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当代码中没有可见的列表时,[String]为什么是推断类型?

type-inference haskell

keithb · 技术社区 · 15 年前

我做了个测试:

testReadFile = runTestTT $ TestLabel "InteractionDomain.readFileContentsToList" (TestList [
    (TestLabel "testing infrastructure: read test file" (TestList [
        TestCase (withTempFileContainingText (\fileHandle ->
            assertEqual "contents as expected" "The dog barks at midnight." (do 
                content <- hGetContents fileHandle
                return content)))]))])

withTempFileContainingText

withTempFileContainingText lambda = 
        bracketOnError 
            (do (path, fileHandle) <- openTempFile "." "input.txt"
                return fileHandle)
            (deleteFile)
            (\fileHandle -> do 
                hPutStr fileHandle "The dog barks at midnight."
                hClose fileHandle --hoping this flushes buffers
                return (lambda fileHandle))

ghc 抱怨说

InteractionDomain.hs:36:4:
    Couldn't match expected type `IO String'
           against inferred type `[String]'
    In a stmt of a 'do' expression: content ≤- hGetContents fileHandle

我不明白。为什么会有这样的推论 content

3 回复 | 直到 15 年前

J Cooper 15 年前

好吧,我现在看到的是你断言中的类型不匹配。

资产等价物 String -> a -> a -> Assertion String 作为第二个论点,意思是 字符串 也应该是第三个论点。然而,你的 do 表达式不返回 字符串 ,但是 IO String

编辑:一旦你搞砸了 IO <- ,然后立即将其包装回 木卫一 具有 return . 如果你想使用这个字符串,你必须在里面做 木卫一

do
  contents <- hGetContents handle
  assertEqual "They're equal" "Expected string" contents

请注意 做 会回来的 IO Assertion

kennytm 15 年前

assertEqual 是

assertEqual :: String -> a -> a -> Assertion

从第二个论点开始 "The dog barks at midnight." 是一根弦,我们知道 a 必须是 String

do 
  content <- hGetContents fileHandle
  return content

必须返回字符串。但是String=[Char],因此Haskell会将其视为列表单子。我不知道为什么它会推断出一个[String]而不是[Char]=String,但这至少解释了列表的来源。

顺便说一句,断言已经是IO()。如果你 return 您将得到一个IO(IO())的断言。也许你是说

withTempFileContainingText :: (Handle -> IO c) -> IO c
withTempFileContainingText lambda = 
    bracketOnError 
        (return . snd =<< openTempFile "." "input.txt")
        (deleteFile)
        (\fileHandle -> do
            hPutStr fileHandle "The dog barks at midnight."
            hClose fileHandle
            lambda fileHandle)   -- # <--- No return here

testCase 可以写成

testCase :: Assertion
testCase = withTempFileContainingText $ \fileHandle -> do
    conts <- hGetContents fileHandle
    assertEqual "contents as expected" "The dog barks at midnight." conts
    -- # ^-- No return here.

keithb 15 年前

谢谢大家的宝贵指点。

它运行并通过:

withTempFileContainingText lambda = 
        bracketOnError 
            (openTempFile "." "input.txt")
            (\(filePath, fileHandle) -> do
                removeFile filePath)
            (\(filePath, fileHandle) -> do
                startOfFile <- hGetPosn fileHandle 
                hPutStr fileHandle "The dog barks at midnight."
                hFlush fileHandle
                hSetPosn startOfFile
                lambda fileHandle
                removeFile filePath)


testReadFile = runTestTT $ TestLabel "InteractionDomain.readFileContentsToList" (TestList [
    (TestLabel "testing infrastructure: read test file" (TestList [
        TestCase (withTempFileContainingText (\fileHandle -> do
            content <- hGetContents fileHandle 
            assertEqual "contents as expected" "The dog barks at midnight." content))]))])