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 #

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 #

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] #

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 #

Proposed replacement for readsPrec using new-style parsers (GHC only).

readListPrec :: ReadPrec [a] #

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 #

Since: 2.1

Read Char #

Since: 2.1

Read Double #

Since: 2.1

Read Float #

Since: 2.1

Read Int #

Since: 2.1

Read Int8 #

Since: 2.1

Read Int16 #

Since: 2.1

Read Int32 #

Since: 2.1

Read Int64 #

Since: 2.1

Read Integer #

Since: 2.1

Read Natural #

Since: 4.8.0.0

Read Ordering #

Since: 2.1

Read Word #

Since: 4.5.0.0

Read Word8 #

Since: 2.1

Read Word16 #

Since: 2.1

Read Word32 #

Since: 2.1

Read Word64 #

Since: 2.1

Read () #

Since: 2.1

Methods

readsPrec :: Int -> ReadS () #

readList :: ReadS [()] #

readPrec :: ReadPrec () #

readListPrec :: ReadPrec [()] #

Read GeneralCategory # 
Read Lexeme #

Since: 2.1

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

Since: 4.7.0.0

Read SomeSymbol #

Since: 4.7.0.0

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

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] #

Since: 2.1

Methods

readsPrec :: Int -> ReadS [a] #

readList :: ReadS [[a]] #

readPrec :: ReadPrec [a] #

readListPrec :: ReadPrec [[a]] #

Read a => Read (Maybe a) #

Since: 2.1

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

Since: 2.1

Read p => Read (Par1 p) # 
Read a => Read (Down a) # 
Read a => Read (Last a) # 
Read a => Read (First a) # 
Read a => Read (Product a) # 
Read a => Read (Sum a) # 
Read a => Read (Dual a) # 
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: 4.8.0.0

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

Since: 4.3.0.0

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

Methods

readsPrec :: Int -> ReadS (V1 k p) #

readList :: ReadS [V1 k p] #

readPrec :: ReadPrec (V1 k p) #

readListPrec :: ReadPrec [V1 k p] #

Read (U1 k p) #

Since: 4.9.0.0

Methods

readsPrec :: Int -> ReadS (U1 k p) #

readList :: ReadS [U1 k p] #

readPrec :: ReadPrec (U1 k p) #

readListPrec :: ReadPrec [U1 k p] #

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

Since: 2.1

Methods

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

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

readPrec :: ReadPrec (a, b) #

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

Read (Proxy k s) #

Since: 4.7.0.0

(Read b, Read a) => Read (Arg a b) # 

Methods

readsPrec :: Int -> ReadS (Arg a b) #

readList :: ReadS [Arg a b] #

readPrec :: ReadPrec (Arg a b) #

readListPrec :: ReadPrec [Arg a b] #

Read (f p) => Read (Rec1 k f p) # 

Methods

readsPrec :: Int -> ReadS (Rec1 k f p) #

readList :: ReadS [Rec1 k f p] #

readPrec :: ReadPrec (Rec1 k f p) #

readListPrec :: ReadPrec [Rec1 k f p] #

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

Since: 2.1

Methods

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

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

readPrec :: ReadPrec (a, b, c) #

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

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

Since: 4.7.0.0

Methods

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

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

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

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

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

Since: 4.7.0.0

Read (f a) => Read (Alt k f a) # 

Methods

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

readList :: ReadS [Alt k f a] #

readPrec :: ReadPrec (Alt k f a) #

readListPrec :: ReadPrec [Alt k f a] #

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

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

Since: 4.8.0.0

Methods

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

readList :: ReadS [Const k a b] #

readPrec :: ReadPrec (Const k a b) #

readListPrec :: ReadPrec [Const k a b] #

Read c => Read (K1 k i c p) # 

Methods

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

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

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

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

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

Methods

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

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

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

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

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

Methods

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

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

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

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

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

Since: 2.1

Methods

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

readList :: ReadS [(a, b, c, d)] #

readPrec :: ReadPrec (a, b, c, d) #

readListPrec :: ReadPrec [(a, b, c, d)] #

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

Since: 4.10.0.0

Methods

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

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

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

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

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

Since: 4.9.0.0

Methods

readsPrec :: Int -> ReadS (Sum * f g a) #

readList :: ReadS [Sum * f g a] #

readPrec :: ReadPrec (Sum * f g a) #

readListPrec :: ReadPrec [Sum * f g a] #

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

Since: 4.9.0.0

Methods

readsPrec :: Int -> ReadS (Product * f g a) #

readList :: ReadS [Product * f g a] #

readPrec :: ReadPrec (Product * f g a) #

readListPrec :: ReadPrec [Product * f g a] #

Read (f p) => Read (M1 k i c f p) # 

Methods

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

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

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

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

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

Methods

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

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

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

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

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

Since: 2.1

Methods

readsPrec :: Int -> ReadS (a, b, c, d, e) #

readList :: ReadS [(a, b, c, d, e)] #

readPrec :: ReadPrec (a, b, c, d, e) #

readListPrec :: ReadPrec [(a, b, c, d, e)] #

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

Since: 4.9.0.0

Methods

readsPrec :: Int -> ReadS (Compose * * f g a) #

readList :: ReadS [Compose * * f g a] #

readPrec :: ReadPrec (Compose * * f g a) #

readListPrec :: ReadPrec [Compose * * f g a] #

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

Since: 2.1

Methods

readsPrec :: Int -> ReadS (a, b, c, d, e, f) #

readList :: ReadS [(a, b, c, d, e, f)] #

readPrec :: ReadPrec (a, b, c, d, e, f) #

readListPrec :: ReadPrec [(a, b, c, d, e, f)] #

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

Since: 2.1

Methods

readsPrec :: Int -> ReadS (a, b, c, d, e, f, g) #

readList :: ReadS [(a, b, c, d, e, f, g)] #

readPrec :: ReadPrec (a, b, c, d, e, f, g) #

readListPrec :: ReadPrec [(a, b, c, d, e, f, g)] #

(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: 2.1

Methods

readsPrec :: Int -> ReadS (a, b, c, d, e, f, g, h) #

readList :: ReadS [(a, b, c, d, e, f, g, h)] #

readPrec :: ReadPrec (a, b, c, d, e, f, g, h) #

readListPrec :: ReadPrec [(a, b, c, d, e, f, g, h)] #

(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: 2.1

Methods

readsPrec :: Int -> ReadS (a, b, c, d, e, f, g, h, i) #

readList :: ReadS [(a, b, c, d, e, f, g, h, i)] #

readPrec :: ReadPrec (a, b, c, d, e, f, g, h, i) #

readListPrec :: ReadPrec [(a, b, c, d, e, f, g, h, i)] #

(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: 2.1

Methods

readsPrec :: Int -> ReadS (a, b, c, d, e, f, g, h, i, j) #

readList :: ReadS [(a, b, c, d, e, f, g, h, i, j)] #

readPrec :: ReadPrec (a, b, c, d, e, f, g, h, i, j) #

readListPrec :: ReadPrec [(a, b, c, d, e, f, g, h, i, j)] #

(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: 2.1

Methods

readsPrec :: Int -> ReadS (a, b, c, d, e, f, g, h, i, j, k) #

readList :: ReadS [(a, b, c, d, e, f, g, h, i, j, k)] #

readPrec :: ReadPrec (a, b, c, d, e, f, g, h, i, j, k) #

readListPrec :: ReadPrec [(a, b, c, d, e, f, g, h, i, j, k)] #

(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: 2.1

Methods

readsPrec :: Int -> ReadS (a, b, c, d, e, f, g, h, i, j, k, l) #

readList :: ReadS [(a, b, c, d, e, f, g, h, i, j, k, l)] #

readPrec :: ReadPrec (a, b, c, d, e, f, g, h, i, j, k, l) #

readListPrec :: ReadPrec [(a, b, c, d, e, f, g, h, i, j, k, l)] #

(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: 2.1

Methods

readsPrec :: Int -> ReadS (a, b, c, d, e, f, g, h, i, j, k, l, m) #

readList :: ReadS [(a, b, c, d, e, f, g, h, i, j, k, l, m)] #

readPrec :: ReadPrec (a, b, c, d, e, f, g, h, i, j, k, l, m) #

readListPrec :: ReadPrec [(a, b, c, d, e, f, g, h, i, j, k, l, m)] #

(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: 2.1

Methods

readsPrec :: Int -> ReadS (a, b, c, d, e, f, g, h, i, j, k, l, m, n) #

readList :: ReadS [(a, b, c, d, e, f, g, h, i, j, k, l, m, n)] #

readPrec :: ReadPrec (a, b, c, d, e, f, g, h, i, j, k, l, m, n) #

readListPrec :: ReadPrec [(a, b, c, d, e, f, g, h, i, j, k, l, m, n)] #

(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: 2.1

Methods

readsPrec :: Int -> ReadS (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o) #

readList :: ReadS [(a, b, c, d, e, f, g, h, i, j, k, l, m, n, o)] #

readPrec :: ReadPrec (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o) #

readListPrec :: ReadPrec [(a, b, c, d, e, f, g, h, i, j, k, l, m, n, o)] #

type ReadS a = String -> [(a, String)] #

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 #

equivalent to readsPrec with a precedence of 0.

read :: Read a => String -> a #

The read function reads input from a string, which must be completely consumed by the input process.

readParen :: Bool -> ReadS a -> ReadS a #

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 #

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 #

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 #

Parse a single lexeme

parens :: ReadPrec a -> ReadPrec a #

(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] #

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] #

A possible replacement definition for the readListPrec method, defined using readPrec (GHC only).

readEither :: Read a => String -> Either String a #

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 #

Parse a string using the Read instance. Succeeds if there is exactly one valid result.

Since: 4.6.0.0