我懒惰地使用此代码编码列表(取自此SO question):
import Data.Binary
newtype Stream a = Stream { unstream :: [a] }
instance Binary a => Binary (Stream a) where
put (Stream []) = putWord8 0
put (Stream (x:xs)) = putWord8 1 >> put x >> put (Stream xs)
问题是解码实现不是懒惰的:
get = do
t <- getWord8
case t of
0 -> return (Stream [])
1 -> do x <- get
Stream xs <- get
return (Stream (x:xs))
这让我觉得它应该是懒惰的,但是如果我们运行这个测试代码:
head $ unstream (decode $ encode $ Stream [1..10000000::Integer] :: Stream Integer)
内存使用量爆炸。出于某种原因,在让我查看第一个元素之前,它想要解码整个列表。
为什么这不是懒惰,我怎么能让它变得懒惰?
答案 0 :(得分:6)
它不是懒惰的,因为Get
monad是一个严格的状态monad(在binary-0.5.0.2 to 0.5.1.1中;它之前是一个懒惰状态monad,而在binary-0.6.*它已成为一个延续monad,我没有分析这种变化对严格性的影响):
-- | The parse state
data S = S {-# UNPACK #-} !B.ByteString -- current chunk
L.ByteString -- the rest of the input
{-# UNPACK #-} !Int64 -- bytes read
-- | The Get monad is just a State monad carrying around the input ByteString
-- We treat it as a strict state monad.
newtype Get a = Get { unGet :: S -> (# a, S #) }
-- Definition directly from Control.Monad.State.Strict
instance Monad Get where
return a = Get $ \s -> (# a, s #)
{-# INLINE return #-}
m >>= k = Get $ \s -> case unGet m s of
(# a, s' #) -> unGet (k a) s'
{-# INLINE (>>=) #-}
因此最后的递归
get >>= \x ->
get >>= \(Stream xs) ->
return (Stream (x:xs))
强制整个Stream
在被返回之前被阅读。
我认为懒惰地解码Stream
monad中的Get
是不可能的(所以更不用Binary
实例)。但是你可以使用runGetState
编写一个延迟解码函数:
-- | Run the Get monad applies a 'get'-based parser on the input
-- ByteString. Additional to the result of get it returns the number of
-- consumed bytes and the rest of the input.
runGetState :: Get a -> L.ByteString -> Int64 -> (a, L.ByteString, Int64)
runGetState m str off =
case unGet m (mkState str off) of
(# a, ~(S s ss newOff) #) -> (a, s `join` ss, newOff)
首先编写一个Get
解析器,返回Maybe a
,
getMaybe :: Binary a => Get (Maybe a)
getMaybe = do
t <- getWord8
case t of
0 -> return Nothing
_ -> fmap Just get
然后使用它来创建(ByteString,Int64) -> Maybe (a,(ByteString,Int64))
类型的函数:
step :: Binary a => (ByteString,Int64) -> Maybe (a,(ByteString,Int64))
step (xs,offset) = case runGetState getMaybe xs offset of
(Just v, ys, newOffset) -> Just (v,(ys,newOffset))
_ -> Nothing
然后您可以使用Data.List.unfoldr
懒惰地解码列表
lazyDecodeList :: Binary a => ByteString -> [a]
lazyDecodeList xs = unfoldr step (xs,0)
并将其包装在Stream
lazyDecodeStream :: Binary a => ByteString -> Stream a
lazyDecodeStream = Stream . lazyDecodeList