haskell - (Show (FilePath -> IO [FilePath])) 没有因使用“打印”而产生的实例
问题描述
我正在尝试修复并运行 Real World Haskell 书中的每个示例,并在此过程中学习一些东西,但我被困在了第 9 章。通过阅读评论,我得到了以下代码进行编译:
FoldDir.hs
:
import ControlledVisit
import Data.Char (toLower)
import Data.Time.Clock (UTCTime(..))
import System.Directory (Permissions(..))
import System.FilePath ((</>), takeExtension, takeFileName)
data Iterate seed
= Done { unwrap :: seed }
| Skip { unwrap :: seed }
| Continue { unwrap :: seed }
deriving (Show)
type Iterator seed = seed -> Info -> Iterate seed
foldTree :: Iterator a -> a -> FilePath -> IO a
foldTree iter initSeed path = do
endSeed <- fold initSeed path
return (unwrap endSeed)
where
fold seed subpath = getUsefulContents subpath >>= walk seed
walk seed (name : names) = do
let path' = path </> name
info <- getInfo path'
case iter seed info of
done @ (Done _) -> return done
Skip seed' -> walk seed' names
Continue seed'
| isDirectory info -> do
next <- fold seed' path'
case next of
done @ (Done _) -> return done
seed'' -> walk (unwrap seed'') names
| otherwise -> walk seed' names
walk seed _ = return (Continue seed)
atMostThreePictures :: Iterator [FilePath]
atMostThreePictures paths info
| length paths == 3
= Done paths
| isDirectory info && takeFileName path == ".svn"
= Skip paths
| extension `elem` [".jpg", ".png"]
= Continue (path : paths)
| otherwise
= Continue paths
where
extension = map toLower (takeExtension path)
path = infoPath info
countDirectories count info =
Continue (if isDirectory info then count + 1 else count)
ControlledVisit.hs
:
module ControlledVisit where
import Control.Monad (forM, liftM)
import Data.Time.Clock (UTCTime(..))
import System.FilePath ((</>))
import System.Directory
( Permissions(..)
, getModificationTime
, getPermissions
, getDirectoryContents
)
import Control.Exception
( bracket
, handle
, SomeException(..)
)
import System.IO
( IOMode(..)
, hClose
, hFileSize
, openFile
)
data Info = Info
{ infoPath :: FilePath
, infoPerms :: Maybe Permissions
, infoSize :: Maybe Integer
, infoModTime :: Maybe UTCTime
} deriving (Eq, Ord, Show)
getInfo :: FilePath -> IO Info
getInfo path = do
perms <- maybeIO (getPermissions path)
size <- maybeIO (bracket (openFile path ReadMode) hClose hFileSize)
modified <- maybeIO (getModificationTime path)
return (Info path perms size modified)
traverseDirs :: ([Info] -> [Info]) -> FilePath -> IO [Info]
traverseDirs order path = do
names <- getUsefulContents path
contents <- mapM getInfo (path : map (path </>) names)
liftM concat $ forM (order contents) $ \ info -> do
if isDirectory info && infoPath info /= path
then traverseDirs order (infoPath info)
else return [info]
getUsefulContents :: FilePath -> IO [String]
getUsefulContents path = do
names <- getDirectoryContents path
return (filter (`notElem` [".", ".."]) names)
isDirectory :: Info -> Bool
isDirectory = maybe False searchable . infoPerms
maybeIO :: IO a -> IO (Maybe a)
maybeIO act = handle (\ (SomeException _) -> return Nothing) (Just `liftM` act)
traverseVerbose order path = do
names <- getDirectoryContents path
let usefulNames = filter (`notElem` [".", ".."]) names
contents <- mapM getEntryName ("" : usefulNames)
recursiveContents <- mapM recurse (order contents)
return (concat recursiveContents)
where
getEntryName name = getInfo (path </> name)
isDirectory info = case infoPerms info of
Nothing -> False
Just perms -> searchable perms
recurse info = do
if isDirectory info && infoPath info /= path
then traverseVerbose order (infoPath info)
else return [info]
但是,当我尝试按照书中的说明在 GHCi 中运行它时,它会失败并出现一个奇怪的错误,据我所知,这是关于 GHCi 本身的:
Prelude> :l FoldDir.hs
[1 of 2] Compiling ControlledVisit ( ControlledVisit.hs, interpreted )
[2 of 2] Compiling Main ( FoldDir.hs, interpreted )
Ok, two modules loaded.
*Main> foldTree atMostThreePictures []
<interactive>:2:1: error:
• No instance for (Show (FilePath -> IO [FilePath]))
arising from a use of ‘print’
(maybe you haven't applied a function to enough arguments?)
• In a stmt of an interactive GHCi command: print it
我想我理解这No instance for (Show (FilePath -> IO [FilePath]))
部分,但我对print it
. 我知道it
是 GHCi 中的一个特殊变量,它存储最后一个表达式的评估结果,我猜代码正在尝试打印一个函数或一个 monad,但我不知道它发生在哪里。
解决方案
尽可能简单 - 您的功能的签名foldTree
是:
foldTree :: Iterator a -> a -> FilePath -> IO a
您为它提供了两个参数,一个是 type Iterator [FilePath]
,第二个是 type FilePath
。由于默认的部分应用程序,这样的调用返回带有签名的函数:
FilePath -> IO [FilePath]
GHCI 想要显示 Your call 的结果,但它不能,因为这种类型没有定义 typeclass 的实例Show
。所以,它给了你一个错误,告诉你这个。
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