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Re: How to detect finite/infinite lists?
by Juanma Barranquero 18 Sep '03

18 Sep '03

On Thu, 18 Sep 2003 14:20:46 -0400, "David Barton" <dlb(a)patriot.net> wrote: > that's halting problem equivalent. Yes, I know it's a halting problem, in the general case. But the compiler could try to say if it knows it *is* a finite list. So: x = [1..10] y = [1..] z = someFunctionOrOther knownToBeFinite x -> should return Just True knownToBeFinite y -> should return Just False knownToBeFinite z -> should return Nothing ("I don't know"). or something along these lines. > Nope. Ok. Thanks, /L/e/k/t/u

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RE: Fast I/O with sockets
by Simon Marlow 17 Sep '03

17 Sep '03

> I was wondering how to read and write from a Socket in the most > performant way. According to the library documentation from GHC, > 'hGetBuf' and 'hPutBuf' are the way to go. But my problem with these > functions is that they expect to read exactly the number of bytes I > gave them! > > The IO computation > > hGetBuf sock ptr 1024 > > wants to read 1024 bytes, no less. This doesn't work for me, because I > need to drive an interactive line-based protocol session. I need the > read operation to consume as much data as is available and then to > return immediately, so that I can process the command right away. > > As an alternative, I looked at 'hGetLine', but this doesn't work for > me either, at least not very well, because in my protocol, lines are > terminated by "\r\n" -- not just "\n", like hGetLine expects it. hGetLine should work fine, indeed that's what I used in the Haskell Web Server. Each line will have an extra '\r' at the end, but you just need to allow for that. We really should have non-blocking versions of hGetBuf and hPutBuf, and in fact I have the code sitting in my tree. I'll take a look and see if it can be committed. Cheers, Simon

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Re: Improvement on this function
by Jon Fairbairn 17 Sep '03

17 Sep '03

On 2003-09-17 at 10:31EDT Gordon James Miller wrote: Something I think is more café than language: > Hello all. > > I'd be interested in getting some feedback on how to do this linear > interpolation function a little more cleanly. The normal way that this > is taught is to find the set of indices in the x list that bracket the > input value, then use the slope between these points to calculate the y > value for the input value. > > I had a version working at one point using !! to access particular > elements but I wasn't sure which one was the better solution. !! can be expensive. > linterp :: [Double] -> [Double] -> Double -> Double > linterp (x1:x2:xs) (y1:y2:ys) x > | x <= x2 || xs == [] = linterpPair x1 x2 y1 y2 x > | otherwise = linterp (x2:xs) (y2:ys) x > where linterpPair x1 x2 y1 y2 x = x1 + (x - x1) * (y2 - y1) / (x2 - > x1) It seems to me that you have too much going on in one function. It would be better to break it into the part that finds the point where the interpolation is supposed to happen and the function that does the interpolation. You sort-of do this, but don't go far enough for my taste. You don't say whether the xs are supposed to be in increasing order. Also it's not obvious that your version does the right thing in the case where x < head xs, and linterpPair doesn't look right to me. Something like: linterp :: [Double] -> [Double] -> Double -> Double linterp xs ys x = linterpPair (neighbours (zip xs ys)) where linterpPair ((x1,y1),(x2,y2)) = y1 + (x - x1) * (y2 - y1) / (x2 - x1) neighbours all@(p@(x1,y1):rest) | x >= x1 = head (dropWhile ((<x). fst . snd) (all `zip` rest)) would be more to my taste, though it would be better to handle the errors rather than leave them to the runtime system. I think I would also define the function as linterp :: [(Double, Double)] -> Double -> Double since that seems more natural as the data are connected that way, and it would eliminate a zip. -- Jón Fairbairn Jon.Fairbairn(a)cl.cam.ac.uk

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Querying DNS from Haskell
by Peter Simons 16 Sep '03

16 Sep '03

Hi, is anyone aware of a library for Haskell, which allows to query domain name servers for MX, A, and PTR records in a non-blocking way? My first guess was to create an Interface to ADNS [1], but I figured, maybe someone else has already solved this problem? Peter [1] http://www.gnu.org/software/adns/

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RE: simpler I/O buffering [was: RE: An IO Question from a Newbie]
by Simon Marlow 16 Sep '03

16 Sep '03

> Writing a sequence of characters across the interface I'm > proposing is a > request by the writing program for those characters to appear > at their > destination "soon". Ideally, "soon" would be "immediately"; > however, the > characters' appearance may deliberately be delayed ("buffered"), for > efficiency, as long as such delay is "unobtrusive" to a human > user of the > program. Buffering timeouts would depend on the device; for > a terminal, > perhaps 50-100 ms would be appropriate. Such an interval > would tend not > to be noticeable to a human user but would be long enough to > effectively > collect, say, an entire line of output for output "in one > piece". The use > of a reasonable timeout would avoid the confusing behavior where a > newline-less prompt doesn't appear until the prompted data is entered. So, let me see if I've understood: the situation would be exactly as now, except that the buffer automatically gets flushed if it stays non-empty for more than a certain time interval. I don't think I like it: whatever time interval you choose will be too long. If you're doing communication with another program, for example, you really don't want to have the dialog stalled for 100ms doing nothing. Sure, you could insert a flush, but only a programmer knowledgeable about buffering will know to do that, or a programmer who has debugged the program in detail to find out why it is sitting idle most of the time. So, in the end the programmer still needs to know about buffering. I'd prefer it if the program didn't work at all when the flush is missing, because it tells you immediately that something is wrong. Cheers, Simon

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Fast I/O with sockets
by Peter Simons 16 Sep '03

16 Sep '03

Hi, I was wondering how to read and write from a Socket in the most performant way. According to the library documentation from GHC, 'hGetBuf' and 'hPutBuf' are the way to go. But my problem with these functions is that they expect to read exactly the number of bytes I gave them! The IO computation hGetBuf sock ptr 1024 wants to read 1024 bytes, no less. This doesn't work for me, because I need to drive an interactive line-based protocol session. I need the read operation to consume as much data as is available and then to return immediately, so that I can process the command right away. As an alternative, I looked at 'hGetLine', but this doesn't work for me either, at least not very well, because in my protocol, lines are terminated by "\r\n" -- not just "\n", like hGetLine expects it. Finally, I considered 'hGetContents', which has the nice property of returning a lazy-evaluated list of the data read from the socket. But unfortunately, I don't see how I could implement something like a "read timeout" on the socket when using this function. Any ideas, anybody? Peter

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RE: An IO Question from a Newbie
by Simon Marlow 16 Sep '03

16 Sep '03

> > But there's one significant difference between C and > Haskell, which is > > applicable in the case of Matt's program. In C, any line-buffered > > output streams are automatically flushed when a read from an > > unbuffered or line-buffered stream can't be satisfied from > its buffer. > > Interesting. I didn't know this. Maybe we should match this > behaviour, or > provide a write-string-and-flush function. It seems like this issue > is causing an undue amound of trouble. I wrote GHC's IO library, and deliberately didn't include this feature. The previous version of the library did have such a feature, specifically for stdin/stdout. Note that the report doesn't say we must do this. The reason I didn't include the feature is because I can't see a way to do it right. Flushing *all* line-buffered handles (the ANSI C way) doesn't seem right. Flushing stdout just because we read from stdin is not right, because the two streams might refer to completely different I/O objects. Perhaps we should attempt to detect when there are two streams connected to the same I/O object (file, pipe, tty, whatever) and enable the magic flushing then. But now do I have to explain to people how this works? I suppose we could take the view that extra flushing is basically harmless, so it doesn't matter that we flush a bunch of Handles more often than we need to. The advantage of the current scheme is that it is easy to understand and explain; the library doesn't try to do clever stuff behind your back. The disadvantage is that it catches people out, and it sometimes requires you to import IO in an otherwise Prelude-only program. I'm more-or-less agnostic - if there's a way to avoid catching people out without introducing too much overhead or complicated rules that we have to explain, then I'll happily implement it. Cheers, Simon

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threaded red-black tree
by Lex Stein 15 Sep '03

15 Sep '03

Hi, No one responded to my question several weeks ago about a purely functional implementation of a threaded, red-black tree. My message was sent about the same time as that flurry of silly emails about "how to respond to homework questions". Was my message not responded to because it was classified as a homework question? I assure you this is officework, not homework. I ended up porting Okasaki's red-black tree implementation; hacking it apart with a bunch of mutation for the threading of the list through the tree. However, I'm still missing a deletion function and I haven't been able to find a prototype (Okasaki's red-black tree module lacks delete). My study of the RB-tree deletion routine in CLR hasn't yet yielded an adaptation for a functional environment. Any advice would be much appreciated. Thanks, Lex -- Lex Stein http://www.eecs.harvard.edu/~stein/ stein(a)eecs.harvard.edu cell: 617-233-0246

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Instance checking and phantom types
by Nick Name 15 Sep '03

15 Sep '03

Hi all, I have an example wich I don't understand: ---------------begin class C t data T = T instance C T data C t => T1 t = T1 f1 :: T1 () f1 = T1 data C t => T2 t = T2 t f2 :: T2 () f2 = T2 () ----------------end The first function, f1, is accepted both by hugs and ghc, unlike the second wich is rejected. Why does this happen? Shouldn't f1 be rejected with "no instance C ()" Thanks for help Vincenzo

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Read file problems
by Frederic BELLOC 15 Sep '03

15 Sep '03

HI, I have a recurrent probleme with function readFile and Monad : it is that i try to get a line, transform it and stock it in a list and what i want is that function return me the list. But hugs say me that in readFile my_file >>= \s -> map cons_line (lines s) readFile is a IO String type but is used here as [[Char]] type... and i don't know what to do... Ghc say me that is the lambda abstraction that fails with "map cons_line (lines s)" , he couldn't match IO against [] ! Help me please, i understand well (i hope) how Monad work but here i don't see a solution. Sorry for my poor english but i'm already a student. Thanks for all. -- C, socks and sun ;-) and haskell bugs :-(

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