| base-3.0.0.0: Basic libraries | Contents | Index

Text.Read |

Portability | non-portable (uses Text.ParserCombinators.ReadP)
Stability | provisional
Maintainer | libraries@haskell.org

Contents

The `Read` class
Haskell 98 functions
New parsing functions

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 98 `Read`. In particular, writing parsers is easier, and the parsers are much more efficient.
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
| Int Integer
| Rat Rational
| EOF

lexP :: ReadPrec Lexeme
parens :: ReadPrec a -> ReadPrec a
readListDefault :: Read a => ReadS [a]
readListPrecDefault :: Read a => ReadPrec [a]

The `Read` class
class Read a where

Parsing of `String`s, producing values. Minimal complete definition: `readsPrec` (or, for GHC only, `readPrec`) 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 98 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

Methods

readsPrec

:: 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]
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 All
Read Any
Read Bool
Read BufferMode
Read CChar
Read CClock
Read CDev
Read CDouble
Read CFloat
Read CIno
Read CInt
Read CIntMax
Read CIntPtr
Read CLDouble
Read CLLong
Read CLong
Read CMode
Read COff
Read CPid
Read CPtrdiff
Read CSChar
Read CShort
Read CSigAtomic
Read CSize
Read CSsize
Read CTime
Read CUChar
Read CUInt
Read CUIntMax
Read CUIntPtr
Read CULLong
Read CULong
Read CUShort
Read CWchar
Read Char
Read ConsoleEvent
Read Double
Read ExitCode
Read Fd
Read Float
Read GeneralCategory
Read IOMode
Read Int
Read Int16
Read Int32
Read Int64
Read Int8
Read IntPtr
Read Integer
Read Lexeme
Read Ordering
Read SeekMode
Read Version
Read Word
Read Word16
Read Word32
Read Word64
Read Word8
Read WordPtr
Read ()
(Read a, Read b) => Read (a, b)
(Read a, Read b, Read c) => Read (a, b, c)
(Read a, Read b, Read c, Read d) => Read (a, b, c, d)
(Read a, Read b, Read c, Read d, Read e) => Read (a, b, c, d, e)
(Read a, Read b, Read c, Read d, Read e, Read f) => Read (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)
(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)
(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)
(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)
(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)
(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)
(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)
(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)
(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)
(RealFloat a, Read a) => Read (Complex a)
Read a => Read (Dual a)
Read a => Read (First a)
Read a => Read (Last a)
Read a => Read (Maybe a)
Read a => Read (Product a)
(Integral a, Read a) => Read (Ratio a)
Read a => Read (Sum a)
Read a => Read [a]
(Ix a, Read a, Read b) => Read (Array a b)
(Read a, Read b) => Read (Either a b)

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 98 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
module Text.ParserCombinators.ReadPrec
data Lexeme

Haskell lexemes.
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. `>>`, `:%`
Int Integer | Integer literal
Rat Rational | Floating point literal
EOF |

Instances

Eq Lexeme
Read Lexeme
Show Lexeme

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).
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