base-4.14.0.0: Basic libraries
Copyright(c) The University of Glasgow 1994-2002
Licensesee libraries/base/LICENSE
Maintainercvs-ghc@haskell.org
Stabilityinternal
Portabilitynon-portable (GHC Extensions)
Safe HaskellTrustworthy
LanguageHaskell2010

GHC.Read

Description

The Read class and instances for basic data types.

Synopsis

Documentation

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 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 Version #

Since: base-2.1

Instance details

Defined in Data.Version

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 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 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 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 CNfds # 
Instance details

Defined in System.Posix.Types

Read CSocklen # 
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 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) #

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 (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

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 #

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

Since: base-2.1

Instance details

Defined in GHC.Read

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

HasResolution a => Read (Fixed a) #

Since: base-4.3.0.0

Instance details

Defined in Data.Fixed

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 getConst 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).

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

lexLitChar :: ReadS String Source #

Read a string representation of a character, using Haskell source-language escape conventions. For example:

lexLitChar  "\\nHello"  =  [("\\n", "Hello")]

readLitChar :: ReadS Char Source #

Read a string representation of a character, using Haskell source-language escape conventions, and convert it to the character that it encodes. For example:

readLitChar "\\nHello"  =  [('\n', "Hello")]

lexDigits :: ReadS String Source #

Reads a non-empty string of decimal digits.

lexP :: ReadPrec Lexeme Source #

Parse a single lexeme

paren :: ReadPrec a -> ReadPrec a Source #

(paren p) parses "(P0)" where p parses "P0" in precedence context zero

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

list :: ReadPrec a -> ReadPrec [a] Source #

(list p) parses a list of things parsed by p, using the usual square-bracket syntax.

choose :: [(String, ReadPrec a)] -> ReadPrec a Source #

Parse the specified lexeme and continue as specified. Esp useful for nullary constructors; e.g. choose [("A", return A), ("B", return B)] We match both Ident and Symbol because the constructor might be an operator eg (:~:)

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).

readField :: String -> ReadPrec a -> ReadPrec a Source #

Read parser for a record field, of the form fieldName=value. The fieldName must be an alphanumeric identifier; for symbols (operator-style) field names, e.g. (#), use readSymField). The second argument is a parser for the field value.

readFieldHash :: String -> ReadPrec a -> ReadPrec a Source #

Read parser for a record field, of the form fieldName#=value. That is, an alphanumeric identifier fieldName followed by the symbol #. The second argument is a parser for the field value.

Note that readField does not suffice for this purpose due to #5041.

readSymField :: String -> ReadPrec a -> ReadPrec a Source #

Read parser for a symbol record field, of the form (###)=value (where ### is the field name). The field name must be a symbol (operator-style), e.g. (#). For regular (alphanumeric) field names, use readField. The second argument is a parser for the field value.

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.