base-compat-0.13.1: A compatibility layer for base

Safe Haskell | Safe-Inferred
Language | Haskell2010

Text.Read.Compat

Contents

- The `Read` class
- Haskell 2010 functions
- New parsing functions

Synopsis

- class Read a where
  - readsPrec :: Int -> ReadS a
  - readList :: ReadS [a]
  - readPrec :: ReadPrec a
  - readListPrec :: ReadPrec [a]
- type ReadS a = String -> [(a, String)]
- reads :: Read a => ReadS a
- read :: Read a => String -> a
- readParen :: Bool -> ReadS a -> ReadS a
- lex :: ReadS String
- module Text.ParserCombinators.ReadPrec
- data Lexeme
  - = Char Char
  - | String String
  - | Punc String
  - | Ident String
  - | Symbol String
  - | Number Number
  - | EOF
- lexP :: ReadPrec Lexeme
- parens :: ReadPrec a -> ReadPrec a
- readListDefault :: Read a => ReadS [a]
- readListPrecDefault :: Read a => ReadPrec [a]
- readEither :: Read a => String -> Either String a
- readMaybe :: Read a => String -> Maybe a

# The `Read` class

class Read a where Source #

Parsing of `String`s, 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:

- `(x,"")` is an element of `(`readsPrec` d (`showsPrec` d x ""))`.

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 | Since: base-2.1
Instance details Defined in Data.Semigroup.Internal Methods readsPrec :: Int -> ReadS All Source # readList :: ReadS [All] Source # readPrec :: ReadPrec All Source # readListPrec :: ReadPrec [All] Source #
Read Any | Since: base-2.1
Instance details Defined in Data.Semigroup.Internal Methods readsPrec :: Int -> ReadS Any Source # readList :: ReadS [Any] Source # readPrec :: ReadPrec Any Source # readListPrec :: ReadPrec [Any] Source #
Read Version | Since: base-2.1
Instance details Defined in Data.Version Methods readsPrec :: Int -> ReadS Version Source # readList :: ReadS [Version] Source # readPrec :: ReadPrec Version Source # readListPrec :: ReadPrec [Version] Source #
Read IntPtr |
Instance details Defined in Foreign.Ptr Methods readsPrec :: Int -> ReadS IntPtr Source # readList :: ReadS [IntPtr] Source # readPrec :: ReadPrec IntPtr Source # readListPrec :: ReadPrec [IntPtr] Source #
Read WordPtr |
Instance details Defined in Foreign.Ptr Methods readsPrec :: Int -> ReadS WordPtr Source # readList :: ReadS [WordPtr] Source # readPrec :: ReadPrec WordPtr Source # readListPrec :: ReadPrec [WordPtr] Source #
Read Void | Reading a `Void` value is always a parse error, considering `Void` as a data type with no constructors. Since: base-4.8.0.0
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS Void Source # readList :: ReadS [Void] Source # readPrec :: ReadPrec Void Source # readListPrec :: ReadPrec [Void] Source #
Read SeekMode | Since: base-4.2.0.0
Instance details Defined in GHC.IO.Device Methods readsPrec :: Int -> ReadS SeekMode Source # readList :: ReadS [SeekMode] Source # readPrec :: ReadPrec SeekMode Source # readListPrec :: ReadPrec [SeekMode] Source #
Read ExitCode |
Instance details Defined in GHC.IO.Exception Methods readsPrec :: Int -> ReadS ExitCode Source # readList :: ReadS [ExitCode] Source # readPrec :: ReadPrec ExitCode Source # readListPrec :: ReadPrec [ExitCode] Source #
Read BufferMode | Since: base-4.2.0.0
Instance details Defined in GHC.IO.Handle.Types Methods readsPrec :: Int -> ReadS BufferMode Source # readList :: ReadS [BufferMode] Source # readPrec :: ReadPrec BufferMode Source # readListPrec :: ReadPrec [BufferMode] Source #
Read Newline | Since: base-4.3.0.0
Instance details Defined in GHC.IO.Handle.Types Methods readsPrec :: Int -> ReadS Newline Source # readList :: ReadS [Newline] Source # readPrec :: ReadPrec Newline Source # readListPrec :: ReadPrec [Newline] Source #
Read NewlineMode | Since: base-4.3.0.0
Instance details Defined in GHC.IO.Handle.Types Methods readsPrec :: Int -> ReadS NewlineMode Source # readList :: ReadS [NewlineMode] Source # readPrec :: ReadPrec NewlineMode Source # readListPrec :: ReadPrec [NewlineMode] Source #
Read IOMode | Since: base-4.2.0.0
Instance details Defined in GHC.IO.IOMode Methods readsPrec :: Int -> ReadS IOMode Source # readList :: ReadS [IOMode] Source # readPrec :: ReadPrec IOMode Source # readListPrec :: ReadPrec [IOMode] Source #
Read Int16 | Since: base-2.1
Instance details Defined in GHC.Int Methods readsPrec :: Int -> ReadS Int16 Source # readList :: ReadS [Int16] Source # readPrec :: ReadPrec Int16 Source # readListPrec :: ReadPrec [Int16] Source #
Read Int32 | Since: base-2.1
Instance details Defined in GHC.Int Methods readsPrec :: Int -> ReadS Int32 Source # readList :: ReadS [Int32] Source # readPrec :: ReadPrec Int32 Source # readListPrec :: ReadPrec [Int32] Source #
Read Int64 | Since: base-2.1
Instance details Defined in GHC.Int Methods readsPrec :: Int -> ReadS Int64 Source # readList :: ReadS [Int64] Source # readPrec :: ReadPrec Int64 Source # readListPrec :: ReadPrec [Int64] Source #
Read Int8 | Since: base-2.1
Instance details Defined in GHC.Int Methods readsPrec :: Int -> ReadS Int8 Source # readList :: ReadS [Int8] Source # readPrec :: ReadPrec Int8 Source # readListPrec :: ReadPrec [Int8] Source #
Read GeneralCategory | Since: base-2.1
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS GeneralCategory Source # readList :: ReadS [GeneralCategory] Source # readPrec :: ReadPrec GeneralCategory Source # readListPrec :: ReadPrec [GeneralCategory] Source #
Read Word16 | Since: base-2.1
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS Word16 Source # readList :: ReadS [Word16] Source # readPrec :: ReadPrec Word16 Source # readListPrec :: ReadPrec [Word16] Source #
Read Word32 | Since: base-2.1
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS Word32 Source # readList :: ReadS [Word32] Source # readPrec :: ReadPrec Word32 Source # readListPrec :: ReadPrec [Word32] Source #
Read Word64 | Since: base-2.1
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS Word64 Source # readList :: ReadS [Word64] Source # readPrec :: ReadPrec Word64 Source # readListPrec :: ReadPrec [Word64] Source #
Read Word8 | Since: base-2.1
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS Word8 Source # readList :: ReadS [Word8] Source # readPrec :: ReadPrec Word8 Source # readListPrec :: ReadPrec [Word8] Source #
Read Lexeme | Since: base-2.1
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS Lexeme Source # readList :: ReadS [Lexeme] Source # readPrec :: ReadPrec Lexeme Source # readListPrec :: ReadPrec [Lexeme] Source #
Read Ordering | Since: base-2.1
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS Ordering Source # readList :: ReadS [Ordering] Source # readPrec :: ReadPrec Ordering Source # readListPrec :: ReadPrec [Ordering] Source #
Read Integer | Since: base-2.1
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS Integer Source # readList :: ReadS [Integer] Source # readPrec :: ReadPrec Integer Source # readListPrec :: ReadPrec [Integer] Source #
Read Natural | Since: base-4.8.0.0
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS Natural Source # readList :: ReadS [Natural] Source # readPrec :: ReadPrec Natural Source # readListPrec :: ReadPrec [Natural] Source #
Read () | Since: base-2.1
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS () Source # readList :: ReadS [()] Source # readPrec :: ReadPrec () Source # readListPrec :: ReadPrec [()] Source #
Read Bool | Since: base-2.1
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS Bool Source # readList :: ReadS [Bool] Source # readPrec :: ReadPrec Bool Source # readListPrec :: ReadPrec [Bool] Source #
Read Char | Since: base-2.1
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS Char Source # readList :: ReadS [Char] Source # readPrec :: ReadPrec Char Source # readListPrec :: ReadPrec [Char] Source #
Read Double | Since: base-2.1
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS Double Source # readList :: ReadS [Double] Source # readPrec :: ReadPrec Double Source # readListPrec :: ReadPrec [Double] Source #
Read Float | Since: base-2.1
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS Float Source # readList :: ReadS [Float] Source # readPrec :: ReadPrec Float Source # readListPrec :: ReadPrec [Float] Source #
Read Int | Since: base-2.1
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS Int Source # readList :: ReadS [Int] Source # readPrec :: ReadPrec Int Source # readListPrec :: ReadPrec [Int] Source #
Read Word | Since: base-4.5.0.0
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS Word Source # readList :: ReadS [Word] Source # readPrec :: ReadPrec Word Source # readListPrec :: ReadPrec [Word] Source #
Read a => Read (ZipList a) | Since: base-4.7.0.0
Instance details Defined in Control.Applicative Methods readsPrec :: Int -> ReadS (ZipList a) Source # readList :: ReadS [ZipList a] Source # readPrec :: ReadPrec (ZipList a) Source # readListPrec :: ReadPrec [ZipList a] Source #
Read a => Read (And a) | Since: base-4.16
Instance details Defined in Data.Bits Methods readsPrec :: Int -> ReadS (And a) Source # readList :: ReadS [And a] Source # readPrec :: ReadPrec (And a) Source # readListPrec :: ReadPrec [And a] Source #
Read a => Read (Iff a) | Since: base-4.16
Instance details Defined in Data.Bits Methods readsPrec :: Int -> ReadS (Iff a) Source # readList :: ReadS [Iff a] Source # readPrec :: ReadPrec (Iff a) Source # readListPrec :: ReadPrec [Iff a] Source #
Read a => Read (Ior a) | Since: base-4.16
Instance details Defined in Data.Bits Methods readsPrec :: Int -> ReadS (Ior a) Source # readList :: ReadS [Ior a] Source # readPrec :: ReadPrec (Ior a) Source # readListPrec :: ReadPrec [Ior a] Source #
Read a => Read (Xor a) | Since: base-4.16
Instance details Defined in Data.Bits Methods readsPrec :: Int -> ReadS (Xor a) Source # readList :: ReadS [Xor a] Source # readPrec :: ReadPrec (Xor a) Source # readListPrec :: ReadPrec [Xor a] Source #
Read a => Read (Complex a) | Since: base-2.1
Instance details Defined in Data.Complex Methods readsPrec :: Int -> ReadS (Complex a) Source # readList :: ReadS [Complex a] Source # readPrec :: ReadPrec (Complex a) Source # readListPrec :: ReadPrec [Complex a] Source #
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 Methods readsPrec :: Int -> ReadS (Identity a) Source # readList :: ReadS [Identity a] Source # readPrec :: ReadPrec (Identity a) Source # readListPrec :: ReadPrec [Identity a] Source #
Read a => Read (First a) | Since: base-2.1
Instance details Defined in Data.Monoid Methods readsPrec :: Int -> ReadS (First a) Source # readList :: ReadS [First a] Source # readPrec :: ReadPrec (First a) Source # readListPrec :: ReadPrec [First a] Source #
Read a => Read (Last a) | Since: base-2.1
Instance details Defined in Data.Monoid Methods readsPrec :: Int -> ReadS (Last a) Source # readList :: ReadS [Last a] Source # readPrec :: ReadPrec (Last a) Source # readListPrec :: ReadPrec [Last a] Source #
Read a => Read (First a) | Since: base-4.9.0.0
Instance details Defined in Data.Semigroup Methods readsPrec :: Int -> ReadS (First a) Source # readList :: ReadS [First a] Source # readPrec :: ReadPrec (First a) Source # readListPrec :: ReadPrec [First a] Source #
Read a => Read (Last a) | Since: base-4.9.0.0
Instance details Defined in Data.Semigroup Methods readsPrec :: Int -> ReadS (Last a) Source # readList :: ReadS [Last a] Source # readPrec :: ReadPrec (Last a) Source # readListPrec :: ReadPrec [Last a] Source #
Read a => Read (Max a) | Since: base-4.9.0.0
Instance details Defined in Data.Semigroup Methods readsPrec :: Int -> ReadS (Max a) Source # readList :: ReadS [Max a] Source # readPrec :: ReadPrec (Max a) Source # readListPrec :: ReadPrec [Max a] Source #
Read a => Read (Min a) | Since: base-4.9.0.0
Instance details Defined in Data.Semigroup Methods readsPrec :: Int -> ReadS (Min a) Source # readList :: ReadS [Min a] Source # readPrec :: ReadPrec (Min a) Source # readListPrec :: ReadPrec [Min a] Source #
Read m => Read (WrappedMonoid m) | Since: base-4.9.0.0
Instance details Defined in Data.Semigroup Methods readsPrec :: Int -> ReadS (WrappedMonoid m) Source # readList :: ReadS [WrappedMonoid m] Source # readPrec :: ReadPrec (WrappedMonoid m) Source # readListPrec :: ReadPrec [WrappedMonoid m] Source #
Read a => Read (Dual a) | Since: base-2.1
Instance details Defined in Data.Semigroup.Internal Methods readsPrec :: Int -> ReadS (Dual a) Source # readList :: ReadS [Dual a] Source # readPrec :: ReadPrec (Dual a) Source # readListPrec :: ReadPrec [Dual a] Source #
Read a => Read (Product a) | Since: base-2.1
Instance details Defined in Data.Semigroup.Internal Methods readsPrec :: Int -> ReadS (Product a) Source # readList :: ReadS [Product a] Source # readPrec :: ReadPrec (Product a) Source # readListPrec :: ReadPrec [Product a] Source #
Read a => Read (Sum a) | Since: base-2.1
Instance details Defined in Data.Semigroup.Internal Methods readsPrec :: Int -> ReadS (Sum a) Source # readList :: ReadS [Sum a] Source # readPrec :: ReadPrec (Sum a) Source # readListPrec :: ReadPrec [Sum a] Source #
Read a => Read (NonEmpty a) | Since: base-4.11.0.0
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS (NonEmpty a) Source # readList :: ReadS [NonEmpty a] Source # readPrec :: ReadPrec (NonEmpty a) Source # readListPrec :: ReadPrec [NonEmpty a] Source #
(Integral a, Read a) => Read (Ratio a) | Since: base-2.1
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS (Ratio a) Source # readList :: ReadS [Ratio a] Source # readPrec :: ReadPrec (Ratio a) Source # readListPrec :: ReadPrec [Ratio a] Source #
Read a => Read (Maybe a) | Since: base-2.1
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS (Maybe a) Source # readList :: ReadS [Maybe a] Source # readPrec :: ReadPrec (Maybe a) Source # readListPrec :: ReadPrec [Maybe a] Source #
Read a => Read (a) | Since: base-4.15
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS (a) Source # readList :: ReadS [(a)] Source # readPrec :: ReadPrec (a) Source # readListPrec :: ReadPrec [(a)] Source #
Read a => Read [a] | Since: base-2.1
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS [a] Source # readList :: ReadS [[a]] Source # readPrec :: ReadPrec [a] Source # readListPrec :: ReadPrec [[a]] Source #
(Read a, Read b) => Read (Either a b) | Since: base-3.0
Instance details Defined in Data.Either Methods readsPrec :: Int -> ReadS (Either a b) Source # readList :: ReadS [Either a b] Source # readPrec :: ReadPrec (Either a b) Source # readListPrec :: ReadPrec [Either a b] Source #
Read (Proxy t) | Since: base-4.7.0.0
Instance details Defined in Data.Proxy Methods readsPrec :: Int -> ReadS (Proxy t) Source # readList :: ReadS [Proxy t] Source # readPrec :: ReadPrec (Proxy t) Source # readListPrec :: ReadPrec [Proxy t] Source #
(Read a, Read b) => Read (Arg a b) | Since: base-4.9.0.0
Instance details Defined in Data.Semigroup Methods readsPrec :: Int -> ReadS (Arg a b) Source # readList :: ReadS [Arg a b] Source # readPrec :: ReadPrec (Arg a b) Source # readListPrec :: ReadPrec [Arg a b] Source #
(Ix a, Read a, Read b) => Read (Array a b) | Since: base-2.1
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS (Array a b) Source # readList :: ReadS [Array a b] Source # readPrec :: ReadPrec (Array a b) Source # readListPrec :: ReadPrec [Array a b] Source #
(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 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 Methods readsPrec :: Int -> ReadS (Const a b) Source # readList :: ReadS [Const a b] Source # readPrec :: ReadPrec (Const a b) Source # readListPrec :: ReadPrec [Const a b] Source #
Read (f a) => Read (Ap f a) | Since: base-4.12.0.0
Instance details Defined in Data.Monoid Methods readsPrec :: Int -> ReadS (Ap f a) Source # readList :: ReadS [Ap f a] Source # readPrec :: ReadPrec (Ap f a) Source # readListPrec :: ReadPrec [Ap f a] Source #
Read (f a) => Read (Alt f a) | Since: base-4.8.0.0
Instance details Defined in Data.Semigroup.Internal Methods readsPrec :: Int -> ReadS (Alt f a) Source # readList :: ReadS [Alt f a] Source # readPrec :: ReadPrec (Alt f a) Source # readListPrec :: ReadPrec [Alt f a] Source #
Coercible a b => Read (Coercion a b) | Since: base-4.7.0.0
Instance details Defined in Data.Type.Coercion Methods readsPrec :: Int -> ReadS (Coercion a b) Source # readList :: ReadS [Coercion a b] Source # readPrec :: ReadPrec (Coercion a b) Source # readListPrec :: ReadPrec [Coercion a b] Source #
a ~ b => Read (a :~: b) | Since: base-4.7.0.0
Instance details Defined in Data.Type.Equality 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 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 #
(Read (f a), Read (g a)) => Read (Product f g a) | Since: base-4.18.0.0
Instance details Defined in Data.Functor.Product Methods readsPrec :: Int -> ReadS (Product f g a) Source # readList :: ReadS [Product f g a] Source # readPrec :: ReadPrec (Product f g a) Source # readListPrec :: ReadPrec [Product f g a] Source #
(Read (f a), Read (g a)) => Read (Sum f g a) | Since: base-4.18.0.0
Instance details Defined in Data.Functor.Sum Methods readsPrec :: Int -> ReadS (Sum f g a) Source # readList :: ReadS [Sum f g a] Source # readPrec :: ReadPrec (Sum f g a) Source # readListPrec :: ReadPrec [Sum f g a] Source #
a ~~ b => Read (a :~~: b) | Since: base-4.10.0.0
Instance details Defined in Data.Type.Equality 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 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 #
Read (f (g a)) => Read (Compose f g a) | Since: base-4.18.0.0
Instance details Defined in Data.Functor.Compose Methods readsPrec :: Int -> ReadS (Compose f g a) Source # readList :: ReadS [Compose f g a] Source # readPrec :: ReadPrec (Compose f g a) Source # readListPrec :: ReadPrec [Compose f g a] 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 #
(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

Instances details

Read Lexeme | Since: base-2.1
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS Lexeme Source # readList :: ReadS [Lexeme] Source # readPrec :: ReadPrec Lexeme Source # readListPrec :: ReadPrec [Lexeme] Source #
Show Lexeme | Since: base-2.1
Instance details Defined in Text.Read.Lex Methods showsPrec :: Int -> Lexeme -> ShowS Source # show :: Lexeme -> String Source # showList :: [Lexeme] -> ShowS Source #
Eq Lexeme | 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