| Copyright | (c) The University of Glasgow 2001 |
|---|---|
| License | BSD-style (see the file libraries/base/LICENSE) |
| Maintainer | [email protected] |
| Stability | provisional |
| Portability | non-portable (uses Text.ParserCombinators.ReadP) |
| Safe Haskell | Trustworthy |
| Language | Haskell2010 |
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.
Parsing of Strings, producing values.
Derived instances of Read make the following assumptions, which derived instances of Show obey:
Read instance will parse only infix applications of the constructor (not the prefix form).Read will parse only the record-syntax form, and furthermore, the fields must be given in the same order as the original declaration.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:
instanceReadT wherereadPrec= ...readListPrec=readListPrecDefault
| :: Int | the operator precedence of the enclosing context (a number from |
| -> 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.
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.
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.
| 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 |
| 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 |
| Read a => Read [a] | Since: 2.1 |
| 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 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) | |
| Read (U1 k p) | Since: 4.9.0.0 |
| (Read a, Read b) => Read (a, b) | Since: 2.1 |
| Read (Proxy k s) | Since: 4.7.0.0 |
| (Read b, Read a) => Read (Arg a b) | |
| Read (f p) => Read (Rec1 k f p) | |
| (Read a, Read b, Read c) => Read (a, b, c) | Since: 2.1 |
| (~) k a b => Read ((:~:) k a b) | Since: 4.7.0.0 |
| Coercible k a b => Read (Coercion k a b) | Since: 4.7.0.0 |
| Read (f a) => Read (Alt k f a) | |
| Read a => Read (Const k a b) |
This instance would be equivalent to the derived instances of the Since: 4.8.0.0 |
| Read c => Read (K1 k i c p) | |
| (Read (g p), Read (f p)) => Read ((:+:) k f g p) | |
| (Read (g p), Read (f p)) => Read ((:*:) k f g p) | |
| (Read a, Read b, Read c, Read d) => Read (a, b, c, d) | Since: 2.1 |
| (~~) k1 k2 a b => Read ((:~~:) k1 k2 a b) | Since: 4.10.0.0 |
| (Read1 f, Read1 g, Read a) => Read (Sum * f g a) | Since: 4.9.0.0 |
| (Read1 f, Read1 g, Read a) => Read (Product * f g a) | Since: 4.9.0.0 |
| Read (f p) => Read (M1 k i c f p) | |
| Read (f (g p)) => Read ((:.:) k2 k1 f g p) | |
| (Read a, Read b, Read c, Read d, Read e) => Read (a, b, c, d, e) | Since: 2.1 |
| (Read1 f, Read1 g, Read a) => Read (Compose * * f g a) | Since: 4.9.0.0 |
| (Read a, Read b, Read c, Read d, Read e, Read f) => Read (a, b, c, d, e, f) | Since: 2.1 |
| (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 |
| (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 |
| (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 |
| (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 |
| (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 |
| (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 |
| (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 |
| (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 |
| (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 |
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).
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.
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:
module Text.ParserCombinators.ReadPrec
| 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. |
| 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
© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/8.2.1/docs/html/libraries/base-4.10.0.0/Text-Read.html