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fixed point
by Harris, Andrew 29 Oct '03

29 Oct '03

Hi - I am trying to do Exercise 9.9 in HSOE; and I've come up with an answer that works but I'm not sure if it answers the question properly. The problem is: The Question: ------------- Suppose we define a function "fix" as: fix f = f (fix f) Suppose further we have a recursive function: remainder :: Integer -> Integer -> Integer remainder a b = if a < b then a else remainder (a - b) b Rewrite this function using "fix" so that it's not recursive. My Answer: ---------- wierdFunc x y z = if y - z > z then ((\a b c -> a b c) x (y-z) z) else ((\a b c -> a b c) (\d e -> d) (y-z) z) myRemainder = fix wierdFunc My Question: ------------ Does the ((\a b c -> a b c) x (y-z) z) mean that I've not removed the recursion? I was assuming that I was returning a function that is to be evaluated and not actually doing any recursion. That's why I thought I answered the question. However, I have a headache now and would like another opinion. thanks, -andrew

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Cast from and to CChar
by Christian Buschmann 29 Oct '03

29 Oct '03

Hi! I've got following problem. If I enter in ghci following line: Prelude Foreign.C> castCCharToChar $ castCharToCChar 'ü' I would expect that this returns 'ü', but it returns '\252'. Is this the correct behaviour? Or am I doing something wrong? Or are there any problems with language specific characters and CChar? thanks Christian B.

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I/O multiplexing (repost)
by Peter Simons 28 Oct '03

28 Oct '03

[ I'm posting this article here again, because the general ] [ mailing list seems to be closed to non-members, and I'm ] [ reading/posting through gmane.org. Pardon me, if you see ] [ this on both lists, please. -peter ] Hi, I have a question concerning "manual" I/O multiplexing in Haskell, or specifically with GHC. I have written an interface to the C library ADNS, which performs asynchronous DNS queries. Everything is fine and dandy, but now comes the hard part: The function foreign import ccall adns_beforepoll :: State -- ADNS context -> Ptr Pollfd -- space for 'n' Pollfd -> Ptr CInt -- n -> Ptr CInt -- max timeout -> Ptr (Timeval) -- current time -> IO CInt generates an array of file descriptors, suitable for calling poll(2). The function foreign import ccall adns_afterpoll :: State -> Ptr Pollfd -> CInt -> Ptr (Timeval) -> IO CInt scans this array (after poll has been called), performs I/O on all readable/writable sockets, and processes any packets that have been received. Obviously, the library wants me to call poll myself, though. But since poll is not supported by Haskell's run-time system, the call would block _all_ running threads in my process, not just the thread doing the DNS resolving. I have considered the following options to solve this: 1. Run the DNS resolver in a forkOS thread. 2. Declare all imported functions to be thread-safe (what they are) and use forkIO. 3. Call poll(2) without blocking, then conjure some magic to decide when to call it again without busy polling. 4. Register the file descriptors with GHC's internal poll event loop. Option (1) seems to be straight-forward, but I don't want to rely on the non-standard forkOS function unless I really have to. The same goes for (4): This is probably a good solution, but it's not portable among compilers. (3) works fine, but is unsatisfactory because the resolver will have comparably large latency times. (I have been processing DNS queries every, say, second.) So option (2) seems quite attractive compared to the others ... But the question is: Would that even work? Any ideas, thoughts, recommendations? Peter

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RE: fixed point
by Harris, Andrew 27 Oct '03

27 Oct '03

> > Notice that, (\x -> x) a reduces to a, so (\a b c -> a b c) x (y-z) z > reduces to x (y-z) z. You can therefore simplify your > function quite a > bit. > wierdFunc x y z = if y-z > z then x (y-z) z else (\d e -> d) (y-z) z > and you can still apply that lambda abstraction (beta-reduce) > wierdFunc x y z = if y-z > z then x (y-z) z else y-z > None of these (except, of course, fix and remainder) are recursive. A > recursive function is just one that calls itself. For wierdFunc to be > recursive, the identifier wierdFunc would have to occur in the > right-hand side of it's definition. > Thanks for your help. For some reason I didn't think "x (y - z) z" and "y - z" had the same type. I am still trying to understand how they do. Also, had a feeling the fix function was related to the "Y" combinator; it seems they're the same thing! http://en.wikipedia.org/wiki/Y_combinator thanks again, -andrew

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replacing guile with haskell?
by David Roundy 25 Oct '03

25 Oct '03

I'm wondering what the possibilities are for replacing the use of guile with a haskell interpereter? I'd like to be able to embed a haskell interpereter (presumably hugs) withing my program, so that the input file could be a haskell program. The application is a numerical simulation code, for which the input is (among other things) a photonic crystal structure. Practically, this means that the input file needs to be a program (or the user needs to write a program to generate the input file--ugly). Currently my code works as a C++ library, but that isn't particularly convenient (except for development). A similar code (mpb--it's available in debian) by a coworker has the input format in scheme using guile. That's nice in many ways, except I can't stand scheme. So I'm wondering whether there might be some convenient way to accomplish the same goal with haskell. Currently, my best (and only) idea is to make the input file a script with #!/usr/bin/env runhugs at the top (and compiling an interface module with ffihugs). There are a couple of issues with this. The first is that I've heard that hugs isn't intended for numerical work (which is what I'm doing). I'm not sure if this will be a problem, since hugs won't be doing any of the real work anyways. The other is that according to the man page, it seems that hugs only supports ffi on x86, powerpc and sparc, which seems likely to be a show stopper. Since I'll need to run the code on supercomputers, I won't always have a choice of architectures, and at least support for POWER (maybe comes free with powerpc?) would be necesary--IBM SP machines are quite nice. In this regard, ghc seems worse than hugs (the thought of bootstrapping ghc on a supercomputer gives me the shivers), and nhc98 last time I looked didn't support 64 bit platforms. Is there a solution to these problems, or will I be stuck with thousands of parentheses in my input file? -- David Roundy http://www.abridgegame.org/darcs

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random numbers
by Jacinta Mary FITZGERALD 24 Oct '03

24 Oct '03

I'm very much a beginning programmer, studying Haskell. I have two questions - Firstly, I am trying to generate random numbers in Haskell, but although I have found a 'random' library on www.zvon.org, I don't really know how to include library functions, and the documentation given doesn't really tell me the effective difference between all the functions provided in the library. In another interesting development, I couldn't find any mention of this library in haskell.org's list of libraries. Secondly, but perhaps more importantly, is there a more 'beginnerish' list that I should be addressing this to? I've been following discussions on this one and they don't seem to be at quite this level. Thanks Jac

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RE: "hSetBuffering stdin NoBuffering" messes up terminal
by Simon Marlow 22 Oct '03

22 Oct '03

> > If I understand recent changes correctly, this is a > deliberate decision. The > > idea is that if you wanted to write a program like 'stty' > in Haskell, you'd > > be very disappointed if the terminal settings got switched > back the moment > > your Haskell program terminated. > > It's deliberate, but (AFAIK) not for those reasons. The idea is so > that "simple, stupid programs" do what the programmer expects (and > saving the list from lots of "my program doesn't get any input until > the user hits Return" questions). > > If you were writing "stty" in Haskell, you would presumably use the > PosixTTY functions directly. GHC 6.2 will have slightly better behaviour in this regard. If the terminal settings are changed as a result of hSetBuffering, then they will be restored on exit. This still allows stty-type programs to be written, as long as you don't also call hSetBuffering on the same Handle. Cheers, Simon

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Strictness annotations
by Derek Elkins 21 Oct '03

21 Oct '03

I've wondered for a while why Haskell 98 didn't provide strictness annotations for functions a la Clean and no implementations provide it as an extension (actually I don't know about NHC, but I'm pretty sure neither GHC nor Hugs do). Clean's solution seems easier to read and write and seems like it would be trivial (though possibly tedious) to implement. So my questions are: What was the motivation for not providing them in Haskell 98? Are there any technical problems with adding them to Haskell now (or then)? If there aren't any issues, are strictness annotations on any implementors' wish-list to be added when there is sufficient demand or no more pressing matters? Googling the haskell archives didn't (immediately at least) turn up any answers to these questions, but did turn up a couple posts suggesting the desirability this. http://www.haskell.org/pipermail/haskell/2002-March/009176.html http://www.haskell.org/pipermail/haskell-cafe/2001-June/001995.html

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tracing Haskell programs
by wvasconc＠csd.abdn.ac.uk 20 Oct '03

20 Oct '03

Hi folk Is there a way to follow the "execution" of a Haskell program? What I mean here is a way to see which function definition is being used at any moment, and how the execution control moves around a script file. If this is a preposterous question, just tell me nicely and I will go away! Oh, and if this has been discussed before, please tell me where and I will chase it up. Thanks! -- Wamberto Vasconcelos, PhD wvasconcelos(a)acm.org Department of Computing Science http://www.csd.abdn.ac.uk/~wvasconc University of Aberdeen, Aberdeen AB24 3UE, Scotland, UK Phone +44 (0)1224 272283 Fax +44 (0)1224 273422

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"listProduct" -- is this a standard function?
by Graham Klyne 20 Oct '03

20 Oct '03

I've constructed a "listProduct" function that I think I've seen somewhere else... is it a standard function? If so, where is it defined? My code is: [[ -- |Given a list of lists, construct a new list of lists where -- each member of the new list is the same length as the original -- list, and each member corresponds to a different choice of -- one element from each of the original members in the -- corresponding position. Thus: -- -- listProduct [[a1,a2],[b1],[c1,c2]] = -- [ [a1,b1,c1], [a1,b1,c2], [a2,b1,c1], [a2,b1,c2] ] -- -- Note: The length of the resulting list is the product of -- lengths of the components of the original list. Thus, if -- any member of the original list is empty then so is the -- resulting list: -- -- listProduct [[a1,a2],[],[c1,c2]] = [] -- listProduct :: [[a]] -> [[a]] listProduct [] = [[]] listProduct (as:ass) = concat [ map (a:) (listProduct ass) | a <- as ] test1 = listProduct [["a1","a2"],["b1"],["c1","c2"]] test2 = listProduct [["a1","a2"],[],["c1","c2"]] ]] And a supplementary point. Following a comment made to me previously, I can replace this list comprehension (not having any guard expressions) with another map function, thus: [[ lp [] = [[]] lp (as:ass) = concatMap (\a -> (map (a:) (lp ass))) as ]] I think I should also be able to eliminate the lambda-abstraction, but I can't see how. I prefer the list comprehension, as I find that easier to read than the \-expression. #g ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact

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