[Haskell-cafe] Re: Where do I put the seq?
bugfact at gmail.com
Fri Aug 21 16:32:36 EDT 2009
typo, "sees the input" => "must enter the input"
On Fri, Aug 21, 2009 at 10:28 PM, Peter Verswyvelen <bugfact at gmail.com>wrote:
> 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>
>> > On Fri, Aug 21, 2009 at 6:53 PM, David Menendez <dave at zednenem.com>
>> >> 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
>> > "truth" when several experts contradict eachother. Because often when
>> > claim something here, they have very good reasons for it, reasons that
>> > 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>
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