Question

我是Haskell的新手，我应该编写一个函数，在其参数中接受一个函数，使用它并返回一个函数，其行为根据我传入的函数而变化，要明确：在我的应用程序中：一个简单的缓存服务器，我读取了一个上下文文件，所有参数中都是“LOG：active”如果日志处于活动状态我必须在屏幕上给出日志以进行调试，否则我什么都不写，我不想使用if-then链，因此我想写一个函数

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

我以这种方式doLog <- setLogging True使用它并且它有效，我对应该返回删除功能的函数有问题我的缓存中的旧元素：它必须包含多少元素由上下文决定;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

当我在doTrimm <- setTrimming "active"使用此功能时出现此错误

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.

我该如何修复这个程序？提前谢谢

Answer 1

首先写出reduceCache的类型可能会有所帮助。您当前的版本具有以下类型：

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 ()类型的日志记录函数，所以有些错误。以下类型更有意义：

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

如果我们正在执行日志记录功能，我们知道我们需要最终进入IO monad。我们只需要对您的实现进行一次更改以匹配新签名：

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

我们还可以简化set...函数：

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

这应该做你想要的。请注意，setLogging和setTrimming不再是monadic，因此您只需在doLog = setLogging True - 表达式中写let doLog = setLogging True（或do）。

传递函数作为参数

1 个答案: