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 <[email protected]> wrote: > On Fri, Aug 21, 2009 at 3:29 PM, Peter Verswyvelen<[email protected]> > wrote: > > On Fri, Aug 21, 2009 at 6:53 PM, David Menendez <[email protected]> > 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 <[email protected]> > <http://www.eyrie.org/~zednenem/> >
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