[Haskell-cafe] Re: Where do I put the seq?

Peter Verswyvelen bugfact at gmail.com
Fri Aug 21 16:28:56 EDT 2009

The question is, in this case when the user gets to see a bit too much of
the output before he sees the input, if that really qualifies as an
"incorrect" program. It's a bit in the gray zone I guess. You could even
argue that it's a feature that input and output are not really synched, they
are lazy, input is only read when evaluated; if you want to sync them, use a
syncIO action ;-) no that's silly of course.
Oh well, thanks for all the input, this was very informative for me hacker.

On Fri, Aug 21, 2009 at 10:20 PM, David Menendez <dave at zednenem.com> wrote:

> On Fri, Aug 21, 2009 at 3:29 PM, Peter Verswyvelen<bugfact at gmail.com>
> wrote:
> > On Fri, Aug 21, 2009 at 6:53 PM, David Menendez <dave at zednenem.com>
> wrote:
> >>
> >> Some people dislike seq because it lets you force strictness
> >> in cases where pattern matching cannot (like function arguments), but
> >> hardly anyone objects to pattern matching.
> >
> > Ah so it's subjective. Okay, it's sometimes hard for a newbie to find the
> > "truth" when several experts contradict eachother. Because often when
> people
> > claim something here, they have very good reasons for it, reasons that
> are
> > not obvious at all to your average newbie!
> You can make a pretty good argument that programs which rely on
> strictness for correctness (as opposed to space/time issues) are
> risky, because it's easy to get things wrong by accident. Internally,
> the IO monad may rely on strictness to make sure things happen in the
> proper sequence, but all of that is hidden so we don't have to worry
> about things like output happening too early because we haven't
> examined some input yet.
> This is also why some people object to getContents.
> For laughs, here's an example of IO code written using Haskell's old
> stream-based IO system, taken from "A History of Haskell":
> main :: Behaviour
> main ~(Success : ~((Str userInput) : ~(Success : ~(r4 : _))))
>  = [ AppendChan stdout "enter filename\n",
>    ReadChan stdin,
>    AppendChan stdout name,
>    ReadFile name,
>    AppendChan stdout
>    (case r4 of
>      Str contents -> contents
>      Failure ioerr -> "can’t open file")
>    ] where (name : _) = lines userInput
> It has a certain elegant purity, but I'm glad I don't have to use it.
> --
> Dave Menendez <dave at zednenem.com>
> <http://www.eyrie.org/~zednenem/>
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