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/Haskell 8

Text.Read

Copyright (c) The University of Glasgow 2001
License BSD-style (see the file libraries/base/LICENSE)
Maintainer [email protected].org
Stability provisional
Portability non-portable (uses Text.ParserCombinators.ReadP)
Safe Haskell Trustworthy
Language Haskell2010

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.

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 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

Since: base-2.1

Instance details

Defined in GHC.Read

Read Char

Since: base-2.1

Instance details

Defined in GHC.Read

Read Double

Since: base-2.1

Instance details

Defined in GHC.Read

Read Float

Since: base-2.1

Instance details

Defined in GHC.Read

Read Int

Since: base-2.1

Instance details

Defined in GHC.Read

Read Int8

Since: base-2.1

Instance details

Defined in GHC.Int

Read Int16

Since: base-2.1

Instance details

Defined in GHC.Int

Read Int32

Since: base-2.1

Instance details

Defined in GHC.Int

Read Int64

Since: base-2.1

Instance details

Defined in GHC.Int

Read Integer

Since: base-2.1

Instance details

Defined in GHC.Read

Read Natural

Since: base-4.8.0.0

Instance details

Defined in GHC.Read

Read Ordering

Since: base-2.1

Instance details

Defined in GHC.Read

Read Word

Since: base-4.5.0.0

Instance details

Defined in GHC.Read

Read Word8

Since: base-2.1

Instance details

Defined in GHC.Read

Read Word16

Since: base-2.1

Instance details

Defined in GHC.Read

Read Word32

Since: base-2.1

Instance details

Defined in GHC.Read

Read Word64

Since: base-2.1

Instance details

Defined in GHC.Read

Read ()

Since: base-2.1

Instance details

Defined in GHC.Read

Read GeneralCategory

Since: base-2.1

Instance details

Defined in GHC.Read

Read Lexeme

Since: base-2.1

Instance details

Defined in GHC.Read

Read IOMode

Since: base-4.2.0.0

Instance details

Defined in GHC.IO.IOMode

Read IntPtr
Instance details

Defined in Foreign.Ptr

Read WordPtr
Instance details

Defined in Foreign.Ptr

Read CUIntMax
Instance details

Defined in Foreign.C.Types

Read CIntMax
Instance details

Defined in Foreign.C.Types

Read CUIntPtr
Instance details

Defined in Foreign.C.Types

Read CIntPtr
Instance details

Defined in Foreign.C.Types

Read CSUSeconds
Instance details

Defined in Foreign.C.Types

Read CUSeconds
Instance details

Defined in Foreign.C.Types

Read CTime
Instance details

Defined in Foreign.C.Types

Read CClock
Instance details

Defined in Foreign.C.Types

Read CSigAtomic
Instance details

Defined in Foreign.C.Types

Read CWchar
Instance details

Defined in Foreign.C.Types

Read CSize
Instance details

Defined in Foreign.C.Types

Read CPtrdiff
Instance details

Defined in Foreign.C.Types

Read CDouble
Instance details

Defined in Foreign.C.Types

Read CFloat
Instance details

Defined in Foreign.C.Types

Read CBool
Instance details

Defined in Foreign.C.Types

Read CULLong
Instance details

Defined in Foreign.C.Types

Read CLLong
Instance details

Defined in Foreign.C.Types

Read CULong
Instance details

Defined in Foreign.C.Types

Read CLong
Instance details

Defined in Foreign.C.Types

Read CUInt
Instance details

Defined in Foreign.C.Types

Read CInt
Instance details

Defined in Foreign.C.Types

Read CUShort
Instance details

Defined in Foreign.C.Types

Read CShort
Instance details

Defined in Foreign.C.Types

Read CUChar
Instance details

Defined in Foreign.C.Types

Read CSChar
Instance details

Defined in Foreign.C.Types

Read CChar
Instance details

Defined in Foreign.C.Types

Read SomeNat

Since: base-4.7.0.0

Instance details

Defined in GHC.TypeNats

Read SomeSymbol

Since: base-4.7.0.0

Instance details

Defined in GHC.TypeLits

Read DecidedStrictness

Since: base-4.9.0.0

Instance details

Defined in GHC.Generics

Read SourceStrictness

Since: base-4.9.0.0

Instance details

Defined in GHC.Generics

Read SourceUnpackedness

Since: base-4.9.0.0

Instance details

Defined in GHC.Generics

Read Associativity

Since: base-4.6.0.0

Instance details

Defined in GHC.Generics

Read Fixity

Since: base-4.6.0.0

Instance details

Defined in GHC.Generics

Read Any

Since: base-2.1

Instance details

Defined in Data.Semigroup.Internal

Read All

Since: base-2.1

Instance details

Defined in Data.Semigroup.Internal

Read SeekMode

Since: base-4.2.0.0

Instance details

Defined in GHC.IO.Device

Read NewlineMode

Since: base-4.3.0.0

Instance details

Defined in GHC.IO.Handle.Types

Read Newline

Since: base-4.3.0.0

Instance details

Defined in GHC.IO.Handle.Types

Read BufferMode

Since: base-4.2.0.0

Instance details

Defined in GHC.IO.Handle.Types

Read ExitCode
Instance details

Defined in GHC.IO.Exception

Read Fd
Instance details

Defined in System.Posix.Types

Read CKey
Instance details

Defined in System.Posix.Types

Read CId
Instance details

Defined in System.Posix.Types

Read CFsFilCnt
Instance details

Defined in System.Posix.Types

Read CFsBlkCnt
Instance details

Defined in System.Posix.Types

Read CClockId
Instance details

Defined in System.Posix.Types

Read CBlkCnt
Instance details

Defined in System.Posix.Types

Read CBlkSize
Instance details

Defined in System.Posix.Types

Read CRLim
Instance details

Defined in System.Posix.Types

Read CTcflag
Instance details

Defined in System.Posix.Types

Read CSpeed
Instance details

Defined in System.Posix.Types

Read CCc
Instance details

Defined in System.Posix.Types

Read CUid
Instance details

Defined in System.Posix.Types

Read CNlink
Instance details

Defined in System.Posix.Types

Read CGid
Instance details

Defined in System.Posix.Types

Read CSsize
Instance details

Defined in System.Posix.Types

Read CPid
Instance details

Defined in System.Posix.Types

Read COff
Instance details

Defined in System.Posix.Types

Read CMode
Instance details

Defined in System.Posix.Types

Read CIno
Instance details

Defined in System.Posix.Types

Read CDev
Instance details

Defined in System.Posix.Types

Read Version

Since: base-2.1

Instance details

Defined in Data.Version

Read ByteOrder

Since: base-4.11.0.0

Instance details

Defined in GHC.ByteOrder

Read GCDetails

Since: base-4.10.0.0

Instance details

Defined in GHC.Stats

Read RTSStats

Since: base-4.10.0.0

Instance details

Defined in GHC.Stats

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 Data.Void

Read a => Read [a]

Since: base-2.1

Instance details

Defined in GHC.Read

Read a => Read (Maybe a)

Since: base-2.1

Instance details

Defined in GHC.Read

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

Since: base-2.1

Instance details

Defined in GHC.Read

Read p => Read (Par1 p)

Since: base-4.7.0.0

Instance details

Defined in GHC.Generics

Read a => Read (NonEmpty a)

Since: base-4.11.0.0

Instance details

Defined in GHC.Read

Read a => Read (Down a)

Since: base-4.7.0.0

Instance details

Defined in Data.Ord

Read a => Read (Product a)

Since: base-2.1

Instance details

Defined in Data.Semigroup.Internal

Read a => Read (Sum a)

Since: base-2.1

Instance details

Defined in Data.Semigroup.Internal

Read a => Read (Dual a)

Since: base-2.1

Instance details

Defined in Data.Semigroup.Internal

Read a => Read (Last a)

Since: base-2.1

Instance details

Defined in Data.Monoid

Read a => Read (First a)

Since: base-2.1

Instance details

Defined in Data.Monoid

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 Data.Functor.Identity

Read a => Read (ZipList a)

Since: base-4.7.0.0

Instance details

Defined in Control.Applicative

Read a => Read (Option a)

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

Read m => Read (WrappedMonoid m)

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

Read a => Read (Last a)

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

Read a => Read (First a)

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

Read a => Read (Max a)

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

Read a => Read (Min a)

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

HasResolution a => Read (Fixed a)

Since: base-4.3.0.0

Instance details

Defined in Data.Fixed

Read a => Read (Complex a)

Since: base-2.1

Instance details

Defined in Data.Complex

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

Since: base-3.0

Instance details

Defined in Data.Either

Read (V1 p)

Since: base-4.9.0.0

Instance details

Defined in GHC.Generics

Read (U1 p)

Since: base-4.9.0.0

Instance details

Defined in GHC.Generics

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

Since: base-2.1

Instance details

Defined in GHC.Read

Methods

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

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

readPrec :: ReadPrec (a, b) Source

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

Read (Proxy t)

Since: base-4.7.0.0

Instance details

Defined in Data.Proxy

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

Since: base-4.9.0.0

Instance details

Defined in Data.Semigroup

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

Since: base-4.7.0.0

Instance details

Defined in GHC.Generics

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

Since: base-2.1

Instance details

Defined in GHC.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

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

Since: base-4.7.0.0

Instance details

Defined in Data.Type.Equality

Coercible a b => Read (Coercion a b)

Since: base-4.7.0.0

Instance details

Defined in Data.Type.Coercion

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

Since: base-4.8.0.0

Instance details

Defined in Data.Semigroup.Internal

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

Since: base-4.12.0.0

Instance details

Defined in Data.Monoid

Read a => Read (Const a b)

This instance would be equivalent to the derived instances of the Const newtype if the runConst field were removed

Since: base-4.8.0.0

Instance details

Defined in Data.Functor.Const

Read c => Read (K1 i c p)

Since: base-4.7.0.0

Instance details

Defined in GHC.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 (f p), Read (g p)) => Read ((f :+: g) p)

Since: base-4.7.0.0

Instance details

Defined in GHC.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.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 a, Read b, Read c, Read d) => Read (a, b, c, d)

Since: base-2.1

Instance details

Defined in GHC.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

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

Since: base-4.10.0.0

Instance details

Defined in Data.Type.Equality

(Read1 f, Read1 g, Read a) => Read (Sum f g a)

Since: base-4.9.0.0

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

(Read1 f, Read1 g, Read a) => Read (Product f g a)

Since: base-4.9.0.0

Instance details

Defined in Data.Functor.Product

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

Since: base-4.7.0.0

Instance details

Defined in GHC.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 (f (g p)) => Read ((f :.: g) p)

Since: base-4.7.0.0

Instance details

Defined in GHC.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 a, Read b, Read c, Read d, Read e) => Read (a, b, c, d, e)

Since: base-2.1

Instance details

Defined in GHC.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

(Read1 f, Read1 g, Read a) => Read (Compose f g a)

Since: base-4.9.0.0

Instance details

Defined in Data.Functor.Compose

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

Since: base-2.1

Instance details

Defined in GHC.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-2.1

Instance details

Defined in GHC.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-2.1

Instance details

Defined in GHC.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-2.1

Instance details

Defined in GHC.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-2.1

Instance details

Defined in GHC.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-2.1

Instance details

Defined in GHC.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-2.1

Instance details

Defined in GHC.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-2.1

Instance details

Defined in GHC.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-2.1

Instance details

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)

Since: base-2.1

Instance details

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

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

module Text.ParserCombinators.ReadPrec

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
Eq Lexeme

Since: base-2.1

Instance details

Defined in Text.Read.Lex

Methods

(==) :: Lexeme -> Lexeme -> Bool Source

(/=) :: Lexeme -> Lexeme -> Bool Source

Read Lexeme

Since: base-2.1

Instance details

Defined in GHC.Read

Show Lexeme

Since: base-2.1

Instance details

Defined in 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

© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/8.6.1/docs/html/libraries/base-4.12.0.0/Text-Read.html