传递函数作为参数

Question

我是新来的Haskell，我应该写一个函数，它的参数中的功能，使用它，并返回其行为变化取决于我传递的功能， 要明确一个功能：在我的应用程序：一个简单的缓存服务器，我读取一个上下文文件，并且在所有参数中，如果日志处于活动状态，则为“LOG：active”我必须在屏幕上给出日志以用于调试目的，否则我不写任何内容，我不想使用IF-THEN，因此，我认为写一个函数

setLogging a = do 
    if a 
    then let logging x = putStrLn x 
    in return logging 
    else let logging x = putStrLn "" 
     in return logging 

我使用这种方式doLog <- setLogging True和它的作品的链，我与功能的问题，应返回的功能去除 我的缓存中的旧元素：它必须包含的元素的数量由上下文决定;这是我的功能

--ritorna la funzione che riduce la cache in funzione dell environment 
setTrimming a = do 
    if a=="active" 
      then let trimming c logging = reduceCache 9 logging c --è attivo lowbandwidth 
       in return trimming 
      else let trimming c logging = reduceCache 5 logging c --non è attivo lowbandwidth 
       in return trimming 

reduceCache s log cache 
    | (M.size cache) >= s = do 
     log "LOG: ciao" 
     reduceCache s log (snd (M.deleteFindMin cache)) 
    | otherwise = cache 

当我用doTrimm <- setTrimming "active"这个功能我得到这个错误

ghc --make "trimmer.hs" (in directory: /media/Volume/doc/progetti/haskell) 
[1 of 1] Compiling Main    (trimmer.hs, trimmer.o) 
trimmer.hs:33:31: 
Couldn't match expected type `t1 -> t' 
     against inferred type `M.Map k a' 
In the expression: reduceCacheLow 9 logging c 
In the definition of `trimming': 
    trimming c logging = reduceCacheLow 9 logging c 
In the expression: 
    let trimming c logging = reduceCacheLow 9 logging c 
    in return trimming 
trimmer.hs:35:30: 
Couldn't match expected type `t1 -> t' 
     against inferred type `M.Map k a' 
In the expression: reduceCacheHigh 4 logging c 
In the definition of `trimming': 
    trimming c logging = reduceCacheHigh 4 logging c 
In the expression: 
    let trimming c logging = reduceCacheHigh 4 logging c 
    in return trimming 
Compilation failed. 

我如何可以解决此计划？ 由于事先

Answer 1

这可能有助于通过写出来的reduceCache类型开始。您当前的版本有以下类型：

reduceCache :: (Monad (M.Map k)) => Int -> ([Char] -> M.Map k a1) -> M.Map k a -> M.Map k a 

但我们知道，第二个参数是String -> IO()类型的日志记录功能，所以什么是错的。以下类型更有意义：

reduceCache :: Int -> (String -> IO()) -> M.Map k v -> IO (M.Map k v) 

如果我们执行日志记录功能，我们知道我们需要在IO单子就结了。我们只需要做出一个改变你实现以匹配新的签名：

reduceCache s log' cache 
    | M.size cache >= s = do 
    log' "LOG: ciao" 
    reduceCache s log' (snd $ M.deleteFindMin cache) 
    | otherwise = return cache 

我们也可以简化set...功能：

setLogging :: Bool -> String -> IO() 
setLogging True = putStrLn 
setLogging False = const $ return() 

setTrimming :: String -> M.Map k v -> (String -> IO()) -> IO (M.Map k v) 
setTrimming "active" = \c logging -> reduceCache 9 logging c 
setTrimming _  = \c logging -> reduceCache 5 logging c 

这应该做你想要什么。需要注意的是setLogging和setTrimming不再单子，所以你只写doLog = setLogging True（或let doLog = setLogging True在do -expression）。