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

21 Aug 2009


      typo, "sees the input" => "must enter the input"
On Fri, Aug 21, 2009 at 10:28 PM, Peter Verswyvelen 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 wrote:
...
...
On Fri, Aug 21, 2009 at 6:53 PM, David Menendez 
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
...
"truth" when several experts contradict eachother. Because often when
On Fri, Aug 21, 2009 at 3:29 PM, Peter Verswyvelen
wrote:
the
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 
http://www.eyrie.org/~zednenem/