base-4.12.0.0: Basic libraries

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 HaskellTrustworthy
LanguageHaskell2010

Text.Read

Contents

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

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 Source #

Since: 2.1

Instance details

Defined in GHC.Read

Read Char Source #

Since: 2.1

Instance details

Defined in GHC.Read

Read Double Source #

Since: 2.1

Instance details

Defined in GHC.Read

Read Float Source #

Since: 2.1

Instance details

Defined in GHC.Read

Read Int Source #

Since: 2.1

Instance details

Defined in GHC.Read

Read Int8 Source #

Since: 2.1

Instance details

Defined in GHC.Int

Read Int16 Source #

Since: 2.1

Instance details

Defined in GHC.Int

Read Int32 Source #

Since: 2.1

Instance details

Defined in GHC.Int

Read Int64 Source #

Since: 2.1

Instance details

Defined in GHC.Int

Read Integer Source #

Since: 2.1

Instance details

Defined in GHC.Read

Read Natural Source #

Since: 4.8.0.0

Instance details

Defined in GHC.Read

Read Ordering Source #

Since: 2.1

Instance details

Defined in GHC.Read

Read Word Source #

Since: 4.5.0.0

Instance details

Defined in GHC.Read

Read Word8 Source #

Since: 2.1

Instance details

Defined in GHC.Read

Read Word16 Source #

Since: 2.1

Instance details

Defined in GHC.Read

Read Word32 Source #

Since: 2.1

Instance details

Defined in GHC.Read

Read Word64 Source #

Since: 2.1

Instance details

Defined in GHC.Read

Read () Source #

Since: 2.1

Instance details

Defined in GHC.Read

Read GeneralCategory Source #

Since: 2.1

Instance details

Defined in GHC.Read

Read Lexeme Source #

Since: 2.1

Instance details

Defined in GHC.Read

Read IOMode Source #

Since: 4.2.0.0

Instance details

Defined in GHC.IO.IOMode

Read IntPtr Source # 
Instance details

Defined in Foreign.Ptr

Read WordPtr Source # 
Instance details

Defined in Foreign.Ptr

Read CUIntMax Source # 
Instance details

Defined in Foreign.C.Types

Read CIntMax Source # 
Instance details

Defined in Foreign.C.Types

Read CUIntPtr Source # 
Instance details

Defined in Foreign.C.Types

Read CIntPtr Source # 
Instance details

Defined in Foreign.C.Types

Read CSUSeconds Source # 
Instance details

Defined in Foreign.C.Types

Read CUSeconds Source # 
Instance details

Defined in Foreign.C.Types

Read CTime Source # 
Instance details

Defined in Foreign.C.Types

Read CClock Source # 
Instance details

Defined in Foreign.C.Types

Read CSigAtomic Source # 
Instance details

Defined in Foreign.C.Types

Read CWchar Source # 
Instance details

Defined in Foreign.C.Types

Read CSize Source # 
Instance details

Defined in Foreign.C.Types

Read CPtrdiff Source # 
Instance details

Defined in Foreign.C.Types

Read CDouble Source # 
Instance details

Defined in Foreign.C.Types

Read CFloat Source # 
Instance details

Defined in Foreign.C.Types

Read CBool Source # 
Instance details

Defined in Foreign.C.Types

Read CULLong Source # 
Instance details

Defined in Foreign.C.Types

Read CLLong Source # 
Instance details

Defined in Foreign.C.Types

Read CULong Source # 
Instance details

Defined in Foreign.C.Types

Read CLong Source # 
Instance details

Defined in Foreign.C.Types

Read CUInt Source # 
Instance details

Defined in Foreign.C.Types

Read CInt Source # 
Instance details

Defined in Foreign.C.Types

Read CUShort Source # 
Instance details

Defined in Foreign.C.Types

Read CShort Source # 
Instance details

Defined in Foreign.C.Types

Read CUChar Source # 
Instance details

Defined in Foreign.C.Types

Read CSChar Source # 
Instance details

Defined in Foreign.C.Types

Read CChar Source # 
Instance details

Defined in Foreign.C.Types

Read SomeNat Source #

Since: 4.7.0.0

Instance details

Defined in GHC.TypeNats

Read SomeSymbol Source #

Since: 4.7.0.0

Instance details

Defined in GHC.TypeLits

Read DecidedStrictness Source #

Since: 4.9.0.0

Instance details

Defined in GHC.Generics

Read SourceStrictness Source #

Since: 4.9.0.0

Instance details

Defined in GHC.Generics

Read SourceUnpackedness Source #

Since: 4.9.0.0

Instance details

Defined in GHC.Generics

Read Associativity Source #

Since: 4.6.0.0

Instance details

Defined in GHC.Generics

Read Fixity Source #

Since: 4.6.0.0

Instance details

Defined in GHC.Generics

Read Any Source #

Since: 2.1

Instance details

Defined in Data.Semigroup.Internal

Read All Source #

Since: 2.1

Instance details

Defined in Data.Semigroup.Internal

Read SeekMode Source #

Since: 4.2.0.0

Instance details

Defined in GHC.IO.Device

Read NewlineMode Source #

Since: 4.3.0.0

Instance details

Defined in GHC.IO.Handle.Types

Read Newline Source #

Since: 4.3.0.0

Instance details

Defined in GHC.IO.Handle.Types

Read BufferMode Source #

Since: 4.2.0.0

Instance details

Defined in GHC.IO.Handle.Types

Read ExitCode Source # 
Instance details

Defined in GHC.IO.Exception

Read Fd Source # 
Instance details

Defined in System.Posix.Types

Read CId Source # 
Instance details

Defined in System.Posix.Types

Read CFsFilCnt Source # 
Instance details

Defined in System.Posix.Types

Read CFsBlkCnt Source # 
Instance details

Defined in System.Posix.Types

Read CBlkCnt Source # 
Instance details

Defined in System.Posix.Types

Read CBlkSize Source # 
Instance details

Defined in System.Posix.Types

Read CRLim Source # 
Instance details

Defined in System.Posix.Types

Read CTcflag Source # 
Instance details

Defined in System.Posix.Types

Read CSpeed Source # 
Instance details

Defined in System.Posix.Types

Read CCc Source # 
Instance details

Defined in System.Posix.Types

Read CUid Source # 
Instance details

Defined in System.Posix.Types

Read CNlink Source # 
Instance details

Defined in System.Posix.Types

Read CGid Source # 
Instance details

Defined in System.Posix.Types

Read CSsize Source # 
Instance details

Defined in System.Posix.Types

Read CPid Source # 
Instance details

Defined in System.Posix.Types

Read COff Source # 
Instance details

Defined in System.Posix.Types

Read CMode Source # 
Instance details

Defined in System.Posix.Types

Read CIno Source # 
Instance details

Defined in System.Posix.Types

Read CDev Source # 
Instance details

Defined in System.Posix.Types

Read Version Source #

Since: 2.1

Instance details

Defined in Data.Version

Read ByteOrder Source #

Since: 4.11.0.0

Instance details

Defined in GHC.ByteOrder

Read GCDetails Source #

Since: 4.10.0.0

Instance details

Defined in GHC.Stats

Read RTSStats Source #

Since: 4.10.0.0

Instance details

Defined in GHC.Stats

Read Void Source #

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

Since: 4.8.0.0

Instance details

Defined in Data.Void

Read a => Read [a] Source #

Since: 2.1

Instance details

Defined in GHC.Read

Read a => Read (Maybe a) Source #

Since: 2.1

Instance details

Defined in GHC.Read

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

Since: 2.1

Instance details

Defined in GHC.Read

Read p => Read (Par1 p) Source #

Since: 4.7.0.0

Instance details

Defined in GHC.Generics

Read a => Read (NonEmpty a) Source #

Since: 4.11.0.0

Instance details

Defined in GHC.Read

Read a => Read (Down a) Source #

Since: 4.7.0.0

Instance details

Defined in Data.Ord

Read a => Read (Product a) Source #

Since: 2.1

Instance details

Defined in Data.Semigroup.Internal

Read a => Read (Sum a) Source #

Since: 2.1

Instance details

Defined in Data.Semigroup.Internal

Read a => Read (Dual a) Source #

Since: 2.1

Instance details

Defined in Data.Semigroup.Internal

Read a => Read (Last a) Source #

Since: 2.1

Instance details

Defined in Data.Monoid

Read a => Read (First a) Source #

Since: 2.1

Instance details

Defined in Data.Monoid

Read a => Read (Identity a) Source #

This instance would be equivalent to the derived instances of the Identity newtype if the runIdentity field were removed

Since: 4.8.0.0

Instance details

Defined in Data.Functor.Identity

Read a => Read (ZipList a) Source #

Since: 4.7.0.0

Instance details

Defined in Control.Applicative

Read a => Read (Option a) Source #

Since: 4.9.0.0

Instance details

Defined in Data.Semigroup

Read m => Read (WrappedMonoid m) Source #

Since: 4.9.0.0

Instance details

Defined in Data.Semigroup

Read a => Read (Last a) Source #

Since: 4.9.0.0

Instance details

Defined in Data.Semigroup

Read a => Read (First a) Source #

Since: 4.9.0.0

Instance details

Defined in Data.Semigroup

Read a => Read (Max a) Source #

Since: 4.9.0.0

Instance details

Defined in Data.Semigroup

Read a => Read (Min a) Source #

Since: 4.9.0.0

Instance details

Defined in Data.Semigroup

HasResolution a => Read (Fixed a) Source #

Since: 4.3.0.0

Instance details

Defined in Data.Fixed

Read a => Read (Complex a) Source #

Since: 2.1

Instance details

Defined in Data.Complex

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

Since: 3.0

Instance details

Defined in Data.Either

Read (V1 p) Source #

Since: 4.9.0.0

Instance details

Defined in GHC.Generics

Read (U1 p) Source #

Since: 4.9.0.0

Instance details

Defined in GHC.Generics

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

Since: 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) Source #

Since: 4.7.0.0

Instance details

Defined in Data.Proxy

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

Since: 4.9.0.0

Instance details

Defined in Data.Semigroup

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

Since: 4.7.0.0

Instance details

Defined in GHC.Generics

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

Since: 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) Source #

Since: 4.7.0.0

Instance details

Defined in Data.Type.Equality

Coercible a b => Read (Coercion a b) Source #

Since: 4.7.0.0

Instance details

Defined in Data.Type.Coercion

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

Since: 4.8.0.0

Instance details

Defined in Data.Semigroup.Internal

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

Since: 4.12.0.0

Instance details

Defined in Data.Monoid

Read a => Read (Const a b) Source #

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

Since: 4.8.0.0

Instance details

Defined in Data.Functor.Const

Read c => Read (K1 i c p) Source #

Since: 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) Source #

Since: 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) Source #

Since: 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) Source #

Since: 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) Source #

Since: 4.10.0.0

Instance details

Defined in Data.Type.Equality

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

Since: 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) Source #

Since: 4.9.0.0

Instance details

Defined in Data.Functor.Product

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

Since: 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) Source #

Since: 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) Source #

Since: 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) Source #

Since: 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) Source #

Since: 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) Source #

Since: 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) Source #

Since: 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) Source #

Since: 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) Source #

Since: 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) Source #

Since: 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) Source #

Since: 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) Source #

Since: 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) Source #

Since: 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) Source #

Since: 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

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: 4.6.0.0

EOF 
Instances
Eq Lexeme Source #

Since: 2.1

Instance details

Defined in Text.Read.Lex

Read Lexeme Source #

Since: 2.1

Instance details

Defined in GHC.Read

Show Lexeme Source #

Since: 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: 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: 4.6.0.0