haskell / 9 / libraries / base-4.17.0.0 / text-read.html

Text.Read

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

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
Instances details
Read All Source

Since: base-2.1

Instance details

Defined in Data.Semigroup.Internal

Read Any Source

Since: base-2.1

Instance details

Defined in Data.Semigroup.Internal

Read Version Source

Since: base-2.1

Instance details

Defined in Data.Version

Read Void Source

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 CBool Source
Instance details

Defined in Foreign.C.Types

Read CChar Source
Instance details

Defined in Foreign.C.Types

Read CClock 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 CInt Source
Instance details

Defined in Foreign.C.Types

Read CIntMax Source
Instance details

Defined in Foreign.C.Types

Read CIntPtr Source
Instance details

Defined in Foreign.C.Types

Read CLLong Source
Instance details

Defined in Foreign.C.Types

Read CLong Source
Instance details

Defined in Foreign.C.Types

Read CPtrdiff Source
Instance details

Defined in Foreign.C.Types

Read CSChar Source
Instance details

Defined in Foreign.C.Types

Read CSUSeconds Source
Instance details

Defined in Foreign.C.Types

Read CShort Source
Instance details

Defined in Foreign.C.Types

Read CSigAtomic Source
Instance details

Defined in Foreign.C.Types

Read CSize Source
Instance details

Defined in Foreign.C.Types

Read CTime Source
Instance details

Defined in Foreign.C.Types

Read CUChar Source
Instance details

Defined in Foreign.C.Types

Read CUInt Source
Instance details

Defined in Foreign.C.Types

Read CUIntMax Source
Instance details

Defined in Foreign.C.Types

Read CUIntPtr Source
Instance details

Defined in Foreign.C.Types

Read CULLong Source
Instance details

Defined in Foreign.C.Types

Read CULong Source
Instance details

Defined in Foreign.C.Types

Read CUSeconds Source
Instance details

Defined in Foreign.C.Types

Read CUShort Source
Instance details

Defined in Foreign.C.Types

Read CWchar Source
Instance details

Defined in Foreign.C.Types

Read IntPtr Source
Instance details

Defined in Foreign.Ptr

Read WordPtr Source
Instance details

Defined in Foreign.Ptr

Read ByteOrder Source

Since: base-4.11.0.0

Instance details

Defined in GHC.ByteOrder

Read Associativity Source

Since: base-4.6.0.0

Instance details

Defined in GHC.Generics

Read DecidedStrictness Source

Since: base-4.9.0.0

Instance details

Defined in GHC.Generics

Read Fixity Source

Since: base-4.6.0.0

Instance details

Defined in GHC.Generics

Read SourceStrictness Source

Since: base-4.9.0.0

Instance details

Defined in GHC.Generics

Read SourceUnpackedness Source

Since: base-4.9.0.0

Instance details

Defined in GHC.Generics

Read SeekMode Source

Since: base-4.2.0.0

Instance details

Defined in GHC.IO.Device

Read ExitCode Source
Instance details

Defined in GHC.IO.Exception

Read BufferMode Source

Since: base-4.2.0.0

Instance details

Defined in GHC.IO.Handle.Types

Read Newline Source

Since: base-4.3.0.0

Instance details

Defined in GHC.IO.Handle.Types

Read NewlineMode Source

Since: base-4.3.0.0

Instance details

Defined in GHC.IO.Handle.Types

Read IOMode Source

Since: base-4.2.0.0

Instance details

Defined in GHC.IO.IOMode

Read Int16 Source

Since: base-2.1

Instance details

Defined in GHC.Int

Read Int32 Source

Since: base-2.1

Instance details

Defined in GHC.Int

Read Int64 Source

Since: base-2.1

Instance details

Defined in GHC.Int

Read Int8 Source

Since: base-2.1

Instance details

Defined in GHC.Int

Read GCDetails Source

Since: base-4.10.0.0

Instance details

Defined in GHC.Stats

Read RTSStats Source

Since: base-4.10.0.0

Instance details

Defined in GHC.Stats

Read SomeChar Source
Instance details

Defined in GHC.TypeLits

Read SomeSymbol Source

Since: base-4.7.0.0

Instance details

Defined in GHC.TypeLits

Read SomeNat Source

Since: base-4.7.0.0

Instance details

Defined in GHC.TypeNats

Read GeneralCategory Source

Since: base-2.1

Instance details

Defined in GHC.Read

Read Word16 Source

Since: base-2.1

Instance details

Defined in GHC.Read

Read Word32 Source

Since: base-2.1

Instance details

Defined in GHC.Read

Read Word64 Source

Since: base-2.1

Instance details

Defined in GHC.Read

Read Word8 Source

Since: base-2.1

Instance details

Defined in GHC.Read

Read CBlkCnt Source
Instance details

Defined in System.Posix.Types

Read CBlkSize Source
Instance details

Defined in System.Posix.Types

Read CCc Source
Instance details

Defined in System.Posix.Types

Read CClockId Source
Instance details

Defined in System.Posix.Types

Read CDev Source
Instance details

Defined in System.Posix.Types

Read CFsBlkCnt Source
Instance details

Defined in System.Posix.Types

Read CFsFilCnt Source
Instance details

Defined in System.Posix.Types

Read CGid Source
Instance details

Defined in System.Posix.Types

Read CId Source
Instance details

Defined in System.Posix.Types

Read CIno Source
Instance details

Defined in System.Posix.Types

Read CKey Source
Instance details

Defined in System.Posix.Types

Read CMode Source
Instance details

Defined in System.Posix.Types

Read CNfds Source
Instance details

Defined in System.Posix.Types

Read CNlink Source
Instance details

Defined in System.Posix.Types

Read COff Source
Instance details

Defined in System.Posix.Types

Read CPid Source
Instance details

Defined in System.Posix.Types

Read CRLim Source
Instance details

Defined in System.Posix.Types

Read CSocklen Source
Instance details

Defined in System.Posix.Types

Read CSpeed Source
Instance details

Defined in System.Posix.Types

Read CSsize Source
Instance details

Defined in System.Posix.Types

Read CTcflag Source
Instance details

Defined in System.Posix.Types

Read CUid Source
Instance details

Defined in System.Posix.Types

Read Fd Source
Instance details

Defined in System.Posix.Types

Read Lexeme Source

Since: base-2.1

Instance details

Defined in GHC.Read

Read Ordering Source

Since: base-2.1

Instance details

Defined in GHC.Read

Read Integer Source

Since: base-2.1

Instance details

Defined in GHC.Read

Read Natural Source

Since: base-4.8.0.0

Instance details

Defined in GHC.Read

Read () Source

Since: base-2.1

Instance details

Defined in GHC.Read

Read Bool Source

Since: base-2.1

Instance details

Defined in GHC.Read

Read Char Source

Since: base-2.1

Instance details

Defined in GHC.Read

Read Double Source

Since: base-2.1

Instance details

Defined in GHC.Read

Read Float Source

Since: base-2.1

Instance details

Defined in GHC.Read

Read Int Source

Since: base-2.1

Instance details

Defined in GHC.Read

Read Word Source

Since: base-4.5.0.0

Instance details

Defined in GHC.Read

Read a => Read (ZipList a) Source

Since: base-4.7.0.0

Instance details

Defined in Control.Applicative

Read a => Read (And a) Source

Since: base-4.16

Instance details

Defined in Data.Bits

Read a => Read (Iff a) Source

Since: base-4.16

Instance details

Defined in Data.Bits

Read a => Read (Ior a) Source

Since: base-4.16

Instance details

Defined in Data.Bits

Read a => Read (Xor a) Source

Since: base-4.16

Instance details

Defined in Data.Bits

Read a => Read (Complex a) Source

Since: base-2.1

Instance details

Defined in Data.Complex

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: base-4.8.0.0

Instance details

Defined in Data.Functor.Identity

Read a => Read (First a) Source

Since: base-2.1

Instance details

Defined in Data.Monoid

Read a => Read (Last a) Source

Since: base-2.1

Instance details

Defined in Data.Monoid

Read a => Read (Down a) Source

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 Data.Ord

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 (Dual a) Source

Since: base-2.1

Instance details

Defined in Data.Semigroup.Internal

Read a => Read (Product a) Source

Since: base-2.1

Instance details

Defined in Data.Semigroup.Internal

Read a => Read (Sum a) Source

Since: base-2.1

Instance details

Defined in Data.Semigroup.Internal

Read p => Read (Par1 p) Source

Since: base-4.7.0.0

Instance details

Defined in GHC.Generics

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

Since: base-2.1

Instance details

Defined in GHC.Read

Read a => Read (NonEmpty a) Source

Since: base-4.11.0.0

Instance details

Defined in GHC.Read

Read a => Read (Maybe a) Source

Since: base-2.1

Instance details

Defined in GHC.Read

Read a => Read (a) Source

Since: base-4.15

Instance details

Defined in GHC.Read

Read a => Read [a] Source

Since: base-2.1

Instance details

Defined in GHC.Read

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

Since: base-3.0

Instance details

Defined in Data.Either

HasResolution a => Read (Fixed a) Source

Since: base-4.3.0.0

Instance details

Defined in Data.Fixed

Read (Proxy t) Source

Since: base-4.7.0.0

Instance details

Defined in Data.Proxy

(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) Source

Since: base-2.1

Instance details

Defined in GHC.Read

Read (U1 p) Source

Since: base-4.9.0.0

Instance details

Defined in GHC.Generics

Read (V1 p) Source

Since: base-4.9.0.0

Instance details

Defined in GHC.Generics

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

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 a => Read (Const a b) Source

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 Data.Functor.Const

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

Since: base-4.12.0.0

Instance details

Defined in Data.Monoid

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

Since: base-4.8.0.0

Instance details

Defined in Data.Semigroup.Internal

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

Since: base-4.7.0.0

Instance details

Defined in Data.Type.Coercion

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

Since: base-4.7.0.0

Instance details

Defined in Data.Type.Equality

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

Since: base-4.7.0.0

Instance details

Defined in GHC.Generics

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

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

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

Since: base-4.9.0.0

Instance details

Defined in Data.Functor.Product

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

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

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

Since: base-4.10.0.0

Instance details

Defined in Data.Type.Equality

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

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) Source

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 c => Read (K1 i c p) Source

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

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

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

Since: base-4.9.0.0

Instance details

Defined in Data.Functor.Compose

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

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 (M1 i c f p) Source

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

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

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

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) Source

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) Source

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) Source

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) Source

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) Source

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) Source

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) Source

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) Source

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) Source

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
Instances details
Read Lexeme Source

Since: base-2.1

Instance details

Defined in GHC.Read

Show Lexeme Source

Since: base-2.1

Instance details

Defined in Text.Read.Lex

Eq Lexeme Source

Since: base-2.1

Instance details

Defined in Text.Read.Lex

Methods

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

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

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/9.4.2/docs/libraries/base-4.17.0.0/Text-Read.html