Haskell的hGetContents不返回任何内容，即使我知道该文件包含文本

Question

I have a small bit of code that isn't behaving as I expected it too:

tempTest = do
    (_,tHand) <- openTempFile "." "tempTest.txt"
    hPutStr tHand "Test!"
    read <- hGetContents tHand
    putStrLn read

It writes to the file correctly, but when asked to display the contents to the screen, it shows only a new line (without any quotes). Given how simple this is, I'm assuming I'm misunderstanding how getContents works. From what I understand, it reads the file in chunks instead of all at once; which prevents the memory from being filled up by a massive file being read. Why is "read' not receiving anything from hGetContents? Any help here would be appreciated.

Answer 1

The problem here is that the handle of your file points to the end of the file. This should work:

tempTest = do
    (_,tHand) <- openTempFile "." "tempTest.txt"
    hPutStr tHand "Test!"
    hSeek tHand AbsoluteSeek 0 -- move the file handle to beginning of file
    read <- hGetContents tHand
    putStrLn read

Demo:

λ> tempTest
Test!

Here using the hSeek function, you move the Handle to the beginning of the file. Also, I would suggest you to close the file handle using hClose function after your work is done.