[Haskell-cafe] unexpected compiler behavior

Donald Bruce Stewart dons at cse.unsw.edu.au
Tue Dec 5 06:15:49 EST 2006

> Hi,
> I am newbie in Haskell and do not understand why the interpreted mode
> differs from the compiled program.
> I run this code
> main =  do
>           putStr "line1"
>           x<-getLine
>           putStr "line2"
>           y<-getLine
>           return (x++y)
> either under runhugs or runghc or in a console under ghci and works
> properly, that is, first displays "line1"  and then prompts for the
> corresponding inputs through the keyboard, and so on. However, if I
> compile the code with the command line
> ghc --make Main.hs -o main
> and launch the compiled program main, then at first prompts for the
> input lines through the keyboard and then displays the strings "line1"
> "line2".

> I have read in the tutorials that the command "do" implies an ordered
> execution and this only occurs in interpreted mode.
> Using the monadic notation >> , >>= occurs the same unordered behavior.
> ?How can I avoid the unordered execution of this code in a compiled
> program?

Sounds like the difference in buffering between the interactive
environments and compiled code.


    import System.IO

    main =  do
        hSetBuffering stdout NoBuffering
        putStr "line1 "
        x <- getLine
        putStr "line2 "
        y <- getLine
        print (x++y)

Running this:

    $ ./a.out 
    line1 1
    line2 2

-- Don

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