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Text.Read
Copyright | (c) The University of Glasgow 2001 |
---|---|
License | BSD-style (see the file libraries/base/LICENSE) |
Maintainer | libraries@haskell.org |
Stability | provisional |
Portability | non-portable (uses Text.ParserCombinators.ReadP) |
Safe Haskell | Trustworthy |
Language | Haskell2010 |
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.
The Read class
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
Methods
Arguments
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
.
Instances
Read All Source | Since: base-2.1 |
Read Any Source | Since: base-2.1 |
Read Version Source | Since: base-2.1 |
Read Void Source | Reading a Since: base-4.8.0.0 |
Read CBool Source | |
Read CChar Source | |
Read CClock Source | |
Read CDouble Source | |
Read CFloat Source | |
Read CInt Source | |
Read CIntMax Source | |
Read CIntPtr Source | |
Read CLLong Source | |
Read CLong Source | |
Read CPtrdiff Source | |
Read CSChar Source | |
Read CSUSeconds Source | |
Defined in Foreign.C.Types MethodsreadsPrec :: Int -> ReadS CSUSeconds Source readList :: ReadS [CSUSeconds] Source |
|
Read CShort Source | |
Read CSigAtomic Source | |
Defined in Foreign.C.Types MethodsreadsPrec :: Int -> ReadS CSigAtomic Source readList :: ReadS [CSigAtomic] Source |
|
Read CSize Source | |
Read CTime Source | |
Read CUChar Source | |
Read CUInt Source | |
Read CUIntMax Source | |
Read CUIntPtr Source | |
Read CULLong Source | |
Read CULong Source | |
Read CUSeconds Source | |
Read CUShort Source | |
Read CWchar Source | |
Read IntPtr Source | |
Read WordPtr Source | |
Read ByteOrder Source | Since: base-4.11.0.0 |
Read Associativity Source | Since: base-4.6.0.0 |
Defined in GHC.Generics MethodsreadsPrec :: Int -> ReadS Associativity Source readList :: ReadS [Associativity] Source |
|
Read DecidedStrictness Source | Since: base-4.9.0.0 |
Defined in GHC.Generics MethodsreadsPrec :: Int -> ReadS DecidedStrictness Source readList :: ReadS [DecidedStrictness] Source |
|
Read Fixity Source | Since: base-4.6.0.0 |
Read SourceStrictness Source | Since: base-4.9.0.0 |
Defined in GHC.Generics MethodsreadsPrec :: Int -> ReadS SourceStrictness Source readList :: ReadS [SourceStrictness] Source |
|
Read SourceUnpackedness Source | Since: base-4.9.0.0 |
Defined in GHC.Generics MethodsreadsPrec :: Int -> ReadS SourceUnpackedness Source readList :: ReadS [SourceUnpackedness] Source |
|
Read SeekMode Source | Since: base-4.2.0.0 |
Read ExitCode Source | |
Read BufferMode Source | Since: base-4.2.0.0 |
Defined in GHC.IO.Handle.Types MethodsreadsPrec :: Int -> ReadS BufferMode Source readList :: ReadS [BufferMode] Source |
|
Read Newline Source | Since: base-4.3.0.0 |
Read NewlineMode Source | Since: base-4.3.0.0 |
Defined in GHC.IO.Handle.Types MethodsreadsPrec :: Int -> ReadS NewlineMode Source readList :: ReadS [NewlineMode] Source |
|
Read IOMode Source | Since: base-4.2.0.0 |
Read Int16 Source | Since: base-2.1 |
Read Int32 Source | Since: base-2.1 |
Read Int64 Source | Since: base-2.1 |
Read Int8 Source | Since: base-2.1 |
Read GCDetails Source | Since: base-4.10.0.0 |
Read RTSStats Source | Since: base-4.10.0.0 |
Read SomeChar Source | |
Read SomeSymbol Source | Since: base-4.7.0.0 |
Defined in GHC.TypeLits MethodsreadsPrec :: Int -> ReadS SomeSymbol Source readList :: ReadS [SomeSymbol] Source |
|
Read SomeNat Source | Since: base-4.7.0.0 |
Read GeneralCategory Source | Since: base-2.1 |
Defined in GHC.Read MethodsreadsPrec :: Int -> ReadS GeneralCategory Source readList :: ReadS [GeneralCategory] Source |
|
Read Word16 Source | Since: base-2.1 |
Read Word32 Source | Since: base-2.1 |
Read Word64 Source | Since: base-2.1 |
Read Word8 Source | Since: base-2.1 |
Read CBlkCnt Source | |
Read CBlkSize Source | |
Read CCc Source | |
Read CClockId Source | |
Read CDev Source | |
Read CFsBlkCnt Source | |
Read CFsFilCnt Source | |
Read CGid Source | |
Read CId Source | |
Read CIno Source | |
Read CKey Source | |
Read CMode Source | |
Read CNfds Source | |
Read CNlink Source | |
Read COff Source | |
Read CPid Source | |
Read CRLim Source | |
Read CSocklen Source | |
Read CSpeed Source | |
Read CSsize Source | |
Read CTcflag Source | |
Read CUid Source | |
Read Fd Source | |
Read Lexeme Source | Since: base-2.1 |
Read Ordering Source | Since: base-2.1 |
Read Integer Source | Since: base-2.1 |
Read Natural Source | Since: base-4.8.0.0 |
Read () Source | Since: base-2.1 |
Read Bool Source | Since: base-2.1 |
Read Char Source | Since: base-2.1 |
Read Double Source | Since: base-2.1 |
Read Float Source | Since: base-2.1 |
Read Int Source | Since: base-2.1 |
Read Word Source | Since: base-4.5.0.0 |
Read a => Read (ZipList a) Source | Since: base-4.7.0.0 |
Read a => Read (And a) Source | Since: base-4.16 |
Read a => Read (Iff a) Source | Since: base-4.16 |
Read a => Read (Ior a) Source | Since: base-4.16 |
Read a => Read (Xor a) Source | Since: base-4.16 |
Read a => Read (Complex a) Source | Since: base-2.1 |
Read a => Read (Identity a) Source | This instance would be equivalent to the derived instances of the Since: base-4.8.0.0 |
Read a => Read (First a) Source | Since: base-2.1 |
Read a => Read (Last a) Source | Since: base-2.1 |
Read a => Read (Down a) Source | This instance would be equivalent to the derived instances of the Since: base-4.7.0.0 |
Read a => Read (First a) Source | Since: base-4.9.0.0 |
Read a => Read (Last a) Source | Since: base-4.9.0.0 |
Read a => Read (Max a) Source | Since: base-4.9.0.0 |
Read a => Read (Min a) Source | Since: base-4.9.0.0 |
Read m => Read (WrappedMonoid m) Source | Since: base-4.9.0.0 |
Defined in Data.Semigroup MethodsreadsPrec :: 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) Source | Since: base-2.1 |
Read a => Read (Product a) Source | Since: base-2.1 |
Read a => Read (Sum a) Source | Since: base-2.1 |
Read p => Read (Par1 p) Source | Since: base-4.7.0.0 |
(Integral a, Read a) => Read (Ratio a) Source | Since: base-2.1 |
Read a => Read (NonEmpty a) Source | Since: base-4.11.0.0 |
Read a => Read (Maybe a) Source | Since: base-2.1 |
Read a => Read (a) Source | Since: base-4.15 |
Read a => Read [a] Source | Since: base-2.1 |
(Read a, Read b) => Read (Either a b) Source | Since: base-3.0 |
HasResolution a => Read (Fixed a) Source | Since: base-4.3.0.0 |
Read (Proxy t) Source | Since: base-4.7.0.0 |
(Read a, Read b) => Read (Arg a b) Source | Since: base-4.9.0.0 |
(Ix a, Read a, Read b) => Read (Array a b) Source | Since: base-2.1 |
Read (U1 p) Source | Since: base-4.9.0.0 |
Read (V1 p) Source | Since: base-4.9.0.0 |
(Read a, Read b) => Read (a, b) Source | Since: base-2.1 |
Read a => Read (Const a b) Source | This instance would be equivalent to the derived instances of the Since: base-4.8.0.0 |
Read (f a) => Read (Ap f a) Source | Since: base-4.12.0.0 |
Read (f a) => Read (Alt f a) Source | Since: base-4.8.0.0 |
Coercible a b => Read (Coercion a b) Source | Since: base-4.7.0.0 |
a ~ b => Read (a :~: b) Source | Since: base-4.7.0.0 |
Read (f p) => Read (Rec1 f p) Source | Since: base-4.7.0.0 |
(Read a, Read b, Read c) => Read (a, b, c) Source | Since: base-2.1 |
(Read1 f, Read1 g, Read a) => Read (Product f g a) Source | Since: base-4.9.0.0 |
(Read1 f, Read1 g, Read a) => Read (Sum f g a) Source | Since: base-4.9.0.0 |
a ~~ b => Read (a :~~: b) Source | Since: base-4.10.0.0 |
(Read (f p), Read (g p)) => Read ((f :*: g) p) Source | Since: base-4.7.0.0 |
(Read (f p), Read (g p)) => Read ((f :+: g) p) Source | Since: base-4.7.0.0 |
Read c => Read (K1 i c p) Source | Since: base-4.7.0.0 |
(Read a, Read b, Read c, Read d) => Read (a, b, c, d) Source | Since: base-2.1 |
(Read1 f, Read1 g, Read a) => Read (Compose f g a) Source | Since: base-4.9.0.0 |
Read (f (g p)) => Read ((f :.: g) p) Source | Since: base-4.7.0.0 |
Read (f p) => Read (M1 i c f p) Source | Since: base-4.7.0.0 |
(Read a, Read b, Read c, Read d, Read e) => Read (a, b, c, d, e) Source | Since: base-2.1 |
(Read a, Read b, Read c, Read d, Read e, Read f) => Read (a, b, c, d, e, f) Source | Since: base-2.1 |
(Read a, Read b, Read c, Read d, Read e, Read f, Read g) => Read (a, b, c, d, e, f, g) Source | Since: base-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) Source | Since: base-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) Source | Since: base-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) Source | Since: base-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) Source | Since: base-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) Source | Since: base-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) Source | Since: base-2.1 |
Defined in GHC.Read |
|
(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: base-2.1 |
Defined in GHC.Read MethodsreadsPrec :: 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: base-2.1 |
Defined in GHC.Read MethodsreadsPrec :: 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.
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
Constructors
Instances
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
© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/9.4.2/docs/libraries/base-4.17.0.0/Text-Read.html