Prompt message disappears in compiled executable?

Question

I have a Haskell program that shows a prompt and then accepts input from the command line. I'm doing this as:

main = do putStr "Please enter program source file name: "
          programFileName <- getLine
          programFileHandle <- openFile programFileName ReadMode
          program <- hGetContents programFileHandle
          putStr "Please enter initial file configuration file name: "
          initConfigFileName <- getLine
          initConfigFileHandle <- openFile initConfigFileName ReadMode
          initConfigStr <- hGetContents initConfigFileHandle
          print (evaluateProgram (lines program) (readReg initConfigStr))

When I run it on the GHCi interpreter, the prompts show up fine and I am able to enter my inputs (and everything else works).

e.g. *Main> main Please enter program source file name: sum.urm Please enter initial file configuration file name: sum.conf 9

When I compile it though (on Mac OS X or Windows), it produces an executable that does not show my prompts. It waits for the two input strings, and then once I have put in the valid filenames, it prints the prompts and the result.

e.g.

$ ./a.out 
sum.urm
sum.conf
Please enter program source file name: Please enter initial file configuration file name: 9

Any ideas why this is happening?

For the curious, I was implementing an Unlimited Register Machine in Haskell.

dflemstr · Accepted Answer · 2012-03-08T23:17:39.697

5

The standard output stream, stdout, is line buffered by default. That means that it will only be written to the console every time you output a \n character, or finally when the program terminates. You can fix this by importing System.IO and doing hFlush stdout after every putStr that doesn't contain a \n at the end.

edited Mar 08 '12 at 23:17

answered Mar 08 '12 at 19:56

dflemstr

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Note also that GHCi sets stdin and stdout to be unbuffered, which is why you get the behaviour you expect there. – Dan Hulme Mar 08 '12 at 20:02
Another potentially simpler solution is to do `hSetBuffering stdout NoBuffering`. Then `putStr` will do the expected thing. – Daniel Lyons Mar 08 '12 at 20:17
Thanks. I had to do `hFlush stdout` instead of `hFlush stdio`, but it gave me the idea. – rdasxy Mar 08 '12 at 21:28
Oh, sorry, that was a typo. It's of course supposed to be `stdout`. – dflemstr Mar 08 '12 at 23:17

score 0 · Answer 2 · edited May 23 '17 at 10:34

0

This is a buffering issue. Here are related questions, with several choices for solutions:

Why isn't my IO executed in order?

Execution order with (>>=) not what I expected

edited May 23 '17 at 10:34

Community

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answered Mar 08 '12 at 20:59

amindfv

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Prompt message disappears in compiled executable?

2 Answers2