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A friend of mine asked me why was I learning Haskell. To demonstrate the power of Haskell I wrote a small program which displayed a list of prime numbers:

main = do
    putStr "Enter the number of prime numbers to display: "
    number <- fmap read getLine :: IO Int
    print . take number . filter isPrime $ [2..]

isPrime :: Integer -> Bool
isPrime n = not . any ((== 0) . mod n) $ [2..floor . sqrt . fromInteger $ n]

The program works as expected save a minor anomaly. It prints the prompt message after taking an input number from the user resulting in an output like:

12
Enter the number of prime numbers to display: [2,3,5,7,11,13,17,19,23,29,31,37]

Why is Haskell not sequencing the IO actions correctly? Where am I going wrong?

Aadit M Shah
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2 Answers2

25

This looks more like a buffering than a sequencing problem. What platform are you on? Have you tried forcing unbuffered output?

hSetBuffering stdout NoBuffering -- from System.IO
JB.
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    I'm using the Glasgow Haskell Compiler. Thank you. That was indeed the problem. I need to learn more about IO operations in Haskell. – Aadit M Shah Oct 22 '13 at 09:25
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    By platform I meant more like OS and terminal. Usually, stdin and stdout are "tied" in a way that makes stdout flush whenever stdin is read from, precisely to avoid that sort of problem. FWIW, your problem both buffers and sequences properly on my setup. – JB. Oct 22 '13 at 09:28
  • Oh, I'm using the "Terminal" on Ubuntu 13.10. I'm not really sure whether it has a specific name like xterm but it's the same terminal found on a Macintosh OSX. – Aadit M Shah Oct 22 '13 at 10:40
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    **Edit:** I have the same "untied" behavior using Debian and LXTerminal or gnome-terminal. [The C people seem to agree that there isn't an automatic flush](http://stackoverflow.com/questions/2123528/does-reading-from-stdin-flush-stdout) – Adam Schmalhofer Oct 23 '13 at 15:42
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    The tie thing is [more of a C++ thing](http://www.cplusplus.com/reference/ios/ios/tie/). I can't seem to find an authoritative mention of this about Haskell of GHC, so it's reasonable to assume there's no such thing here either. My setup (GHCi in emacs) is ultra-biased. – JB. Oct 23 '13 at 21:02
11

stdin and stdout are two different files that needn't have any connection. Take e.g. the Unix shell command grep:

$ seq 1 100 | grep 2$ | less

seq 1 100 prints the numbers 1 to 100 to its stdout which is greps stdin (| connects the stdout of one command to the stdin of an other). grep then writes the lines that match the given regex to its stdout which is lesss stdin.

To force stdout (or any other file) to be written use hFlush from System.IO:

 hFlush stdout
Adam Schmalhofer
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