February 2009 - Haskell-Cafe

package code duplication
by Louis Wasserman 25 Feb '09

25 Feb '09

There was a question recently about being allowed to get into package internals, and I had a question. I want to use uvector's stream internals in ways that the exposed methods don't permit, but I don't especially want to use another package (e.g. vector, which does expose its internals) or reimplement my own stream fusion. Would it make sense to duplicate uvector's internals, copying licensing information and other stuff of course, inside my package? It's a suboptimal solution, but it seems better than the alternative... >< Louis Wasserman wasserman.louis(a)gmail.com

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Re: [Haskell-cafe] base-4 + gtk2hs-0.10.0 licensing
by Robert Greayer 25 Feb '09

25 Feb '09

Colin Paul Adams wrote: > But IF there is no difference between LGPL and GPL for Haskell > programs, then the licensing of gtk2hs as LGPL is just a smokescreen - > it is effectively GPL, so you have to license your program as GPL. > Which I'm all in favour of :-) I actually don't think this is 100% true. With the LGPL, you can distribute your program with under a non-GPL license, as long as you provide *some mechanism* for replacing the library and recreating the program. Normally this means dynamic linking. But it also allows you (I think) to distribute your program with a GPL-incompatible-but-nevertheless-open-source license, because that provides a mechanism for replacing the library (because you can rebuild the program from source). If you license the library under GPL, you cannot even do that. At least this is my understanding...

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[Colin Paul Adams] Re: [Haskell-cafe] base-4 + gtk2hs-0.10.0 licensing
by Colin Paul Adams 25 Feb '09

25 Feb '09

>>>>> "Wolfgang" == Wolfgang Jeltsch <g9ks157k(a)acme.softbase.org> writes: Wolfgang> Am Mittwoch, 25. Februar 2009 14:33 schrieb Duncan Wolfgang> Coutts: >> Note that some people will tell you that by a strict >> interpretation of the LGPL that statically linked Haskell libs >> under that license are a pain in the backside. When we decided >> on that license for gtk2hs that was not our intention. In other >> words nobody is going to sue you if you statically link gtk2hs >> libs. Of course if you need a cast iron legal guarantee then >> that's not good enough and you'd have to ship .a and .o files >> to let users relink if they wanted to. Wolfgang> I’m not sure whether this would be enough. .a and .o Wolfgang> files are not compatible among GHC versions, as far as I Wolfgang> know. Relinking against newer Gtk2Hs versions might not Wolfgang> work. And a program using Gtk2Hs contains code from the Wolfgang> .hi files of Gtk2Hs through inlining. So it’s not pure Wolfgang> linking. However, the LGPL only allows linking, as far Wolfgang> as I understand. Wolfgang> I want to repeat what I’ve said earlier on this list: Wolfgang> For Haskell, there is no real difference between LGPL Wolfgang> and GPL, as far as I understand it. If you don’t want to Wolfgang> force the users of your library to use an open source Wolfgang> license for their work then use BSD3 or a similar Wolfgang> license for your library. But IF there is no difference between LGPL and GPL for Haskell programs, then the licensing of gtk2hs as LGPL is just a smokescreen - it is effectively GPL, so you have to license your program as GPL. Which I'm all in favour of :-) -- Colin Adams Preston Lancashire

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Cabal: local documentation
by Martijn van Steenbergen 25 Feb '09

25 Feb '09

Hello, Can cabal automatically generate local documentation for packages I install? It'd be awesome if it generated a page like [1] for locally installed packages. Thanks, Martijn. [1] http://www.haskell.org/ghc/dist/current/docs/libraries/

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The community is more important than the product
by Don Stewart 25 Feb '09

25 Feb '09

http://haskell.org/haskellwiki/Protect_the_community Random notes on how to maintain tone, focus and productivity in an online community I took a few years ago. Might be some material there if anyone's seeking to help ensure we remain a constructive, effective community. -- Don P.S. release some code on hackage.haskell.org.

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ONeillPrimes.hs - priority queue broken?
by Eugene Kirpichov 25 Feb '09

25 Feb '09

Hi, I've recently tried to use the priority queue from the ONeillPrimes.hs, which is famous for being a very fast prime generator: actually, I translated the code to Scheme and dropped the values, to end up with a key-only heap implementation. However, the code didn't work quite well, and I decided to check the haskell code itself. Turns out that it is broken. module PQ where import Test.QuickCheck data PriorityQ k v = Lf | Br {-# UNPACK #-} !k v !(PriorityQ k v) !(PriorityQ k v) deriving (Eq, Ord, Read, Show) emptyPQ :: PriorityQ k v emptyPQ = Lf isEmptyPQ :: PriorityQ k v -> Bool isEmptyPQ Lf = True isEmptyPQ _ = False minKeyValuePQ :: PriorityQ k v -> (k, v) minKeyValuePQ (Br k v _ _) = (k,v) minKeyValuePQ _ = error "Empty heap!" minKeyPQ :: PriorityQ k v -> k minKeyPQ (Br k v _ _) = k minKeyPQ _ = error "Empty heap!" minValuePQ :: PriorityQ k v -> v minValuePQ (Br k v _ _) = v minValuePQ _ = error "Empty heap!" insertPQ :: Ord k => k -> v -> PriorityQ k v -> PriorityQ k v insertPQ wk wv (Br vk vv t1 t2) | wk <= vk = Br wk wv (insertPQ vk vv t2) t1 | otherwise = Br vk vv (insertPQ wk wv t2) t1 insertPQ wk wv Lf = Br wk wv Lf Lf siftdown :: Ord k => k -> v -> PriorityQ k v -> PriorityQ k v -> PriorityQ k v siftdown wk wv Lf _ = Br wk wv Lf Lf siftdown wk wv (t @ (Br vk vv _ _)) Lf | wk <= vk = Br wk wv t Lf | otherwise = Br vk vv (Br wk wv Lf Lf) Lf siftdown wk wv (t1 @ (Br vk1 vv1 p1 q1)) (t2 @ (Br vk2 vv2 p2 q2)) | wk <= vk1 && wk <= vk2 = Br wk wv t1 t2 | vk1 <= vk2 = Br vk1 vv1 (siftdown wk wv p1 q1) t2 | otherwise = Br vk2 vv2 t1 (siftdown wk wv p2 q2) deleteMinAndInsertPQ :: Ord k => k -> v -> PriorityQ k v -> PriorityQ k v deleteMinAndInsertPQ wk wv Lf = error "Empty PriorityQ" deleteMinAndInsertPQ wk wv (Br _ _ t1 t2) = siftdown wk wv t1 t2 leftrem :: PriorityQ k v -> (k, v, PriorityQ k v) leftrem (Br vk vv Lf Lf) = (vk, vv, Lf) leftrem (Br vk vv t1 t2) = (wk, wv, Br vk vv t t2) where (wk, wv, t) = leftrem t1 leftrem _ = error "Empty heap!" deleteMinPQ :: Ord k => PriorityQ k v -> PriorityQ k v deleteMinPQ (Br vk vv Lf _) = Lf deleteMinPQ (Br vk vv t1 t2) = siftdown wk wv t2 t where (wk,wv,t) = leftrem t1 deleteMinPQ _ = error "Empty heap!" toDescList :: Ord k => PriorityQ k v -> [(k,v)] toDescList q | isEmptyPQ q = [] | otherwise = (minKeyValuePQ q) : toDescList (deleteMinPQ q) fromList :: Ord k => [(k,v)] -> PriorityQ k v fromList = foldr (uncurry insertPQ) emptyPQ Here goes a test: *PQ> let s = map fst . toDescList . fromList . (`zip` (repeat ())) :: [Int]->[Int] *PQ> s [4,3,1,2] [1,2,3,4] Looks fine. *PQ> s [3,1,4,1,5,9,2,6,5,3,5,8] [1,1,2*** Exception: Empty heap! OK, probably it doesn't like duplicates. *PQ> s [3,1,4,5,9,2,6,8,10] [1,2,3,4,5,9,10] Whoops, 6 and 8 are lost. So, the morale is: don't use the priority queue from ONeillPrimes in your projects. It works for primes by a lucky chance. I haven't yet figured out, however, what exactly the bug is. -- Eugene Kirpichov Web IR developer, market.yandex.ru

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Indentation Question.
by Ramaswamy, Vivek 25 Feb '09

25 Feb '09

Hello All~ I have been reading the book "Haskell for real programmers" and am presently on chapter 03. There was a small program demonstrated by the author on "let". The problem is that when I cut and paste authors code into eclipse, the program works fine, where as when I write my own copy of the program I am getting an error. I figured out that when I remove the function any one function lend or bend, the file compiles and runs fine. My question is why does the Haskell compiler complains, in the case 2 functions with the same signature and logic, even though their names are different. module Lending where {-- snippet lend --} lend amount balance = let reserve = 100 newBalance = balance - amount in if balance < reserve then Nothing else Just newBalance {-- /snippet lend --} bend amount balance = let reserver = 100 newBalance=balance-amount in if balance < reserver then Nothing else Just newBalance Regards -Vivek Ramaswamy- ........................................................................ ........................................................................ ............................................ Disclaimer: This e-mail, including any attachment(s) hereto, is intended only for the individual or entity to whom it is addressed. It may contain proprietary, confidential or privileged information or attorney work product belonging to Fidelity Business Services India Pvt. Ltd. (FBS India) or its affiliates. If you are not the intended recipient of this e-mail, or if you have otherwise received this e-mail in error, please immediately notify the sender via return e-mail and permanently delete the original mail, any print outs and any copies, including any attachments. Any dissemination, distribution, alteration or copying of this e-mail is strictly prohibited. The originator of this e-mail does not guarantee the security of this message and will not be responsible for any damages arising from any dissemination, distribution, alteration or copying of this message and/or any attachments to this message by a third party or as a result of any virus being passed on. Any comments or statements made in this are not necessarily those of FBS India or any other Fidelity entity. All e-mails sent from or to FBS India may be subject to our monitoring and recording procedures. ........................................................................ ........................................................................ ..........................................

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IntMap documentation error
by Louis Wasserman 24 Feb '09

24 Feb '09

In the documentation for Data.IntMap updateMin, a piece of example code both communicates incorrect intuition, and fails to even compile. updateMin :: (a -> a) -> IntMap a -> IntMap a updateMin (\ _ -> Nothing) (fromList [(5,"a"), (3,"b")]) -- code straight from the docs <interactive>:1:49: Couldn't match expected type `Maybe a' against inferred type `[Char]' In the expression: "a" In the expression: (5, "a") In the first argument of `fromList', namely `[(5, "a"), (3, "b")]' As a side note, the sort of operation implied by the example code was really what I was looking for in the documentation -- but such a method no longer exists in IntMap. ::sad:: Who do I tell this to / how do I ask to get it fixed? Louis Wasserman wasserman.louis(a)gmail.com

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Data.Binary poor read performance
by Neil Mitchell 24 Feb '09

24 Feb '09

Hi, In an application I'm writing with Data.Binary I'm seeing very fast write performance (instant), but much slower read performance. Can you advise where I might be going wrong? The data type I'm serialising is roughly: Map String [Either (String,[String]) [(String,Int)]] A lot of the String's are likely to be identical, and the end file size is 1Mb. Time taken with ghc -O2 is 0.4 seconds. Various questions/thoughts I've had: 1) Is reading a lot slower than writing by necessity? 2) The storage for String seems to be raw strings, which is nice. Would I get a substantial speedup by moving to bytestrings instead of strings? If I hashed the strings and stored common ones in a hash table is it likely to be a big win? 3) How long might you expect 1Mb to take to read? Thanks for the library, its miles faster than the Read/Show I was using before - but I'm still hoping that reading 1Mb of data can be instant :-) Thanks Neil

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emacs cedet support for haskell?
by Xiao-Yong Jin 24 Feb '09

24 Feb '09

Hi, I've googled around for possible haskell support in emacs cedet, but couldn't find any. From cedet's website, it claims to support the following Language Parsers Parsers that have already been implemented: Emacs Lisp, Java, C/C++, C#, Python, Erlang, awk, Makefile, Scheme, HTML, Texinfo, Javascript, dot. Also: Semantic's own grammar format (.by or .wy) Is there any plan to support haskell in the near future? Best, Xiao-Yong -- c/* __o/* <\ * (__ */\ <

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