Haskell's hGetContents isn't returning anything, even though I know the file contains text

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.

Note that similar problems often turn up with Haskell's standard IO routines because of _lazyness_. That was completely innocent in this case, but generally it's often a good idea to just go for completely strict IO, i.e. open & read an entire file and immediately close it: the time and memory needed is often neglectable (though sometimes it isn't of course!), and you get much simpler, reliable semantics. — leftaroundabout, Jul 18 '14 at 18:38
I haven't knowingly used anything strict in Haskell yet. The only method I know of for ensuring strictness is the $! operator, which, as far as I know causes `seq` to be called during the application (so I guess I know of `seq` aswell). I have yet to use either though. — Carcigenicate, Jul 18 '14 at 23:28

Sibi · Accepted Answer · 2014-07-18T16:40:01.067

9

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.

edited Jul 18 '14 at 16:40

answered Jul 18 '14 at 16:33

Sibi

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Wow. Now that I think about it, it makes sense that the handle would be at the end of the file, just having written something -_-. Thank you. Quickest answer ever. – Carcigenicate Jul 18 '14 at 16:45
1

@Sibi I'm a little wary of suggesting that one combine `hClose` with lazy IO like `hGetContents`, it can so easily do the wrong thing. – Ørjan Johansen Jul 18 '14 at 23:46
@ØrjanJohansen What can go wrong? Does it have a chance of cutting the text early? – Carcigenicate Jul 19 '14 at 03:58
@Carcigenicate Look on [this](http://stackoverflow.com/questions/5892653/whats-so-bad-about-lazy-i-o) to get the ideas of when lazy IO will be a problem. – Sibi Jul 19 '14 at 08:53
@Carcigenicate Yes, that's it. – Ørjan Johansen Jul 19 '14 at 11:55

Haskell's hGetContents isn't returning anything, even though I know the file contains text

1 Answers1