March 2003 - Haskell-Cafe

Re: is identity the only polymorphic function without typeclasses?
by Jon Cast 04 Mar '03

04 Mar '03

Cagdas Ozgenc <co19(a)cornell.edu> wrote: > Greetings, > Is identity function the only meaningful function one can write > without constraining the type variable using a typeclass? If not, > could you please give a counter-example? Certainly you can write lots of ``meaningful function''s without type classes: not, (&&), (||), as well as many more complicated functions at more complicated types. You can also write useful polymorphic functions without type classes, as long as you specify at least one type. For example, you can write polymorphic functions over/yielding lists, such as repeat, cycle, map and its many relatives, foldr and its many relatives, take and its relatives, takeWhile and its relatives, etc. Similar functions often exist for other types. I'm somewhat curious, though: why do you ask this question? How do you expand your question that makes the answer seem to be ``no''? > Thanks Jon Cast

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speedup help
by Damien R. Sullivan 04 Mar '03

04 Mar '03

So, I'm having to calculate 'n choose k' an awful lot. At the moment I've got this: comb :: Integer -> Integer -> Integer comb m 0 = 1 comb m n = (numerator(toRational (fact m) / toRational (fact n * fact (m-n)))) where fact is a memoized factorial function. It's not perfectly memoized, though; I use lists, since that's easier by default. They should be arrays, and possibly just changing that would speed comb up a lot. (Comb is currently 40% of runtime, fact is 23%.) But I think it should be possible to speed up comb itself, too. comb is only called from here: sumbn n = sum [ bernoulli i * fromIntegral(comb (n+1) i) | i <- [0 .. n-1] ] Here was one try: fcomb :: Integer -> Integer -> Integer fcomb m 0 = 1 fcomb m n = res where res = last * (m-n+1) `div` n last = res except, obviously, this doesn't work. I hope it's clear what I'm trying to do, or what I would be in a more imperative language -- in C I'd probably have some static variable in fcomb. I figure monads are needed, but I've been unable to figure them out enough to apply them here. Will the monadism propagate all the way up and require changing lots of function types? Bleah. I'm using ghc, can I sneak some mutable in here instead? Any advice? Also on using arrays, where my parameters come off the command line. I imagine in C++ I'd just precompute a bunch of tables and then just use those values in the actual algorithm. Thanks! -xx- Damien X-) (if you're curious, this is for a class, but not a class on using Haskell. I chose to use Haskell for this assignment after ghc -O, to my surprise, outperformed ocaml. I suspect GMP deserves the real credit, but hey.)

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RE: Network module problem
by Simon Marlow 04 Mar '03

04 Mar '03

> Hello. I'm running into a problem with the Network module, > which I suspect > is pretty easy to fix, but am not sure how to best do so. > > The problem is that "accept" fails when the reverse DNS > fails, with the > following error: > > Fail: does not exist > Action: getHostByAddr > Reason: no such host entry > > I'm not sure how to get around this. I don't actually need > the hostname of > the client, and would be happy to just substitute its IP > address in that field, but I'm not sure how to do that. We've made this change in the library, the next release of GHC will include the fix. Unfortunately there's no immediate workaround, other than using the Network.Socket interface to accept (which isn't hard, the Network.accept wrapper is fairly simple). Cheers, Simon

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do let in
by Damien R. Sullivan 04 Mar '03

04 Mar '03

main = do args <- System.getArgs let (m, b) = (read (args!!0), read (args!!1)) let lim :: Int lim = read (args!!2) printstate = args!!3 time1 <- getClockTime let n = 2^b let afact = array (0,n) ((0,1):[(i,i*afact!(i-1)) | i<-[1..n]]) in let (glo, ghi) = gamma_tup lim m b time2 <- getClockTime gives the Hugs error ERROR "gamma3_7.hs":141 - Syntax error in expression (unexpected `;', possibly due to bad layout) I tried indenting the last two lines, or just the penulatimate line, but no joy. I've modified my functions to use afact instead of fact, but I'm hoping to have it exist as a global for them, rather than modifying all the function signatures to pass afact down the chain. Help, please? -xx- Damien X-)

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Re: do let in
by Jon Cast 04 Mar '03

04 Mar '03

"Damien R. Sullivan" <dasulliv(a)cs.indiana.edu> wrote: <snip> > For the reason that I'm lazy and don't want to have to modify all my > functions which use afact, or call functions which use afact, and > don't see why I should have to -- they were able to call the 'fact' > function as a global, and can refer to a global 'afact' if I define it > outside of main with a fixed value. I don't see why having a global > dependent on outside input should be so much harder. Never programmed in C++ much, eh? In general, getting the ordering of initialization right in the general case is a harder problem than you might think. <snip> Jon Cast

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Re: is identity the only polymorphic function without typeclasses?
by Jon Cast 03 Mar '03

03 Mar '03

Andrew J Bromage <ajb(a)spamcop.net> wrote: > G'day. > On Mon, Mar 03, 2003 at 11:49:29AM +0200, Cagdas Ozgenc wrote: > > Yes, I thought about these too. Do you find these functions > > practically useful? Can you give an example where I can utilize > > these functions? > Functions like this are useful for plugging into higher-order > functions to tailor them for your specific needs. > Here's an artificial example: > length = sum . map (const 1) Incidentally, here's another artificial example: > indices = zipWith const [0..] This can be used to define scanl, incidentally: > scanl f z xn = [foldl f z (take i xn) | i <- indices (undefined:xn)] (where the undefined: just reflects the fact that we want one more element than xn has). Jon Cast

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Network module problem
by David Roundy 03 Mar '03

03 Mar '03

Hello. I'm running into a problem with the Network module, which I suspect is pretty easy to fix, but am not sure how to best do so. The problem is that "accept" fails when the reverse DNS fails, with the following error: Fail: does not exist Action: getHostByAddr Reason: no such host entry I'm not sure how to get around this. I don't actually need the hostname of the client, and would be happy to just substitute its IP address in that field, but I'm not sure how to do that. If I understand correctly (which I may not), if I were to catch this connection with code like: (myh,_,_) <- accept s `catch` ... there would be no way to actually get the handle itself from the connection, which seems to imply that I'll have to go into the Network.Socket module and use the lower level code (which looks like it would be straightforward). However, it seems that this is a simple enough (and presumably common enough) problem that there should be a more elegant solution which would allow me to use the high level interface. Is there any way to `catch` the exception within getHostByAddr, and substitute the IP address for the output of that function? (I'll admit, I've never thoroughly understood exceptions in any language, in the sense of understanding how really to use them right.) Also, what if I just didn't care about the hostname? That seems like a common enough case that you wouldn't always want to do a reverse DNS lookup on every accept. Does that just mean you would have to use the low level interface? -- David Roundy http://civet.berkeley.edu/droundy/

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modeling out of memory
by Cagdas Ozgenc 03 Mar '03

03 Mar '03

Greetings, 1) How does one model "out of memory" condition in Haskell, perhaps using a Maybe type? 2) Could you give an intutive description of data construction, and how it relates to lamda calculus? Thanks

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is identity the only polymorphic function without typeclasses?
by Cagdas Ozgenc 02 Mar '03

02 Mar '03

Greetings, Is identity function the only meaningful function one can write without constraining the type variable using a typeclass? If not, could you please give a counter-example? Thanks

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