[Haskell-beginners] Strange difference in behaviour between ghc and ghci
matthewmoppett at gmail.com
Thu Jun 7 13:52:43 CEST 2012
OK -- thanks everybody for your enlightening responses.
On Thu, Jun 7, 2012 at 9:40 PM, Henry Lockyer <henry.lockyer at ntlworld.com>wrote:
> in other words, in your version "What's your name?" *has* been put in the
> IO output buffer but this has not been flushed
> to the terminal yet. You then input your name (without having seen the
> prompt you were expecting (yet)) and then
> the newline of your subsequent putStrLn flushes both bits of output at the
> same time.
> On 7 Jun 2012, at 12:32, Henry Lockyer wrote:
> > A very quick reply and without reading carefully considering what may be
> different in GHCI (!),
> > but the output buffer flushes (at least on my system) with newline (if
> you don;t flush it 'manually').
> > I suspect if you use putStrLn for 'what's your name' you may get
> something more like what you want.
> > On 7 Jun 2012, at 12:22, Matthew Moppett wrote:
> >> I have a very simple Haskell file (HelloWorld.hs) that reads like this:
> >> main = do
> >> putStr "What's your name? "
> >> n <- getLine
> >> putStrLn $ "Pleased to meet you, " ++ n
> >> When I load it into ghci, I get the following result, as expected:
> >> [1 of 1] Compiling Main ( HelloWorld.hs, interpreted )
> >> Ok, modules loaded: Main.
> >> *Main> main
> >> What's your name? Matt
> >> Pleased to meet you, Matt
> >> *Main>
> >> However, when I compile the same file using ghc and run it in a
> terminal, I get a very different result:
> >> matt at matt-Lenovo-G575:~/Haskell$ ghc HelloWorld.hs
> >> [1 of 1] Compiling Main ( HelloWorld.hs, HelloWorld.o )
> >> Linking HelloWorld ...
> >> matt at matt-Lenovo-G575:~/Haskell$ ./HelloWorld
> >> Matt
> >> What's your name? Pleased to meet you, Matt
> >> matt at matt-Lenovo-G575:~/Haskell$
> >> -- in other words, the getLine action is being run before the putStr
> action, for some strange reason.
> >> Is this a bug? Can anyone enlighten me as to what might be going on?
> >> Regards,
> >> Matt.
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