base-4.10.1.0: 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

Read Char Source #

Since: 2.1

Read Double Source #

Since: 2.1

Read Float Source #

Since: 2.1

Read Int Source #

Since: 2.1

Read Int8 Source #

Since: 2.1

Read Int16 Source #

Since: 2.1

Read Int32 Source #

Since: 2.1

Read Int64 Source #

Since: 2.1

Read Integer Source #

Since: 2.1

Read Natural Source #

Since: 4.8.0.0

Read Ordering Source #

Since: 2.1

Read Word Source #

Since: 4.5.0.0

Read Word8 Source #

Since: 2.1

Read Word16 Source #

Since: 2.1

Read Word32 Source #

Since: 2.1

Read Word64 Source #

Since: 2.1

Read () Source #

Since: 2.1

Read GeneralCategory Source # 
Read Lexeme Source #

Since: 2.1

Read IOMode Source # 
Read IntPtr Source # 
Read WordPtr Source # 
Read CUIntMax Source # 
Read CIntMax Source # 
Read CUIntPtr Source # 
Read CIntPtr Source # 
Read CSUSeconds Source # 
Read CUSeconds Source # 
Read CTime Source # 
Read CClock Source # 
Read CSigAtomic Source # 
Read CWchar Source # 
Read CSize Source # 
Read CPtrdiff Source # 
Read CDouble Source # 
Read CFloat Source # 
Read CBool Source # 
Read CULLong Source # 
Read CLLong Source # 
Read CULong Source # 
Read CLong Source # 
Read CUInt Source # 
Read CInt Source # 
Read CUShort Source # 
Read CShort Source # 
Read CUChar Source # 
Read CSChar Source # 
Read CChar Source # 
Read SomeNat Source #

Since: 4.7.0.0

Read SomeSymbol Source #

Since: 4.7.0.0

Read DecidedStrictness Source # 
Read SourceStrictness Source # 
Read SourceUnpackedness Source # 
Read Associativity Source # 
Read Fixity Source # 
Read Any Source # 
Read All Source # 
Read SeekMode Source # 
Read NewlineMode Source # 
Read Newline Source # 
Read BufferMode Source # 
Read ExitCode Source # 
Read Fd Source # 
Read CKey Source # 
Read CId Source # 
Read CFsFilCnt Source # 
Read CFsBlkCnt Source # 
Read CClockId Source # 
Read CBlkCnt Source # 
Read CBlkSize Source # 
Read CRLim Source # 
Read CTcflag Source # 
Read CSpeed Source # 
Read CCc Source # 
Read CUid Source # 
Read CNlink Source # 
Read CGid Source # 
Read CSsize Source # 
Read CPid Source # 
Read COff Source # 
Read CMode Source # 
Read CIno Source # 
Read CDev Source # 
Read Version Source # 
Read GCStats Source # 
Read GCDetails Source # 
Read RTSStats Source # 
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

Read a => Read [a] Source #

Since: 2.1

Read a => Read (Maybe a) Source #

Since: 2.1

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

Since: 2.1

Read p => Read (Par1 p) Source # 
Read a => Read (Down a) Source # 
Read a => Read (Last a) Source # 
Read a => Read (First a) Source # 
Read a => Read (Product a) Source # 
Read a => Read (Sum a) Source # 
Read a => Read (Dual a) Source # 
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

Read a => Read (ZipList a) Source # 
Read a => Read (NonEmpty a) Source # 
Read a => Read (Option a) Source # 
Read m => Read (WrappedMonoid m) Source # 
Read a => Read (Last a) Source # 
Read a => Read (First a) Source # 
Read a => Read (Max a) Source # 
Read a => Read (Min a) Source # 
HasResolution a => Read (Fixed a) Source #

Since: 4.3.0.0

Read a => Read (Complex a) Source # 
(Read b, Read a) => Read (Either a b) Source # 
Read (V1 k p) Source # 
Read (U1 k p) Source #

Since: 4.9.0.0

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

Since: 2.1

Methods

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

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

readPrec :: ReadPrec (a, b) Source #

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

Read (Proxy k s) Source #

Since: 4.7.0.0

(Read b, Read a) => Read (Arg a b) Source # 
Read (f p) => Read (Rec1 k f p) Source # 
(Read a, Read b, Read c) => Read (a, b, c) Source #

Since: 2.1

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 #

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

Since: 4.7.0.0

Methods

readsPrec :: Int -> ReadS ((k :~: a) b) Source #

readList :: ReadS [(k :~: a) b] Source #

readPrec :: ReadPrec ((k :~: a) b) Source #

readListPrec :: ReadPrec [(k :~: a) b] Source #

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

Since: 4.7.0.0

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

Methods

readsPrec :: Int -> ReadS (Alt k f a) Source #

readList :: ReadS [Alt k f a] Source #

readPrec :: ReadPrec (Alt k f a) Source #

readListPrec :: ReadPrec [Alt k f a] Source #

Read a => Read (Const k 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

Read c => Read (K1 k i c p) Source # 

Methods

readsPrec :: Int -> ReadS (K1 k i c p) Source #

readList :: ReadS [K1 k i c p] Source #

readPrec :: ReadPrec (K1 k i c p) Source #

readListPrec :: ReadPrec [K1 k i c p] Source #

(Read (g p), Read (f p)) => Read ((:+:) k f g p) Source # 

Methods

readsPrec :: Int -> ReadS ((k :+: f) g p) Source #

readList :: ReadS [(k :+: f) g p] Source #

readPrec :: ReadPrec ((k :+: f) g p) Source #

readListPrec :: ReadPrec [(k :+: f) g p] Source #

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

Methods

readsPrec :: Int -> ReadS ((k :*: f) g p) Source #

readList :: ReadS [(k :*: f) g p] Source #

readPrec :: ReadPrec ((k :*: f) g p) Source #

readListPrec :: ReadPrec [(k :*: f) g p] Source #

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

Since: 2.1

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 #

(~~) k1 k2 a b => Read ((:~~:) k1 k2 a b) Source #

Since: 4.10.0.0

Methods

readsPrec :: Int -> ReadS ((k1 :~~: k2) a b) Source #

readList :: ReadS [(k1 :~~: k2) a b] Source #

readPrec :: ReadPrec ((k1 :~~: k2) a b) Source #

readListPrec :: ReadPrec [(k1 :~~: k2) a b] Source #

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

Since: 4.9.0.0

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

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

Methods

readsPrec :: Int -> ReadS (M1 k i c f p) Source #

readList :: ReadS [M1 k i c f p] Source #

readPrec :: ReadPrec (M1 k i c f p) Source #

readListPrec :: ReadPrec [M1 k i c f p] Source #

Read (f (g p)) => Read ((:.:) k2 k1 f g p) Source # 

Methods

readsPrec :: Int -> ReadS ((k2 :.: k1) f g p) Source #

readList :: ReadS [(k2 :.: k1) f g p] Source #

readPrec :: ReadPrec ((k2 :.: k1) f g p) Source #

readListPrec :: ReadPrec [(k2 :.: k1) f g p] Source #

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

Since: 2.1

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

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

Since: 2.1

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

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

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

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

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

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

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

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

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

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.

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 

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.

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.

Since: 4.6.0.0