- Haskell - Haskell.org

not naming modules Main
by Hal Daume 19 Nov '01

19 Nov '01

I'm really frustrated that modules that you want to compile to executables have to be named Main. I often have a module with a main method that I use for testing or whatever (perhaps I want the gained speed of an executable) but is, for the most part, a module I import into others. I end up having to constantly change the module name whenever I want to compile it and I find this terribly frustrating. Is there any reason you can't just compile things that simply export a main method with the proper type? Is this a ghc specific thing or does nhc also have this restriction? Any chance ghc will change its policy on this? - Hal -- Hal Daume III "Computer science is no more about computers | hdaume(a)isi.edu than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume

5 4

Help
by Juan M. Duran 19 Nov '01

19 Nov '01

Hi, I am developing a software based on Genetic Programming and I am trying that this soft makes source code in Haskell. I've two problems, one is that I only count on The Haskell report and I need more information about haskell's available library. On the other hand, I need to read a file and (as in imperative programming) get each word the file has because they are parameters for future functions. Thanks Juan PS: Here is some code I made: import IO arch = "twoboxes.dat" mode = ReadMode main = do {handler <- openFile arch mode; contents <- hGetContents handler; putStr contents; hClose handler } putList :: a -> [a] -> [a] putList x [] = [x] putList x xs = x:xs

1 0

heap and walking through a very long list
by Lukasz Pankowski 18 Nov '01

18 Nov '01

Hi. I wonder if there are any methods of walking through a very long list without a huge heap. It is good that a laziness makes creation of large stuctures delayed, but it seems that destruction of never again used beginning of a list is also delayed (probably because of other reason). Consider the simple program: main = do putStrLn $ show $ hist_inc 100000 3 hist_inc n b = hist (0,b) $ take n $ repeat b hist bnds is = accumArray (+) 0 bnds [(i, 1) | i <- is, inRange bnds i] and it's output (compiled with GHC): $ ./hist_inc Stack space overflow: current size 1048576 bytes. Use `+RTS -Ksize' to increase it. I am using GHC 5.0, does it have any optimization to overcome it (`-O -fvia-C -O2-for-C' does not do this). It is obvious that the beginning of the list is no longer needed (does garbage collector now this?). If there is a way is it written somewhere in documentation? I feel it insane to have to have let say 256Mb of memory because I have 1 million measurements on a list. Currently I pipe the results to a C program, nice UN!X practice which I would like avoid. -- =*= Lukasz Pankowski =*=

1 0

ANNOUNCE: HOpenGL 1.01 released
by Sven Panne 17 Nov '01

17 Nov '01

HOpenGL, a Haskell binding for OpenGL and GLUT version 1.01 I am pleased to announce the twelfth release of the Haskell binding for GL 1.2.1 / GLU 1.3 / GLUT 3.7beta. It offers easy access to *the* industrial strength 3D graphics API and a GUI toolkit. More details about HOpenGL can be found at http://www.informatik.uni-muenchen.de/~Sven.Panne/haskell_libs/HOpenGL.html This is a compatibility release for >= GHC-5.02. With a little work, HOpenGL can probably tweaked to work with a recent Hugs and NHC98, feedback about this and/or patches are highly welcome. Have fun! Sven

1 0

announce: greencard, version 2.03
by Sigbjorn Finne 17 Nov '01

17 Nov '01

A new release of GreenCard is out, version 2.03. It is a refresh release, fixing a couple of minor bugs + updating the source code to make it work with current Haskell systems. Also new in this release is the inclusion of an example (contributed by Antony Courtney) which demonstrates how to use GreenCard together with Hugs and/or GHC on your favourite platform. More info, distributions etc. at: http://www.haskell.org/greencard/ --sigbjorn

1 0

Your New Account Info
by welcome 17 Nov '01

17 Nov '01

Your E-mail Address Removal/Deletion Instructions at the end of this email: We visited your site and thought the following might benefit you. Dear Advertiser: VnetFind.Com is currently achieving over 1,000,000 hits a month! We had been a meta search engine for quite sometime and just recently went to paid rankings (pay per click)! Most of our search terms are still available for only $.01! And with our Amazing Bonuses of up to 800% you could get highly targeted search engine traffic for as little as $.001 per unique visitor! You have nothing to lose and everything to gain! You pay only for traffic you receive NO OTHER FEES, NO MONTHLY MINIMUMS! Check out what VnetFind.Com has to offer! List your site now and VnetFind.Com will give you: Traffic - We deliver hundreds of thousands of searches every month. Targeting - Directly connect with specific audiences through custom titles and descriptions. Cost effectiveness - Pay when targeted customers click through to your site. Control - Choose how much to pay for each visitor. Easy account management - Get 24 hours a day, 7 days a week account access at Vnetfind.Com Free Money- VnetFind.Com will match your deposit with up to an 800% bonus! When you put in the promo code MPT1310 YOU WILL get $10 free just for signing up plus the following bonuses on deposits! Deposit $10 USD into your account and receive a 100% Bonus ($20 in your account) Deposit $25 USD into your account and receive a 500% Bonus ($150 in your account) Deposit $50 USD into your account and receive a 600% Bonus ($350 in your account) Deposit $100 USD into your account and receive a 700% Bonus ($800 in your account) Deposit $200 USD into your account and receive a 800% Bonus($1800 in your account) Within minutes you can place your site in our top rated search engine and be receiving highly targeted traffic for only pennies a click. Here are the details: We are a bid-for-placement search engine. You bid a certain number of cents you are willing to pay to have people come to your site. The more you pay, the higher you appear in our search. The best part is that you only pay for traffic you receive ... there are no other fees! Register for an account it's easy and only takes a few minutes! Deposit funds into your VnetFind account via check order or by credit card. Select the keywords you wish to place your site under. For example, if you run a site that sells rare coins, you might choose the keywords "rare coins" or "shillings". Create a description of your site for the keywords you chose. Choose how much you wish to pay per click for your sites link. The more you are willing to pay, the higher you will appear in our search engine. You will only be charged for traffic you receive. If you are paying .01 cents per click and receive 8 unique clicks, your account will be debited .08 cents. If a visitor clicks on your entry multiple times in the same day, you will only be charged once. We have special systems in place to make sure that you are only charged for unique clicks and not overcharged. You can request any remaining funds be returned to you at any time and there are no membership fees to use our service! That's it! You will begin receiving highly targeted search engine traffic for as little as $.001 per visitor in no time! Remember this is a special offer and you MUST enter the promo code MPT1310 to receive your bonuses! Thanks and Good Luck! Jennifer C Account Manager $$$$$$$$$$$$$$$$$$$-REMOVAL INSTRUCTION-$$$$$$$$$$ This message is sent in compliance of the new email Bill HR 1910. Under Bill HR 1910 passed by the 106th US Congress on May 24, 1999, this message cannot be considered SPAM as long as I include a valid return address and the way to be removed. ONCE AGAIN THIS IS NOT SPAM. PLEASE IF YOU WANT TO BE REMOVED SEND AN E-MAIL WITH "REMOVE" IN THE SUBJECT TO THE FOLLOWING ADDRESS: removeme(a)vnetdirect.com

1 0

How do I write the type of 'inverse'?
by marku＠cs.waikato.ac.nz 16 Nov '01

16 Nov '01

I have a class whose instances can extract a field of type a. Is there a way of referring to that type 'a' in the signatures of functions that use the class? Sigh! That wasn't very clear. I'll try explaining via example: I am trying to define a `Pairable' class, which is an abstraction of all kinds of pairs: (a,b), Pair a b, etc. > class Ord p => Pairable p where > is_pair :: p -> Bool > pair_fst :: Ord a => p -> a -- Precondition: is_pair > pair_snd :: Ord b => p -> b -- Precondition: is_pair > make_pair :: (Ord a, Ord b) => a -> b -> p > instance (Ord a, Ord b) => Pairable (a,b) where > is_pair (a,b) = True > pair_fst (a,b) = a > pair_snd (a,b) = b > make_pair a b = (a,b) I then want to define a `Relation' to be a set of Pairable values. > newtype Pairable p => Reln p = Reln [p] This works okay for many of my relation operations. But I run into an interesting problem with `inverse' (which swaps the components of each pair). inverse :: (Pairable p, Pairable p2) => Reln p -> Reln p2 inverse rs = Reln[make_pair (pair_snd p) (pair_fst p) | p <- reln2list rs] Hugs gives the error: ERROR /home/utting/jaza/tmp.hs:19 - Cannot justify constraints in explicitly typed binding *** Expression : inverse *** Type : (Pairable a, Pairable b) => Reln a -> Reln b *** Given context : (Pairable a, Pairable b) *** Constraints : (Ord c, Ord d) which I can understand. BUT, I cannot see any way of naming the `component types', c and d, in the signature of inverse! It seems like I want to pass the component types into the class definition somehow: class Pairable (p a b) How does one normally get around this kind of problem? Thanks. Mark Utting Professeur Invité Laboratoire d'Informatique de l'Université de Franch-Comté 16, route de Gray - 25000 Besançon, cedex, FRANCE Tel: (33) 3 81 66 20 69 Fax: (33) 3 81 66 64 50 Email: marku(a)cs.waikato.ac.nz

2 1

Re: How do I write the type of 'inverse'?
by John Hughes 16 Nov '01

16 Nov '01

I'll try explaining via example: I am trying to define a `Pairable' class, which is an abstraction of all kinds of pairs: (a,b), Pair a b, etc. > class Ord p => Pairable p where > is_pair :: p -> Bool > pair_fst :: Ord a => p -> a -- Precondition: is_pair > pair_snd :: Ord b => p -> b -- Precondition: is_pair > make_pair :: (Ord a, Ord b) => a -> b -> p > instance (Ord a, Ord b) => Pairable (a,b) where > is_pair (a,b) = True > pair_fst (a,b) = a > pair_snd (a,b) = b > make_pair a b = (a,b) Funny: I get an error from just this instance declaration! The problem is that the class definition means that pair_fst is *polymorphic* in a, that is, it should be able to return *any* type in class Ord. Of course, your instance can't: it can only return the type of the first component. Thus the instance is not general enough. The solution (using hugs -98 or ghc -fglasgow-exts, because this is an extension to Haskell 98) is to use a multi-parameter class: > class (Ord p, Ord a, Ord b) => Pairable p a b where > is_pair :: p -> Bool > pair_fst :: p -> a -- Precondition: is_pair > pair_snd :: p -> b -- Precondition: is_pair > make_pair :: a -> b -> p > instance Pairable (a,b) a b where > is_pair (a,b) = True > pair_fst (a,b) = a > pair_snd (a,b) = b > make_pair a b = (a,b) This is what you were after here: It seems like I want to pass the component types into the class definition somehow: class Pairable (p a b) Now the instances of pair_fst and pair_snd are for a *particular* component type, and all is fine. This works, but tends to lead to many ambiguities, since the type-checker cannot know that there isn't an instance instance Pairable (a,b) b a where ... pair_fst (a,b) = b ... also. That means it cannot infer, when it sees pair_fst (a,b), that the result is of the same type as a. You can avoid the ambiguities too, by adding a *functional dependency* to the class definition. In this case, the type of the "pair" presumably determines the type of each component, so you could write > class (Ord p, Ord a, Ord b) => Pairable p a b | p->a, p->b where > is_pair :: p -> Bool > pair_fst :: p -> a -- Precondition: is_pair > pair_snd :: p -> b -- Precondition: is_pair > make_pair :: a -> b -> p to declare that for each type p, there can only be one type a and one type b. This permits the type-checker to infer the result type of pair_fst and pair_snd from the argument type. Of course, if you actually *want* to declare pairs with reversed components as "pair-like" also, then you can't do this: you have to put up with the ambiguities (and add types by hand to resolve them) because the ambiguities are really there. John

1 0

RE: GHC installation
by Simon Peyton-Jones 16 Nov '01

16 Nov '01

You don't say which version of GHC... GHC 5.02.1 (which I advise you to use) installs in c:/ghc/ghc-5.02.1 The installer doesn't give you a way to change this. (It should, but we havn't found out how yet.) But once you have put it there you can move it if you want. (Best to mail glasgow-haskell-users rather than the entire Haskell mailing list with GHC-related questions.) Simon | -----Original Message----- | From: Jerzy Karczmarczuk [mailto:karczma@info.unicaen.fr] | Sent: 16 November 2001 11:56 | Cc: haskell(a)haskell.org | Subject: GHC installation | | | I might be dead wrong, in that case I apologize... | | I just took the Windows installer and tried to put GHC etc. | on my laptop. I suspect that the installer absolutely wants | to put the stuff in Program Files and doesn't give the user | the opportunity to install it on another disk. Anyway, I have | plenty of space on another partition, but the installer | complains that it lacks space. | | Any suggestions, please? | | | Jerzy Karczmarczuk | Caen, France | | _______________________________________________ | Haskell mailing list | Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell |

1 0

RE: how strict is Array.array?
by Simon Peyton-Jones 16 Nov '01

16 Nov '01

Well, it's clearly a design choice. But in general laziness does not come for free. The choice made by the Haskell designers was that the 'array' function can consume and explore all its input before producing any result. It just give more scope to the implementor. Nothing too deep. Simon | -----Original Message----- | From: Johannes Waldmann [mailto:joe@isun.informatik.uni-leipzig.de] | Sent: 12 November 2001 20:29 | To: haskell(a)haskell.org | Subject: how strict is Array.array? | | | I was surprised to note that | | bounds $ array (0,1) undefined | | is undefined ( rather than (0,1) ) - why is this? | | just curious, | -- | -- Johannes Waldmann ---- | http://www.informatik.uni-leipzig.de/~joe/ -- | -- | joe(a)informatik.uni-leipzig.de -- phone/fax (+49) 341 9732 204/252 -- | | _______________________________________________ | Haskell mailing list | Haskell(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell |

1 0