On Tue, Feb 3, 2015 at 10:23 PM, Madhu Babu <madhub.bits@gmail.com> wrote:

Basically, in the following code, we print a line first and the read the line from stdin. This behavior works perfectly fine when using “runhaskell guess.hs”. But when i say “./guess”, i had to type in a number directly ( i.e. first prompt is skipped ).

runhaskell is presumably using unbuffered I/O for some reason, so outputting directly to the terminal. The compiled program follows standard Unix buffering conventions: line buffering on output to a terminal, so the putStr is sitting in a buffer waiting to see a newline output.

C / C++ programs also do buffering, but there's a heinous hack which detects reads on stdin and flushes stdout beforehand. (Heinous because there is no guarantee that they are actually related --- but naïve programmers invariably do not learn about line buffering and expect all output to be unbuffered(*), and C standard library programmers eventually gave up and catered to them after years of trying to get them to pay attention. I have a nasty suspicion we're going to end up with a similar horrible hack in Haskell eventually.)

You can use hFlush from System.IO to flush the prompt out to the terminal, or disable output buffering with hSetBuffering in the same module.

(*) At some point someone will pop up and say that on modern computers, buffering is obsolete because it's fast enough that horribly inefficient character-at-a-time I/O is good enough. Yet, I can *still* see visible hesitations when character-at-a-time I/O is used on a modern Intel core i5/i7. And your disk benchmarks will *tank* even with server-class disk subsystems.

brandon s allbery kf8nh sine nomine associates

allbery.b@gmail.com ballbery@sinenomine.net

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