根据Yorgey教授的UPenn类给出以下定义:
newtype Parser a = Parser { runParser :: String -> Maybe (a, String) }
satisfy :: (Char -> Bool) -> Parser Char
satisfy p = Parser f
where
f [] = Nothing -- fail on the empty input
f (x:xs) -- check if x satisfies the predicate
-- if so, return x along with the remainder
-- of the input (that is, xs)
| p x = Just (x, xs)
| otherwise = Nothing -- otherwise, fail
以及以下代数数据类型:
type Key = String
data Json = JObj Key JValue
| Arr [JValue]
deriving Show
data JValue = N Double
| S String
| B Bool
| J Json
deriving Show
我编写了以下函数来解析带有小数点的位置JSON数字:
parseDecimalPoint :: Parser Char
parseDecimalPoint = satisfy (== '.')
type Whole = Integer
type Decimal = Integer
readWholeAndDecimal :: Whole -> Decimal -> Double
readWholeAndDecimal w d = read $ (show w) ++ "." ++ (show d)
parsePositiveDecimal:: Parser JValue
parsePositiveDecimal = (x _ y -> f x y) <$> (
(oneOrMore (satisfy isNumber)) <*> parseDecimalPoint <*>
(zeroOrMore (satisfy isNumber)) )
where
f x [] = N (read x)
f x y = N (-(readWholeAndDecimal (read x) (read y)))
然而,我得到了以下编译时错误:
JsonParser.hs:30:25:
Couldn't match expected type ‘t0 -> [Char] -> JValue’
with actual type ‘JValue’
The lambda expression ‘ x _ y -> f x y’ has three arguments,
but its type ‘String -> JValue’ has only one
In the first argument of ‘(<$>)’, namely ‘( x _ y -> f x y)’
In the expression:
( x _ y -> f x y)
<$>
((oneOrMore (satisfy isNumber)) <*> parseDecimalPoint
<*> (zeroOrMore (satisfy isNumber)))
JsonParser.hs:30:49:
Couldn't match type ‘[Char]’ with ‘Char -> [Char] -> String’
Expected type: Parser (Char -> [Char] -> String)
Actual type: Parser [Char]
In the first argument of ‘(<*>)’, namely
‘(oneOrMore (satisfy isNumber))’
In the first argument of ‘(<*>)’, namely
‘(oneOrMore (satisfy isNumber)) <*> parseDecimalPoint’
在我的parsePositiveDecimal
函数中,我对类型的理解是:
(String -> Char -> String -> JValue) <$> (Parser String <*> Parser Char <*> Parser String)
我已经完成了一些使用<$>
和<*>
制作解析器的示例。但我并没有完全摸索类型。
如果能对理解它们有所帮助,我们将不胜感激。
仙人掌是正确的。我会在类型上做一些扩展。
<$> :: Functor f => (a -> b) -> f a -> f b
这里的f
是Parser
,<$>
的第一个参数的类型是String -> Char -> String -> JValue
。请记住,这可以理解为一个接受String
并返回函数Char -> String -> JValue
的函数,因此a
类型的变量用String
填充。
从中可以看出,<$>
的第二个参数需要是Parser String
类型。oneOrMore (satisfy isNumber)
具有该类型。
综合起来,我们现在有:
(x _ y -> f x y) <$> (oneOrMore (satisfy isNumber)) :: Parser (Char -> String -> JValue)
我们已经从一个3个参数的函数(根本不涉及Parser
)变成了一个包含2个参数的Parser.
函数。要将此函数应用于它的下一个参数Char
,我们需要:
(<*>) :: Applicative f => f (a -> b) -> f a -> f b
f
再次是Parser
,这里的a
是Char
。CCD_ 27具有CCD_ 28右侧所需的类型。
(x _ y -> f x y) <$> (oneOrMore (satisfy isNumber)) <*> parseDecimalPoint :: Parser (String -> JValue)
我们再做一次,得到:
(x _ y -> f x y) <$> oneOrMore (satisfy isNumber) <*> parseDecimalPoint <*> zeroOrMore (satisfy isNumber) :: Parser JValue
我利用知道运算符的优先级和关联性的优势删除了一些括号。这就是我看到的大多数这样的代码的编写方式,但也许Cactus的版本更清晰。甚至是全括号版本,强调关联性:
( ((x _ y -> f x y) <$>
(oneOrMore (satisfy isNumber)))
<*> parseDecimalPoint)
<*> (zeroOrMore (satisfy isNumber)) :: Parser JValue