Parsing of Strings, producing values.
Minimal complete definition: readsPrec (or, for GHC only, readPrec)
Derived instances of Read make the following assumptions, which
derived instances of Text.Show.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 | | | :: 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 Text.Show.Show satisfy the following:
- (x,"") is an element of
(readsPrec d (Text.Show.showsPrec d x "")).
That is, readsPrec parses the string produced by
Text.Show.showsPrec, and delivers the value that
Text.Show.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).
| | | | 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 | | Read Char | | Read Double | | Read Float | | Read Int | | Read Int8 | | Read Int16 | | Read Int32 | | Read Int64 | | Read Integer | | Read Ordering | | Read Word | | Read Word8 | | Read Word16 | | Read Word32 | | Read Word64 | | Read () | | Read BufferMode | | Read CChar | | Read CSChar | | Read CUChar | | Read CShort | | Read CUShort | | Read CInt | | Read CUInt | | Read CLong | | Read CULong | | Read CLLong | | Read CULLong | | Read CFloat | | Read CDouble | | Read CPtrdiff | | Read CSize | | Read CWchar | | Read CSigAtomic | | Read CClock | | Read CTime | | Read CIntPtr | | Read CUIntPtr | | Read CIntMax | | Read CUIntMax | | Read Lexeme | | Read StdGen | | Read LocalTime | | Read ZonedTime | | Read TimeOfDay | | Read TimeZone | | Read UTCTime | | Read Day | | Read a => Read [a] | | (Integral a, Read a) => Read (Ratio a) | | Read a => Read (Maybe a) | | Read a => Read (Lazy a) | | (Read a, Read b) => Read (a, b) | | (Ix a, Read a, Read b) => Read (Array a b) | | (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) |
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