Vyacheslav Akhmechet wrote:
I encountered the following issue last night. Writing a program like this:
main = do putStr "> "
x <- getLine
putStr x
behaves as expected in hugs and ghci (first prints "> ", then waits
for input, then prints the input). However, when compiling and running
an executable via ghc, the behavior was very different. I saw a blank
console screen and when I typed something it was echoed back to me (as
in "> 25") only after I hit enter. This is trivially fixed by
flushing:
main = do putStr "> "
hFlush stdout
x <- getLine
putStr x
or by turning off buffering. Is this the intended behavior?
Yes, it's the intended behaviour.
This
problem does not appear when using putStrLn (as it flushes
automatically) and is quite distracting to newbies. To me it looks
like this behavior is at odds with something one can reasonably expect
and bufferring should be turned off by default. What do you guys
think?
We did at one time have some code that flushed stdout before reading from stdin,
but I wasn't too keen on the idea so I didn't implement that idea in the current
version of the IO library.
Personally I think the current behaviour is ok, despite the fact that it does
trip people up occasionally. If you think about it, it's consistent: stdout is
line bufferred, so it doesn't get flushed until you output a \n.
Cheers,
Simon
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