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我想繼續應用解析器到一個字符串，並添加它的輸出，直到我到達字符串的末尾。不幸的是，當我解析字符串的結尾時，它失敗了，所以我得到一個空列表。我希望能夠檢查解析是否失敗，然後在解析失敗之前從所有解析中獲得輸出。我正試圖將這個字符串解析成一個浮點列表。我怎樣才能阻止這種情況的發生（即獲得一個空的列表而不是我的浮動列表）。如何檢查解析是否失敗？
我寫這種類型的解析器：newtype Parser a = MkP (String -> [(String,a)]) 及其單子實例是這樣的：
-- (>>=) :: m a -> (a -> m b) -> m b
instance Monad Parser where
return x = MkP f
where
f inp = [(inp,x)]
f >>= g = MkP s
where
s inp = [(rest,out) | (firstremain,firstout) <- applyParser f inp, (rest,out) <- applyParser (g firstout) firstremain]
無法解析由空列表[]表示。我有這些主要的解析器：
item :: Parser Char
item = MkP f
where
f [] = []
f (x:xs) = [(xs,x)]
zero :: Parser Char
zero = MkP f
where
f _ = []
我試圖解析這個字符串：
unparsedboeyield = "0.63 0.81 1.01 1.20 1.38 1.54 1.68 1.79 1.89 1.97 2.05 2.11 2.17 2.23 2.28 2.33 2.37 2.42 2.45 2.49 2.53 2.56 2.59 2.62 2.64 2.67 2.69 2.71 2.73 2.75 2.77 2.79 2.80 2.82 2.83 2.84 2.85 2.87 2.87 2.88 2.89 2.90 2.90 2.91 2.91 2.92 2.92 2.92 2.93 2.93"
這個解析器：
numberParser :: Parser Float
numberParser = do
a <- item
b <- item
c <- item
d <- item
let number = read [a,b,c,d] :: Float in return number
yieldParser :: Parser [Float]
yieldParser = do
x <- numberParser
helper [x] where
helper y = do
a <- randomthing
helper (y ++ [a])
randomthing = do
item
item
item
item
numberParser
我不明白什麼是helper發生時，什麼是a當randomthing失敗（它不能[]然後它會引發一個類型錯誤，因爲它不是一個浮點數）。當我嘗試解析字符串出現這種情況：
applyParser yieldParser boeunparsedyield => []
整個代碼是在這裏：
import Control.Monad
import Data.Char
interpolate :: (Ord a, Fractional a) => ((a,a),(a,a)) -> a -> a
interpolate ((x1,y1),(x2,y2)) x = if (x > x2) || (x < x1) then error "value out of range" else y1 + difference * gradient
where
gradient = (y1 -y2)/(x1 - x2)
difference = x - x1
pvt :: Fractional a => a -> Int -> a
pvt x t = x/(m + spotrate t)^t where m = 1 :: Fractional a => a
unparsedboeyield :: String
unparsedboeyield = "0.63 0.81 1.01 1.20 1.38 1.54 1.68 1.79 1.89 1.97 2.05 2.11 2.17 2.23 2.28 2.33 2.37 2.42 2.45 2.49 2.53 2.56 2.59 2.62 2.64 2.67 2.69 2.71 2.73 2.75 2.77 2.79 2.80 2.82 2.83 2.84 2.85 2.87 2.87 2.88 2.89 2.90 2.90 2.91 2.91 2.92 2.92 2.92 2.93 2.93"
newtype Parser a = MkP (String -> [(String,a)])
applyParser :: Parser a -> String -> [(String,a)]
applyParser (MkP x) y = x y
-- (>>=) :: m a -> (a -> m b) -> m b
instance Monad Parser where
return x = MkP f
where
f inp = [(inp,x)]
f >>= g = MkP s
where
s inp = [(rest,out) | (firstremain,firstout) <- applyParser f inp, (rest,out) <- applyParser (g firstout) firstremain]
item :: Parser Char
item = MkP f
where
f [] = []
f (x:xs) = [(xs,x)]
zero :: Parser Char
zero = MkP f
where
f _ = []
sat :: (Char -> Bool) -> Parser Char
sat predicate =
item >>= \x ->
if predicate x then return x else zero
doParser :: Parser a -> Int -> Parser()
doParser x count = helper count
where
helper count = do
if count > 0 then do
x
helper (count - 1)
else return()
numberParser :: Parser Float
numberParser = do
a <- item
b <- item
c <- item
d <- item
let number = read [a,b,c,d] :: Float in return number
yieldParser :: Parser [Float]
yieldParser = do
x <- numberParser
helper [x] where
helper y = do
a <- randomthing
helper (y ++ [a])
randomthing = do
item
item
item
item
numberParser
spotrate :: Fractional a => Int -> a
spotrate = \t -> if (t == 1) then 5 else 2
untilParser :: (Char -> Bool) -> Parser [Char]
untilParser p = helper []
where
helper x = do
y <- item
if p y then helper (x ++ [y]) else return x