] Source #

(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

Instance details

Defined in GHC.Internal.Read

Methods

readsPrec :: Int -> ReadS (a, b, c, d, e, f, g) Source #

readList :: ReadS [(a, b, c, d, e, f, g)] Source #

readPrec :: ReadPrec (a, b, c, d, e, f, g) Source #

readListPrec :: ReadPrec [(a, b, c, d, e, f, g)] Source #

(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

Instance details

Defined in GHC.Internal.Read

Methods

readsPrec :: Int -> ReadS (a, b, c, d, e, f, g, h) Source #

readList :: ReadS [(a, b, c, d, e, f, g, h)] Source #

readPrec :: ReadPrec (a, b, c, d, e, f, g, h) Source #

readListPrec :: ReadPrec [(a, b, c, d, e, f, g, h)] Source #

(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

Instance details

Defined in GHC.Internal.Read

Methods

readsPrec :: Int -> ReadS (a, b, c, d, e, f, g, h, i) Source #

readList :: ReadS [(a, b, c, d, e, f, g, h, i)] Source #

readPrec :: ReadPrec (a, b, c, d, e, f, g, h, i) Source #

readListPrec :: ReadPrec [(a, b, c, d, e, f, g, h, i)] Source #

(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

Instance details

Defined in GHC.Internal.Read

Methods

readsPrec :: Int -> ReadS (a, b, c, d, e, f, g, h, i, j) Source #

readList :: ReadS [(a, b, c, d, e, f, g, h, i, j)] Source #

readPrec :: ReadPrec (a, b, c, d, e, f, g, h, i, j) Source #

readListPrec :: ReadPrec [(a, b, c, d, e, f, g, h, i, j)] Source #

(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

Instance details

Defined in GHC.Internal.Read

Methods

readsPrec :: Int -> ReadS (a, b, c, d, e, f, g, h, i, j, k) Source #

readList :: ReadS [(a, b, c, d, e, f, g, h, i, j, k)] Source #

readPrec :: ReadPrec (a, b, c, d, e, f, g, h, i, j, k) Source #

readListPrec :: ReadPrec [(a, b, c, d, e, f, g, h, i, j, k)] 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 (a, b, c, d, e, f, g, h, i, j, k, l)

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Methods

readsPrec :: Int -> ReadS (a, b, c, d, e, f, g, h, i, j, k, l) Source #

readList :: ReadS [(a, b, c, d, e, f, g, h, i, j, k, l)] Source #

readPrec :: ReadPrec (a, b, c, d, e, f, g, h, i, j, k, l) Source #

readListPrec :: ReadPrec [(a, b, c, d, e, f, g, h, i, j, k, l)] 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 (a, b, c, d, e, f, g, h, i, j, k, l, m)

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Methods

readsPrec :: Int -> ReadS (a, b, c, d, e, f, g, h, i, j, k, l, m) Source #

readList :: ReadS [(a, b, c, d, e, f, g, h, i, j, k, l, m)] Source #

readPrec :: ReadPrec (a, b, c, d, e, f, g, h, i, j, k, l, m) Source #

readListPrec :: ReadPrec [(a, b, c, d, e, f, g, h, i, j, k, l, m)] 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 (a, b, c, d, e, f, g, h, i, j, k, l, m, n)

@since base-2.01

Instance details

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

Instance details

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 #

type ReadS a = String -> [(a, String)] Source #

A parser for a type a, represented as a function that takes a String and returns a list of possible parses as (a,String) pairs.

Note that this kind of backtracking parser is very inefficient; reading a large structure may be quite slow (cf ReadP).

Haskell 2010 functions

reads :: Read a => ReadS a Source #

equivalent to readsPrec with a precedence of 0.

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" :: Int
123
>>> read "hello" :: Int
*** Exception: Prelude.read: no parse

readParen :: Bool -> ReadS a -> ReadS a Source #

readParen True p parses what p parses, but surrounded with parentheses.

readParen False p parses what p parses, but optionally surrounded with parentheses.

lex :: ReadS String 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 "" = [("","")].) If there is no legal lexeme at the beginning of the input string, 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

data Lexeme Source #

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. foo, Baz

Symbol String

Haskell symbol, e.g. >>, :%

Number Number

@since base-4.6.0.0

EOF 

Instances

Instances details
Read Lexeme

@since base-2.01

Instance details

Defined in GHC.Internal.Read

Show Lexeme

@since base-2.01

Instance details

Defined in GHC.Internal.Text.Read.Lex

Eq Lexeme

@since base-2.01

Instance details

Defined in GHC.Internal.Text.Read.Lex

lexP :: ReadPrec Lexeme Source #

Parse a single lexeme

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).

readEither :: Read a => String -> Either String a Source #

Parse a string using the Read instance. Succeeds if there is exactly one valid result. A Left value indicates a parse error.

>>> readEither "123" :: Either String Int
Right 123
>>> readEither "hello" :: Either String Int
Left "Prelude.read: no parse"

@since base-4.6.0.0

readMaybe :: Read a => String -> Maybe a Source #

Parse a string using the Read instance. Succeeds if there is exactly one valid result.

>>> readMaybe "123" :: Maybe Int
Just 123
>>> readMaybe "hello" :: Maybe Int
Nothing

@since base-4.6.0.0