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Asynchronous exceptions and "resume"
by Nick Name 26 Mar '03

26 Mar '03

Hi all, is there a way, or is it planned to, or has anyone published articles on... resuming from asynchronous exceptions? I mean: it would be useful there was a suspend :: ThreadID -> IO () where the result is the remaining computation of the other thread, wich one could forkIO again, or simply run in the current thread. If this is not possible, it would still be useful that the IO action returned by "suspend" would only "resume" the other thread, as an alternative to a brutal "kill" wich destroys any work done by the other thread. The first form seems interesting when one wants to fork off a long computation, but wants the ability to wait for it again if necessary; pointers to articles are always appreciated. Vincenzo

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(no subject)
by Mrs.Sese-seko 26 Mar '03

26 Mar '03

Dear friend, I am Mrs. Sese-Seko widow of late president Mobutu Sese-Seko of zaire? now known as democratic republic of congo (drc). I am moved to write you this letter, this was in confidence considering my present circumstance and situation. I escaped along with my husband and two of our sons Theophilus and Basher out of Democratic Republic of Congo (DRC) to Abidjan, cote d'ivoire where my family and I settled, while we later moved to settled in morroco where my husband later died of cancer disease. However due to this situation we decided to changed most of my husband's billions of dollars deposited in swiss bank and other countries into other forms of money coded for safe purpose because the new head of state of (dr) Mr, laurent kabila has made arrangement with the swiss government and other european countries to freeze all my late husband's treasures deposited in some european countries. hence my children and I decided laying low in Africa to study the situation till when things gets better. Now that President kabila is dead and the son taking over (Joseph kabila). one of my late husband's chateaux in southern france was confiscated by the french government, and as such I had to change my identity so that my investment will not be traced and confiscated. I have deposited the sum Thirty Million United State Dollars(US$30,000,000,00.) with a security company , for safekeeping. The funds are security coded to prevent them from knowing the content. What I want you to do is to indicate your interest that you will assist us by receiving the money on our behalf. Acknowledge this message, so that I can introduce you to my son (Theophilus) who has the out modalities for the claim of the said funds. I want you to assist in investing this money, but I will not want my identity revealed. I will also want to buy properties and stock in multi-national companies and to engage in other safe and non-speculative investments. May I at this point emphasise the high level of confidentiality, which this business demands, and hope you will not betray the trust and confidence, which I repose in you. in conclusion, if you want to assist us , my son shall put you in the true picture of the business, tell you where the funds are currently being maintained and also discuss other modalities including remuneration for your services. For this reason kindly furnish us your contact information, that is your personal telephone and fax number for confidential purpose. Best regards, Mrs,sese-seko

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Re: African money
by Marcus Lindblom 26 Mar '03

26 Mar '03

From: John Peterson <peterson-john(a)cs.yale.edu> > > If the spam storm gets too heavy on haskell-cafe we can switch to > moderator approval as on the main Haskell list. We have the spam > assassin available at haskell.org but I've been reluctant to turn it > loose on the lists. Maybe the time has come ... Many lists use moderator approval on non-list members. Maybe spam assassin could be turned loose on just those non-member mails, and have those passing through be moderator reviewed? /Marcus

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25 Mar '03

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shortest path with Haskell
by tuyen pham 25 Mar '03

25 Mar '03

Hi every one ! I have a big problem with implementation the Dijkstra's shortest path with Haskell .the condition is it must be implemented with priority queue using list (or tree).Any of yo have the code for this ? I really really needs for help !!! thankssssssss a lot!!!!!! pham __________________________________________________ Do You Yahoo!? Everything you'll ever need on one web page from News and Sport to Email and Music Charts http://uk.my.yahoo.com

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(no subject)
by Mrs.Sese-seko 25 Mar '03

25 Mar '03

Dear friend, I am Mrs. Sese-Seko widow of late president Mobutu Sese-Seko of zaire? now known as democratic republic of congo (drc). I am moved to write you this letter, this was in confidence considering my present circumstance and situation. I escaped along with my husband and two of our sons Theophilus and Basher out of Democratic Republic of Congo (DRC) to Abidjan, cote d'ivoire where my family and I settled, while we later moved to settled in morroco where my husband later died of cancer disease. However due to this situation we decided to changed most of my husband's billions of dollars deposited in swiss bank and other countries into other forms of money coded for safe purpose because the new head of state of (dr) Mr, laurent kabila has made arrangement with the swiss government and other european countries to freeze all my late husband's treasures deposited in some european countries. hence my children and I decided laying low in Africa to study the situation till when things gets better. Now that President kabila is dead and the son taking over (Joseph kabila). one of my late husband's chateaux in southern france was confiscated by the french government, and as such I had to change my identity so that my investment will not be traced and confiscated. I have deposited the sum Thirty Million United State Dollars(US$30,000,000,00.) with a security company , for safekeeping. The funds are security coded to prevent them from knowing the content. What I want you to do is to indicate your interest that you will assist us by receiving the money on our behalf. Acknowledge this message, so that I can introduce you to my son (Theophilus) who has the out modalities for the claim of the said funds. I want you to assist in investing this money, but I will not want my identity revealed. I will also want to buy properties and stock in multi-national companies and to engage in other safe and non-speculative investments. May I at this point emphasise the high level of confidentiality, which this business demands, and hope you will not betray the trust and confidence, which I repose in you. in conclusion, if you want to assist us , my son shall put you in the true picture of the business, tell you where the funds are currently being maintained and also discuss other modalities including remuneration for your services. For this reason kindly furnish us your contact information, that is your personal telephone and fax number for confidential purpose. Best regards, Mrs,sese-seko

1 0

Problem with hugs, concurrency and unsafeInterleaveIO (maybe a bug?)
by Nick Name 17 Mar '03

17 Mar '03

I am trying to lazily wait an MVar in hugs, in conjunction with concurrent haskell: ----- import Concurrent import IOExts f = do v <- newEmptyMVar c <- getContents forkIO (putMVar v (head c)) r <- unsafeInterleaveIO (takeMVar v) return v f2 = f >>= unsafeInterleaveIO . takeMVar main1 = f >>= takeMVar >>= print main2 = f2 >>= print main3 = f2 >>= (\ x -> yield >> print x) ----- main1 and main3 work flawlessly, but main2 aborts with "no more threads to run". I think that main2 should work, because in evaluating the result of f2, the main thread should suspend and yield. Is there somebody who wants to enlighten me? Vincenzo

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AW: How to read an initialization file.
by Markus.Schnell＠infineon.com 14 Mar '03

14 Mar '03

I would recommend reading the variables in the main and pack the rest of your program into a monad. But only if you really need to access the variables from everywhere and don't want to pass them around explicitely. Markus > I need to be able to access these values effieiently and from > virtually anywhere in the program. It would be nice to "at some > point" open the file and read all these variables, all at once.

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Redefining methods in a subCLASS
by Oleg 14 Mar '03

14 Mar '03

Hi Is it possible to redefine methods in a subclass (not an instance)? E.g. I get errors in class (Show a) => Employee a where speak :: a -> [Char] speak x = "Employee: " ++ (show x) class (Employee a) => Manager a where speak x = "Manager: " ++ (show x) Thanks Oleg

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Re: How to search for a string sequence in a file a rewrite it???
by Tom Pledger 13 Mar '03

13 Mar '03

(moving to haskell-cafe) Alexandre Weffort Thenorio writes: | Ooops a small error before but here is the right one. | | Great. I got almost everything. My problem now is: | | I got a function called findstr where | | findstr "aaaa" "zzzzz" ["xxxxaaaa","xxxaaaaxxx"] = | ["xxxxzzzzz","xxxzzzzzxxx"] with the inferred type findstr :: String -> String -> [String] -> [String] | and then I try something like | | fillIn lines = do | bkfilled <- (findstr str str2 lines) with the inferred type (findstr str str2 lines) :: [String] Note that [] (the list data constructor) is a monad. So, this 'do' expression is in the [] monad, where perhaps you intended the IO monad. Computations in the [] monad have the effect of iterating over the elements of lists. So, the bound variable gets the inferred type bkfilled :: String which you've noticed in the error message. | write bkfilled | | where write takes a [String] and concatenates it writing it to a file then. | But I get this error saying: | | Expected Type: [String] | Inferred Type: String | In the first argument of 'write' namelly 'bkfilled' | In a do expection pattern binding: write bkfilled | | Any idea?? I mean bkfilled is supposed to be [String] but it says it is a | String, any idea why??? The smallest code change to fix this is to add a 'return', which will wrap another monad (in this case, IO) around findstr's [String] result... > fillIn lines = do > bkfilled <- return (findstr str str2 lines) > write bkfilled ... but you should be able to simplify that code, using one of the monad laws in http://haskell.org/onlinereport/basic.html#sect6.3.6 Regards, Tom

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Learning Haskell, update
by Arjan van IJzendoorn 13 Mar '03

13 Mar '03

Hi all, I've removed the frequently asked questions box at the bottom. It was unused. Fritz Ruehr made a nice Learning Haskell logo. Thank you, Fritz! NHC is compilable under Windows now, so the remark "no windows version" has been removed. Is there someone who wants to make a more decent language comparison table? If not, I'm thinking of removing it. Many things are said about it already in the comp.lang.functional FAQ. Greetings, Arjan

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Explicit function call
by pavel＠joker.botik.ru 12 Mar '03

12 Mar '03

Hello, How can I make an explicit function call in a "do" sequence? Ex: ... do let a = myFunc ... b = myFunc ... c = "Something else" return c ... As I understand myFunc will not be executed, but I need it... Please, help. -- Pavel Zhbanov

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Alternatives to finalization
by Nick Name 11 Mar '03

11 Mar '03

As the result of a conversation on haskell-gui, I have tried to implement the disallocation of resources when a stream is garbage collected. To explain myself: I have a function f :: IO [a] which returns a lazy stream after allocating some resource to feed it (say installing a callback). I wish that the resource could be disallocated when it's no longer used. I did the obvious implementation with Weak.addFinalizer; results are encouraging but not completely satisfying; the scheme I used is: f = do allocateResource l <- makeTheStream addFinalizer l (disallocateResource) return l The problem is that if no memory is allocated, no garbage collection happens; of course finalization is not guaranteed, as the manual states. Another alternative is to make f return an esplicit "close stream" action: f :: IO ([a],IO ()) Is anyone willing to explain me other alternatives if there are, or to tell me that there aren't? Thanks for attention Vincenzo

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ANNOUNCE: Helium, for Learning Haskell, version 1.1
by Arjan van IJzendoorn 11 Mar '03

11 Mar '03

Dear all, We are proud to announce version 1.1 of Helium. It can be found at the Helium website: http://www.cs.uu.nl/~afie/helium/ Version 1.1 has the following improvements: - An installer for Windows systems for an improved out-of- the-box experience. - A cool Java-based graphical interpreter called Hint written by a student at our institute. It has a nice GUI with colours, toolbar buttons and menu's. The best feature is that it integrates with your favourite editor and you can jump to the exact locations of messages by clicking on them! - New warnings and hints for common mistakes: "sin .3", "X = 5" (see below for examples of messages) - Completely new presentation of type errors with more information - Much improved syntax error messages thanks to a new lexer. And thanks to Parsec we can tell not only what was unexpected, but also what would be legal at that position. - The command-line interpreter is now called 'texthint' to avoid conflicts with hmake's 'hi' (sorry!). TextHint is improved in many ways. For one, it now accepts file paths after :l. Other improvements can be found if you type :? - User manuals for the different tools (helium, hint, texthint) and other helpful information on the Helium website - We've compiled Helium with Oxygen (ghc -O2) and this makes the compiler twice as fast. Great work, GHC people! - One well-placed 'seq' makes Helium another 25% faster. - Very many minor bug fixes - The webpage is simpler, wider and easier to navigate thanks to a road map Have fun! the Helium team -- Examples of Helium messages These messages where generated by typing expressions on the texthint prompt. If you compile a file with these errors you will get exact error locations. Prelude> [(1, 3] Unexpected close bracket ']' Hint: Expecting a close bracket for '(' Prelude> sinn .2 Warning: Function composition (.) immediately followed by number Hint: If a Float was meant, write "0.2" Otherwise, insert a space for readability Undefined variable "sinn" Hint: Did you mean "sin" ? Prelude> map [1..10] even Type error in application expression : map [1 .. 10] even term : map type : (a -> b) -> [a] -> [b] does not match : [Int] -> (Int -> Bool) -> c probable fix : re-order arguments Prelude> let X = 5 in X Left hand side pattern defines no variables Undefined constructor "X" Hint: Use identifiers starting with a lower case letter to define a function or a variable Undefined constructor "X" Prelude> 1+chr '0' Type error in variable expression : chr type : Int -> Char expected type : Char -> Int probable fix : use ord instead

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RE: GHC Error Message.
by Simon Peyton-Jones 10 Mar '03

10 Mar '03

------------------------------------------------------------------------ --------------------------- data AParser String = AP {apapply::([String]->[(String,[String])])} Whoa! You can't declare a data type like that! You must have a type variable after the data type name, thus: data AParser a = AP .... It's an egregious bug that GHC does not reject your data type declaration. After that, all bets are off. Some deeply-wired-in invariant is not being obeyed, and all manner of bad things may happen. Thank you for showing up the bug -- I'll fix it. Simon

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GHC Error Message.
by Mansour Al-Mutairi 09 Mar '03

09 Mar '03

Hi, Could someone please explain to me why the following error message happen: When I load the following code into GHC I get an error message: code: --------------------------------------------------------------------------------------------------- data AParser String = AP {apapply::([String]->[(String,[String])])} instance Monad AParser where return v = AP (\inp -> [(v,inp)]) (AP p) >>= f = AP (\inp -> concat [ apapply (f v) inp1 | (v,inp1) <- p inp]) --------------------------------------------------------------------------------------------------- error: --------------------------------------------------------------------------------------------------- Inferred type is less polymorphic than expected Quantified type variable `b' is unified with another quantified type variable `a' When trying to generalise the type inferred for `>>=' Signature type: forall a1 b1. AParser a1 -> (a1 -> AParser b1) -> AParser b1 Type to generalise: forall a1 b1. AParser a1 -> (a1 -> AParser b1) -> AParser b1 In the instance declaration for `Monad AParser' Failed, modules loaded: none. --------------------------------------------------------------------------------------------------- But when I create the type synonym type Stack = [String] data AParser String = AP {apapply::(Stack -> [(String,Stack)])} and use Stack instead of [String], GHC does not complain??? Thanks. Mansour.

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Re: speedup help
by oleg＠pobox.com 08 Mar '03

08 Mar '03

| comb is only called from here: | sumbn n = sum [ bernoulli i * fromIntegral(comb (n+1) i) | i | <- [0 .. n-1] ] Probably I misunderstand what "bernoulli i" stands for. If it is meant Bernoulli number B_i, http://mathworld.wolfram.com/BernoulliNumber.html then the above expression is quite inefficient. Bernoulli numbers with odd indices are all zero, except B_1, which is -1/2. Therefore, the above expression ought to be written as sumbn n = 1 - (fromIntegral (n+1)%(fromIntegral 2)) + sum [ (b' i) * fromIntegral(comb (n+1) i) | i <- [2,4 .. n-1] ] It appears that you compute the sumbn to obtain B_n from the equality sum [bernoulli i * (comb (n+1) i) | i<-[0..n]] == 0 Have you tried bernoulli n = sum [ (sumib k n) % (k+1) | k<-[1..n]] where sumib k n = sum [ (comb k r) * r^n | r<-[2..k]] - sum [ (comb k r) * r^n | r<-[1,3..k]] the advantage of the latter series is that sumib is an Integer, rather than a rational. The powers of r can be memoized. Here's the code -- powers = [[r^n | r<-[1..]] | n<-[1..]] powers = [1..] : map (zipWith (*) (head powers)) powers -- neg_powers = [[(-1)^r * r^n | r<-[1..]] | n<-[1..]] neg_powers = map (zipWith (\n x -> if n then x else -x) (iterate not False)) powers pascal:: [[Integer]] pascal = [1,1] : map (\line -> zipWith (+) (line++[0]) (0:line)) pascal bernoulli n = sum [ fromIntegral (sumib k n) % fromIntegral (k+1) | k<-[1..n]] sumib:: Int -> Int -> Integer sumib k n = sum $ zipWith (*) (neg_powers!!(n-1)) (tail $ pascal!!(k-1)) This code seems to compute 'bernoulli 82' in less then a second, in Hugs (on a 2GHz Pentium IV).

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RE: instance declaration troubles
by Simon Peyton-Jones 07 Mar '03

07 Mar '03

It's in 7.3.5.3. I'm going to make it more prominent S | -----Original Message----- | From: Hal Daume III [mailto:hdaume@ISI.EDU] | Sent: 07 March 2003 15:08 | To: Nick Name | Cc: Haskell Cafe | Subject: RE: instance declaration troubles | | Double Argh! I just noticed that this isn't what I meant to quote and now | I can't find it. But the basic idea of what I wanted to quote was that: | | YOu have to have at least one non-type variable in instance declarations | otherwise we don't necessarily know that reduction will terminate. I.e.: | | instance A a => B a where | instance B a => A a where | | will not terminate if you ever try to reduce A to B. I recall from the | docs that Simon is "looking for a rule which lifts this rule but still | maintains decidability." or something along those lines. | | Sorry about that! | | - Hal | | -- | Hal Daume III | hdaume(a)isi.edu | "Arrest this man, he talks in maths." | www.isi.edu/~hdaume | | On Fri, 7 Mar 2003, Simon Peyton-Jones wrote: | | > Argh. The documentation is out of date, or rather, inconsistent. | > | > In Section 7.3.4 you'll see that GHC -fglasgow-exts lifts the | > restriction that class methods must not constrain only the class type | > variable. | > | > I'll fix 7.3.5.2, which you are quoting. | > | > Simon | > | > | -----Original Message----- | > | From: Hal Daume III [mailto:hdaume@ISI.EDU] | > | Sent: 07 March 2003 03:16 | > | To: Nick Name | > | Cc: haskell-cafe(a)haskell.org | > | Subject: Re: instance declaration troubles | > | | > | From the GHC docs: | > | | > | In the signature of a class operation, every constraint must mention | > at | > | least one type variable that is not a class type variable. Thus: | > | | > | class Collection c a where | > | mapC :: Collection c b => (a->b) -> c a -> c b | > | | > | is OK because the constraint (Collection a b) mentions b, even though | > it | > | also mentions the class variable a. On the other hand: | > | | > | class C a where | > | op :: Eq a => (a,b) -> (a,b) | > | | > | is not OK because the constraint (Eq a) mentions on the class type | > | variable a, but not b. However, any such example is easily fixed by | > moving | > | the offending context up to the superclass context: | > | | > | class Eq a => C a where | > | op ::(a,b) -> (a,b) | > | | > | A yet more relaxed rule would allow the context of a class-op | > signature to | > | mention only class type variables. However, that conflicts with Rule | > | 1(b) for types above. | > | | > | -- | > | Hal Daume III | hdaume(a)isi.edu | > | "Arrest this man, he talks in maths." | www.isi.edu/~hdaume | > | | > | On Fri, 7 Mar 2003, Nick Name wrote: | > | | > | > | > | > I want to declare the following: | > | > | > | > class Get a where | > | > ls :: a b -> IO [b] | > | > mk :: IO [b] -> a b | > | > | > | > instance (Get a) => Functor a where | > | > fmap f x = mk (ls x >>= return . map f) | > | > | > | > | > | > But to have ghc type everything, I have to turn on "-fglasgow-exts | > | > -fallow-undecidable-instances -fallow-overlapping-instances". | > | > | > | > Is there a clean way to state that all types in my type class are | > also | > | > in the "Functor" type class? | > | > | > | > If not, what is the problem? | > | > | > | > Vincenzo | > | > _______________________________________________ | > | > Haskell-Cafe mailing list | > | > Haskell-Cafe(a)haskell.org | > | > http://www.haskell.org/mailman/listinfo/haskell-cafe | > | > | > | | > | _______________________________________________ | > | Haskell-Cafe mailing list | > | Haskell-Cafe(a)haskell.org | > | http://www.haskell.org/mailman/listinfo/haskell-cafe | > _______________________________________________ | > Haskell-Cafe mailing list | > Haskell-Cafe(a)haskell.org | > http://www.haskell.org/mailman/listinfo/haskell-cafe | > | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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instance declaration troubles
by Nick Name 07 Mar '03

07 Mar '03

I want to declare the following: class Get a where ls :: a b -> IO [b] mk :: IO [b] -> a b instance (Get a) => Functor a where fmap f x = mk (ls x >>= return . map f) But to have ghc type everything, I have to turn on "-fglasgow-exts -fallow-undecidable-instances -fallow-overlapping-instances". Is there a clean way to state that all types in my type class are also in the "Functor" type class? If not, what is the problem? Vincenzo

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RE: instance declaration troubles
by Simon Peyton-Jones 07 Mar '03

07 Mar '03

Argh. The documentation is out of date, or rather, inconsistent. In Section 7.3.4 you'll see that GHC -fglasgow-exts lifts the restriction that class methods must not constrain only the class type variable. I'll fix 7.3.5.2, which you are quoting. Simon | -----Original Message----- | From: Hal Daume III [mailto:hdaume@ISI.EDU] | Sent: 07 March 2003 03:16 | To: Nick Name | Cc: haskell-cafe(a)haskell.org | Subject: Re: instance declaration troubles | | From the GHC docs: | | In the signature of a class operation, every constraint must mention at | least one type variable that is not a class type variable. Thus: | | class Collection c a where | mapC :: Collection c b => (a->b) -> c a -> c b | | is OK because the constraint (Collection a b) mentions b, even though it | also mentions the class variable a. On the other hand: | | class C a where | op :: Eq a => (a,b) -> (a,b) | | is not OK because the constraint (Eq a) mentions on the class type | variable a, but not b. However, any such example is easily fixed by moving | the offending context up to the superclass context: | | class Eq a => C a where | op ::(a,b) -> (a,b) | | A yet more relaxed rule would allow the context of a class-op signature to | mention only class type variables. However, that conflicts with Rule | 1(b) for types above. | | -- | Hal Daume III | hdaume(a)isi.edu | "Arrest this man, he talks in maths." | www.isi.edu/~hdaume | | On Fri, 7 Mar 2003, Nick Name wrote: | | > | > I want to declare the following: | > | > class Get a where | > ls :: a b -> IO [b] | > mk :: IO [b] -> a b | > | > instance (Get a) => Functor a where | > fmap f x = mk (ls x >>= return . map f) | > | > | > But to have ghc type everything, I have to turn on "-fglasgow-exts | > -fallow-undecidable-instances -fallow-overlapping-instances". | > | > Is there a clean way to state that all types in my type class are also | > in the "Functor" type class? | > | > If not, what is the problem? | > | > Vincenzo | > _______________________________________________ | > Haskell-Cafe mailing list | > Haskell-Cafe(a)haskell.org | > http://www.haskell.org/mailman/listinfo/haskell-cafe | > | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

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debug with GHC FFI?
by paul＠theV.net 06 Mar '03

06 Mar '03

I am using Green Card to make some FFI modules with GHC, but sometimes it segfaults in the middle of nowhere, leaving no sensible stack trace in gdb. I wonder what is the proper way to debug FFI modules written in C? Trying to add a "-g" flag doesn't seem help at all... Regards, .paul.

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You missed my Birthday again!
by Ami 06 Mar '03

06 Mar '03

Hi ya, Can't believe you missed my Birthday again, give me a call when you get this e-mail. You have to check out this great present Mel sent me, everyone at work is going mad over it! www.deals2go.co.uk/script Ami

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Re: is identity the only polymorphic function without typeclasses?
by Jon Cast 05 Mar '03

05 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.)

6 10

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

1 0

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-)

4 7

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

2 1

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

04 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

1 0

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/

1 0

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

4 4

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

03 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

2 1

Re:
by wr 28 Feb '03

28 Feb '03

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1 0

Hugs' graphics lib
by Daniel Jian 27 Feb '03

27 Feb '03

Hi all, I have just installed Hugs to my system. And then I also compiled its drawing library. My question is how can I include that library in my code? I know this is rather silly a question, but I have tried to play around with the environment variable in .bashrc, and I have added:-P$HOME/local/graphics-2.0.4/lib/x11 to the HUGSFLAGS. But Hugs still complains that it cannot include the lib. I am just wondering, can I make an executable file using hugs? How can I do that (if I can)? Many thanks! Blessings, Daniel Jian __________________________________________________ Do you Yahoo!? Yahoo! Tax Center - forms, calculators, tips, more http://taxes.yahoo.com/

2 2

Buenas noches !!!
by Olga Lucia Quintero Fonseca 25 Feb '03

25 Feb '03

Buenas noches, quisiera consultarles sobre lo siguiente, tengo estas dos funciones -- Calcula Raiz Wn wn n = cos c :+ (-1*sin c) where c=2*pi/n twiddlefactor r k = concat [(map (^i) [(wn rs)^i | i<-[0..s-1]])| i<-[0..r-1]] where rs = (r^k) ; s = (r^(k-1)) la dificultad que tengo es que twiddlefactor no me acepta enteros compila pero no me acepta los parametros, sin embargo si la defino como --Calcula vector de Twiddle Factor twiddlefactor r s rs= concat [(map (^i) diag) | i<-[0..r-1]] where diag = [(wn rs)^i | i<-[0..s-1]] no tengo problema, lo que sucede es que posteriormente esta definicion me es inconveniente pues el valor de rs es r^k y lo necesito en un programa recursivo, que me esta dando inconvenientes precisamente por que debo estar reenviando el valor r^k. Agradezco la ayuda que puedan darme

2 1

Re: Simple problem from Hudak's SOE
by C.Reinke 24 Feb '03

24 Feb '03

> I checked out the type of "snd" in hugs and basically > figured out what it does, but I was wondering where I could find the > definition of this function. Is it possible to look at it in hugs? If your hugs is configured correctly to use an editor, simply type ":find snd". This underappreciated feature is one reason why you want to set the editor in hugs!-) http://cvs.haskell.org/Hugs/pages/hugsman/started.html hth, Claus

1 0

Debugging haskell
by Sengan.Baring-Gould＠nsc.com 24 Feb '03

24 Feb '03

http://www.catb.org/~esr/writings/taoup/html/ch01s06.html states that "debugging often occupies three-quarters or more of development time". I don't think that is my experience in Haskell... more like 1/4 at most. I was wondering what others felt. Sengan

7 7

Simple problem from Hudak's SOE
by M. Parker 24 Feb '03

24 Feb '03

I'm a real newbie to Haskell, and I'm having trouble with a particular problem dealing with higher-order functions. Exercise 5.9 in Hudak's "School of Expression" asks us to write a function, "makeChange," s.t. it makes change for a given amount using coins in a coin supply (represented by a list of decreasing integers). For example, make Change 99 [5,1] ==> [19,4] This chapter is about higher order functions, so I'm assuming he wants us to compute the result using the higher order functions defined in the chapter (map, foldl, foldr). I devised two solutions: {-Solution 1-} makeChange money coinList = zipWith div (scanl mod money coinList) coinList {-Solution 2-} makeChange' money (coin:coins) = let money' = money `mod` coin numCoins = money `div` coin in (numCoins: makeChange' money' coins) makeChange' 0 _ = [] makeChange' _ [] = [] However, my problem is that neither solution uses the higher-order functions defined in the chapter. So is it possible to solve this problem using map and fold? Furthermore, Hudak makes the case that we should strive to find the higher-order solutions instead of the recursive ones because the latter leads to clearer and more concise solutions. Although solution 1 is more concise, I feel like Solution 2 is clearer to me than Solution 1, but maybe this is just because I'm new to haskell and higher order functions. It just seems like its easier to understand the actual algorithm in solution 2 than in solution 1. Thanks, Matt Parker University of North Texas undergrad http://www.cs.unt.edu

2 2

x-type.net
by Domain Monitor 23 Feb '03

23 Feb '03

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1 0

Re: can this be written more easily?
by Dean Herington 23 Feb '03

23 Feb '03

[moved to haskell-cafe] While I largely agree with what Nils said, it does seem that arrays are a good match for your application. It is true, unfortunately, as you're discovering, that mutable arrays are awkward in a pure functional language. I think the most appropriate way to deal with them would depend on the larger perspective of your application's control structure. If the array manipulation is necessarily intermixed with input/output, for example, you might consider an essentially imperative approach using `IOArray`s in the `IO` monad. If the input/ouput and array manipulation tend to alternate, `STArray`s in the `ST` monad might offer a more functional approach. If the amount of array manipulation is small, then the approach you've shown so far might be entirely adequate. Dean "Mike T. Machenry" wrote: > I guess I figured that Arrays were the natural data type for the tickets > since it has a fixed size and the elements all have a specific player > associated with them. I am coming from a Scheme background so I am already > very fluent in list manipulation. I'm not an imperative programer, so that's > not really the problem. I just think that Arrays represent this data much > better. > > -mike > > On Fri, Feb 21, 2003 at 01:00:30PM +0100, Nils Decker wrote: > > "Mike T. Machenry" <dskippy(a)ccs.neu.edu> wrote: > > > Hey Everyone, > > > > > > I am having a hard time making a data structure that I can > > > incrimentally > > > update. Mostly because dealing with arrays is so tough. Does anyone > > > think I'm going about this completely the wrong way? This is what I > > > have. > > IMO there is normally no need to use arrays in haskell. You should > > use lists instead, because it is much more easy to use recursion > > over lists. Once you got the pattern, it feels like the natural > > way in haskell. > > It also helps to define more types than just one large type for everything. > > > > > data GameState = GameState { > > > dTickets :: Array Player (Array Ticket Int), > > > fTickets :: Array FugitiveTicket Int, > > > history :: [Move], > > > dLocations :: Array Player Stop, > > > fLocations :: Set Stop > > > } > > > > > > removeTicket :: GameState -> Detective -> Ticket -> GameState > > > removeTicket s d t = > > > s { tickets = (tickets s) // [(d,[(t,((tickets s)!d!t - 1))])] } > > > > why not use > > > > data Ticket = Ticket Int {-value-} deriving (Eq, Show) > > type Tickets = [Ticket] > > > > removeTicket :: Ticket -> Tickets -> Tickets > > removeTicket _ [] = fail "not there" > > removeTicket x (t:ts) > > | x == t = ts > > | otherwise = t : (removeTicket x ts) > > > > Is there a reason, to have different fields and types for detectives? > > data PlayerNames = MrX | Red | Green | Blue deriving (Eq,Show) > > If MrX needs special treatment ( computed move or information shown > > to the player ) you can patternmatch for MrX. For the usage of tickets > > and the history of moves there should be no difference. > > > > There might even be no reason to have PlayerNames as instance of enum. > > As i understand, you want to use succ(player) to find the next player > > to move. It can be easier to have a function that recurses over a > > list of players to run one round. > > > > > This remove ticket function is just terrible and it's common for me to > > > have to do operations like this. It's been hard to make this a > > > function that I can pattern match on, because which piece of data is > > > manipulated depends on the parmeter d (Detective) > > Just split up the huge record and have tiny functions to deal with > > every specific part of it > > > > Summary in a few words: Use many small functions instead of a few > > big ones. Use lists instead of arrays. If you use arrays, first understand > > why you can not use lists in that specific case. Learn to recurse > > over lists! Learn to use map, foldl and foldr. They save you a lot of > > typing and make most functions dealing with lists short and clear. > > > > You should derive Show for all your types and test every new function > > in hugs or ghci. > > > > Regards > > Nils Decker > > > > PS: I have learned haskell a year ago after using imperative languages > > all of my life. At first it is hard to get used to some concepts, > > but then they are wonderful. > > > > PPS: There is another list called haskell-cafe. It is used for discussion > > of problems while this list is meant for short threads and announcements. > > You might want to subscribe to it. > > > > > > -- > > Freedom of speech is wonderful - right up there with the freedom > > not to listen. > > > > Nils Decker <nils(a)ndecker.de>

3 3

a monadic if or case?
by David Roundy 21 Feb '03

21 Feb '03

Hello all. I was just thinking that there ought to be a better way to write the following code. It seems to be a common case that within a 'do' I bind a variable that I only intend to use once, in an if or case statement. It occurred to me that there ought to be a better way to do this. For example, it seems like the following code: whatisit :: String -> IO String whatisit f = do isdir <- doesDirectoryExist f if isdir then return "dir" else do isfile <- doesFileExist f if isfile then return "file" else return "nothing" could be replaced by something like: whatisit :: String -> IO String whatisit f = do ifM doesDirectoryExist f then return "dir" else ifM doesFileExist f then return "file" else return "nothing" Is there any way I could do something like this? -- David Roundy http://civet.berkeley.edu/droundy/

6 7

CDirent definition.
by Thaddeus L. Olczyk 19 Feb '03

19 Feb '03

Looking through the sources of ghc, I could not find the definition/devclaration of CDirent used by the FFI version of Posix directory commands (readdir, opendir etc... ). All I could find was type CDirent = () I'm confused by this. Can anyone clarify?

1 0

Re: Graphics output from Haskell as eps
by Sarah Thompson 18 Feb '03

18 Feb '03

> Has anyone done much work on outputting graphics in eps format > directly from > Haskell? Hmm... Actually it turns out to be really easy to generate EPS directly, so long as you're mostly just interested in drawing lines, which I am. I've already got something working which will probably be good enough for my purposes. Thanks to all who replied... Sarah PS: I'll post a link to my library once it does something more interesting than drawing a box in Ghostview! :)

1 0

Graphics output from Haskell as eps
by Sarah Thompson 17 Feb '03

17 Feb '03

Hi all, Has anyone done much work on outputting graphics in eps format directly from Haskell? My intention is to be able to generate publication quality diagrams for inclusion in TeX/LaTeX documents, without the size or quality penalties of using bitmaps. I want to write an automatic circuit visualisation utility somewhat analogous to the AT&T GraphViz tools, but which generates circuit diagrams that look 'right' to an electronic engineer (i.e. me). The idea is that a raw netlist (effectively a directed graph) will be generated by some other code, then fed into this tool, which will then generate a reasonably understandable circuit diagram. My most immediate requirement is to generate circuit diagrams, although longer term I'll probably also want to be able to generate chip floor plan diagrams too. I'm not too bothered about higher level drawing primitives - just something that can draw lines of appropriate widths and output text would be enough. Has anyone done anything already, or should I just pull down the PDF specs from Adobe and weigh in and write a library from scratch? If I do the latter, will anyone be likely to want to use it also? TIA, Sarah PS: To all who helped (esp. Sigbjorn Finne and Simon Peyton-Jones), the in-memory SQL database I was working on did actually work very well in the end. Unfortunately, my boss has insisted on a C++ rewrite - the C++ version (using strict evaluation and STL data structures roughly modelled on the Haskell version) is (I think, I've not measured this) a little slower, and about 5 - 10 times the size. It's been extended a little beyond the Haskell version, so it is difficult to be exact about the size difference, but gut feeling is that it took about five times as much code and about three times as long to write. And that's on the basis of someone who has over 10 years experience coding C++, but very little prior exposure to Haskell beyond the occasional recreational use. I think this proves Haskell's case as far as I'm concerned - keep up the good work, folks!

3 2

RE: Yet another weakly defined bug report
by Simon Marlow 17 Feb '03

17 Feb '03

> "Simon Marlow" <simonmar(a)microsoft.com> writes: > > To fix these > > problems you have to think carefully about strictness and > demand in your > > program. For memory we have heap profilers to help out, > but we don't > > have I/O descriptor profilers for lazy I/O! > > Surely I/O descriptors are just a type of heap-value, and as such > can be profiled in the normal way (producer, retainer, etc), just > like any other heap construction? File descriptors are held in the kernel, not the heap. Hmmm, I suppose you could track Handles, which notionally "hold on to" a file descriptor, but it's not completely reliable because a Handle will be held by a finalizer for an unspecified period of time before the descriptor is released. In practice it might not be a serious problem unless there are lots of other threads. Cheers, Simon

1 0

Haskell-Tutorial
by Gerhard Navratil 17 Feb '03

17 Feb '03

At the department we use Haskell for several courses and as a modelling tool in master- and PhD-thesis. A few years ago Damir Medak held a short internal workshop to answer most of the beginners questions and I created a tutorial text from his material. The tutorial could be useful for beginners because it contains lots of examples to explain the concepts. Included are function definition, lists, data types and polymorphism. Excluded are aspects of efficiency and user interaction (esp. monadic programming). The tutorial can be found here as a PDF-File: ftp://ftp.geoinfo.tuwien.ac.at/navratil/HaskellTutorial.pdf I hope you find the tutorial useful and I would be glad to get some feedback for further improvement. Gerhard Navratil ======================================== Gerhard Navratil Teaching- And Research-Assistant Technical University Vienna, Austria Institute for Geoinformation Gusshausstr. 27-29 1040 Vienna Tel.: ++43 (0) 1 / 58 801 - 12721 Fax.: ++43 (0) 1 / 58 801 - 12799 Cel.: ++43 (0) 699 / 197 44 761 http://www.geoinfo.tuwien.ac.at

2 1

RE: Yet another weakly defined bug report
by Simon Marlow 17 Feb '03

17 Feb '03

> >File reading is not a pure operation: running out > >of file descriptors is a good counter-example. > > How does this differ from running out of memory whilst trying > to evaluate something? You're absolutely right, of course. I think the point is that by using implicit memory allocation we've accepted that "running out of memory" is going to be a failure condition that we cannot reasonably avoid or prepare for. This is a bad thing - but we deem the tradeoff to be worth it, because on the other hand we don't suffer from memory allocation bugs of the same kind that you get in C programs. I/O is similar in some ways: lazy I/O might lead to file descriptor leaks, which are similar to memory leaks but aren't something that Haskell programmers "expect" to have to worry about. To fix these problems you have to think carefully about strictness and demand in your program. For memory we have heap profilers to help out, but we don't have I/O descriptor profilers for lazy I/O! Also, by using lazy I/O one elects to ignore I/O errors - which I think is not something we should be encouraging. Cheers, Simon

2 1

Re: Global variables?
by Jon Cast 16 Feb '03

16 Feb '03

Richard Uhtenwoldt <ru(a)river.org> wrote: <snip> > I do not think it is nice: I do not like any of the solutions Hughes > considers in that paper because this problem can be handled much more > simply with lexical scope and the IO monad. can be /= should be > Just to get our bearings, let us first consider the solution that uses > unsafePerformIO, which neither Hughes nor I prefer: > >globalVar :: IORef Int > >globalVar = unsafePerformIO $ newIORef 0 > >foo = fff aaa bbb ccc > >bar = ggg xxx yyy zzz > >main = mmm >> nnn >> ooo > where the lines > >foo = fff aaa bbb ccc > >bar = ggg xxx yyy zzz > stand in for a typically much larger chunk of code --the bulk of the > program, let us call it. I, personally, haven't written a program whose bulk will fit in a single file in several years, and I doubt I ever will again. So, support for separate compilation is a necessity. How do you intend to handle this? <snip> Jon Cast

3 2

big rationals question
by Damien R. Sullivan 16 Feb '03

16 Feb '03

So, for my Algorithms class in grad school we have to calculate some number to as many digits as possible in a minute. I figured this would be a good project to try in multiple languages, to see how they compared in ease and speed. In particular I've been intrigued by the functionality and strong typing of Haskell and O'caml for a while, but never had an incentive to really learn them without a project. So I tackled ocaml first, looking at their tutorial, and without too much trouble got a logarithm program working, confirming in the process that (a) the syntax sucks (I like operator overloading for math) and (b) it seems pretty fast, although I need to try Java or Perl or some C++ library for comparison. Not as fast as 'bc', but it might be using better algorithms. In particular I was able to print both the exact form of the big rational and a decimal form to as many digits as I wanted easily. Then I moved to Haskell, largely rewriting the ML code. The result is much nicer looking, being able to say 'sum + powx' instead of 'Num.add_num sum powx', or having to define 'num_2' for a bignum version of 2. It took longer to actually run properly -- I got bit by laziness, but John Meacham tipped me off. And now I have '34283788463231218 % 20536892788086675' being printed out, although I seem to have a bug since if that's a numerator and denominator the result is not the natural logarithm of 2. But my real question is can I get Haskell to actually do that division itself? I get complaints about 'Fractional Integer'; I had to use bc to do the division. -xx- Damien X-)

2 1

Re: big rationals question
by Jon Cast 16 Feb '03

16 Feb '03

"Damien R. Sullivan" <dasulliv(a)cs.indiana.edu> wrote: <snip> > Then I moved to Haskell, largely rewriting the ML code. The result is > much nicer looking, being able to say 'sum + powx' instead of > 'Num.add_num sum powx', or having to define 'num_2' for a bignum > version of 2. It took longer to actually run properly -- I got bit by > laziness, but John Meacham tipped me off. And now I have > '34283788463231218 % 20536892788086675' being printed out, although I > seem to have a bug since if that's a numerator and denominator the > result is not the natural logarithm of 2. But my real question is can > I get Haskell to actually do that division itself? I get complaints > about 'Fractional Integer'; I had to use bc to do the division. Sure: > rationalToDouble :: Rational -> Double > rationalToDouble rat = fromInteger (numerator rat) > / fromInteger (denominator rat) HTH > -xx- Damien X-) Jon Cast

1 0

Re: Haskell-Cafe digest, Vol 1 #616 - 12 msgs
by Chevalier Dr. Dr. Ruediger Marcus Flaig 15 Feb '03

15 Feb '03

Am Donnerstag, 13. Februar 2003 18:01 schrieb haskell-cafe-request(a)haskell.org: > Message: 5 > Date: Thu, 13 Feb 2003 10:06:53 +0100 > From: Jerzy Karczmarczuk <karczma(a)info.unicaen.fr> > Organization: =?ISO-8859-1?Q?Universit=E9_de_Caen=2C_France?= > Cc: haskell(a)haskell.org, haskell-cafe(a)haskell.org > Subject: Re: ANNOUNCE: Learning Haskell portal, version 0.1 > > Arjan van IJzendoorn wrote: > > Often we see messages from people who want to learn Haskell (something we > > applaud), but don't know where to begin. > > ... > > > Enough talk: http://www.cs.uu.nl/~afie/haskell/LearningHaskell.html > > Thanks, Arjan, nice work. > I would add some significant papers, such as John Hughes' > "Why functional programming matters", etc. > > and perhaps - for some, a little bit advanced readers - some > other papers, introducing type classes, perhaps monads (Wadler) > etc. Everything can be found through the Home of Haskell, but > gathering essential references on your page would shorten the > search path. > > I believe - from some discussions here and on comp.lang.functional > (some of them quite annoying...) that it would perhaps be a good > idea to put down a relatively comprehensive, easy comparison > between Haskell and other languages, notably functional: Clean, > also: Scheme, absolutely: ML variants, and Erlang. > Such questions are, and will continue to be recurring. > > > Jerzy Karczmarczuk > Excellent idea -- if you need a volunteer, contact me (I am familiar with Erlang, the Lisp/Scheme group and CAML). But I would also suggest to include the declarative languages such as Prolog and Mercury, if we can find anyone who knows -- the Mercury homepage (sorry, I don't have the URL right now, it was something long and winding) contains an explicit comparison between Mercury and Haskell. (A rather fair and objective one, it must be pointed out!) And maybe one should also add a few lines about hybrids such as Python? Yours, Ruediger M. Flaig -- "I was always told that death and taxes are no subjects for discussion." "Well... the one does not concern me." "And the other?" "I admit that I do not pay taxes either." -- Diane Duane: "The Wounded Sky"

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