- Haskell-Cafe - Haskell.org

Map Monoid instance (was commutative monoid?)
by Jens Blanck 25 Jun '11

25 Jun '11

I don't think the original question really is about commutativity, but rather the choice of Monoid instance. Not being especially mathematically inclined, every once and a while I > get a little panicked when I notice that, e.g. Data.Map mappend is a > plain left-biased union, and doesn't actually mappend the values of > the map. > As noted in the reply > > So there's a range of possible Monoid instances for each type, > > More for some types than for others. For Maybe there are three: > > * always take the first/left value; > * always take the last/right value; > * or, use a semigroup operation defined on the values. > > The first two options are provided by the First and Last newtypes, and the > third option is provided by the instance for Maybe itself (except that it > spuriously requires a Monoid instance instead of just a semigroup). > > But why does the Map instance of Monoid _not_ mimic the one chosen for Maybe. I claim this causes unnecessary surprises for users (and lifting the Monoid instance seems more useful, but that is harder to substantiate). Jens

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Why aren't there anonymous sum types in Haskell?
by Elliot Stern 25 Jun '11

25 Jun '11

A tuple is basically an anonymous product type. It's convenient to not have to spend the time making a named product type, because product types are so obviously useful. Is there any reason why Haskell doesn't have anonymous sum types? If there isn't some theoretical problem, is there any practical reason why they haven't been implemented? It seems to me that type inference on anonymous sum types would give you a lot of the power that you get from heterogeneous lists. Whenever you use a heterogeneous list, you've essentially got a homogenous list of some sum type. If you don't handle all of the cases of the sum type, then you're going to eventually run into a type error. It seems to me that having type inference on anonymous sums would allow you to safely get the power of a heterogeneous list. Am I missing something here, or is this correct?

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Haskell Logo
by Michael Xavier 25 Jun '11

25 Jun '11

I wondered if anyone knew the legalities of using the haskell logo, in particular, this one: http://media.nokrev.com/junk/haskell-logos/logo1.png on a website, a personal blog in particular. While I am not yet a primarily haskell coder, I'm using it more and more. I find this logo in particular to be quite beautiful and would probably modify it a bit to emphasize the lambda, which has broader application to computer science beyond 1 language. I can't find any information on what the license is for this image. Could anyone offer any advice on this? -- Michael Xavier http://www.michaelxavier.net

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Re: [Haskell-cafe] Iteratee IO examples
by John Lato 25 Jun '11

25 Jun '11

From: Eric Rasmussen <ericrasmussen(a)gmail.com> > > Hi, > > Examples are very helpful to me too -- thank you for sharing. I'm > especially > curious to see if there are any examples that allow you to use or convert > non-iteratee-based functions. I have only just begun reading about > iteratees > and might be missing the point, but it seems like many of the examples so > far rely on explicit recursion or special functions from one of the > iteratee > modules. > You might be interested in the attoparsec-enumerator and attoparsec-iteratee packages, which adapt attoparsec parsers to work with iteratees. They're small, self-contained, and quite readable. Since attoparsec works with partial parses, it's a natural fit for iteratees. Honestly I'm quite dis-satisfied with the current state of code which depends on iteratee/enumerator. It's nearly all written in a very low-level style, i.e. directly writing 'liftI step', or 'case x of Yield -> ...'. This is exactly what I would hope users could avoid, by using the functions in e.g. Data.Iteratee.ListLike. I've recently added more functions to iteratee which greatly reduce the need for this type of code. I don't know about enumerator, but I expect it isn't rich enough since most user code I've seen is pretty low-level. For some other iteratee examples, you can 'darcs get http://www.tiresiaspress.us/haskell/sndfile-enumerators/' and look at the examples directory (or browse online, of course). > > Is there a way to take a simple function (example below) and use an > enumerator to feed it a ByteString from a file, or do you have to write > functions explicitly to work with a given iteratee implementation? > > import qualified Data.ByteString.Char8 as B > sortLines = B.unlines . sort . B.lines > For this case, there's no point to using iteratees at all. Just read the file in directly to a strict bytestring. Since you're sorting, you'll need to see all the lines before results can be returned. If the file is too big to fit into memory, you'd need a more sophisticated algorithm for which you could use iteratees. In the general case, you need to write for a given iteratee implementation, but in many specific cases it's not necessary. If you want to transform each line of a file, for instance (with iteratee): import Data.ByteString.Char8 as B import Data.Iteratee as I import Data.Iteratee.Char import System.IO import Control.Monad.IO.Class transform :: (ByteString -> ByteString) -> FilePath -> Iteratee [ByteString] IO () transform tFunc oFile = do h <- liftIO $ openFile oFile WriteMode joinI $ rigidMapStream tFunc $ I.mapM_ (B.hPutStrLn h) liftIO $ hClose h rewriteFile :: (ByteString -> ByteString) -> FilePath -> FilePath -> IO () rewriteFile tFunc iFile oFile = fileDriver (joinI $ enumLinesBS (transform tFunc oFile)) iFile An unfolding version would be possible too, which would take a parameter tFunc :: (s -> ByteString -> (s, ByteString)) Maybe I'll add these as utilities in the next version of iteratee. John

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Re: [Haskell-cafe] Software patents covered in GHC?
by austin seipp 25 Jun '11

25 Jun '11

*sigh* CC'ing to the rest of haskell-cafe for completeness. I need to change 'reply all' to a default in my email I guess. On Mon, Jun 20, 2011 at 12:19 PM, austin seipp <as(a)hacks.yi.org> wrote: > Hello, > > Realistically, there probably is. Considering everything down to > linked lists are patented by the USPO, I'd be willing to bet (at least > a dollar or two) that there are many patents out there that apply to > the code used in GHC. I don't know if you could say anything different > about *any* nontrivial software, to be quite honest. > > IANAL however, so I won't attempt to claim in what countries these > patents apply, nor could I cite you patent numbers (legalese being > terse and software patents so seemingly open to interpretation > sometimes, it would be hard to narrow down anyway I think.) > > On Mon, Jun 20, 2011 at 12:10 PM, Shakthi Kannan <shakthimaan(a)gmail.com> wrote: >> Hi, >> >> I would like to know if there are any software algorithms used in the >> Glasgow Haskell Compiler that have been patented? If yes, in which >> countries do they apply? >> >> Just curious to know. >> >> SK >> >> -- >> Shakthi Kannan >> http://www.shakthimaan.com >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > > > > -- > Regards, > Austin > -- Regards, Austin

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Re: [Haskell-cafe] toSql and fromSql, for Algebraic Data Type
by Daniel Patterson 25 Jun '11

25 Jun '11

(forgot to CC the list) On Jun 24, 2011, at 8:27 PM, Daniel Patterson wrote: > It looks, from looking at the provided instances (not from having done it myself), that you need to define safeConvert functions to and from your value. Here is a partial list of conversions to and from String. > instance Convertible String SqlValue where > safeConvert = return . SqlString > instance Convertible SqlValue String where > safeConvert (SqlString x) = return x > safeConvert (SqlByteString x) = return . BUTF8.toString $ x > safeConvert (SqlInt32 x) = return . show $ x > safeConvert (SqlInt64 x) = return . show $ x > safeConvert (SqlWord32 x) = return . show $ x > safeConvert (SqlWord64 x) = return . show $ x > safeConvert (SqlInteger x) = return . show $ x > safeConvert (SqlChar x) = return [x] > safeConvert (SqlBool x) = return . show $ x > safeConvert (SqlDouble x) = return . show $ x > safeConvert (SqlRational x) = return . show $ x > > On Jun 24, 2011, at 8:02 PM, Tom Murphy wrote: > >> Hi *, >> The title is self-explanatory. I'd like to store information from >> an algebraic data type in an SQL database, but the type signature of >> toSql (toSql >> :: Data.Convertible.Base.Convertible a SqlValue => a -> SqlValue) >> doesn't make sense to me. >> How is this done (how do I make an instance of a typeclass like that?) >> >> Thanks for your time, >> Tom >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >

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None brute-force BWT algorithm
by larry.liuxinyu 24 Jun '11

24 Jun '11

Hi, I read a previous thread about BWT implementation in Haskell: http://www.mail-archive.com/haskell-cafe@haskell.org/msg25609.html and http://sambangu.blogspot.com/2007/01/burrows-wheeler-transform-in-haskell They are all in a `brute-force' way, that is implement based on Burrows-Wheeler's definition like below: BWT: sort the rotations of input S to get a matrix M', return the last column L, and the row I, where S appears in M' -- O( n^2 \lg n) bwt :: (Ord a)=>[a]->([a], Int) bwt s = (map last m, i) where m = sort [ drop i s ++ take i s | i<-[1..length s]] (Just i) = elemIndex s m And the IBWT: Re-construct M' by iteratively sort on input L, add one column at a time, and pick the I-th row in M' -- O( n^3 \lg n ) ibwt :: (Ord a)=> ([a], Int) -> [a] ibwt (r, i) = m !! i where m = iterate f (replicate n []) !! n f = sort . zipWith (:) r n = length r I read Richard Bird's book, `Pearls of functional algorithm design', there is another solution. Although it is deduced step by step, the core idea is based on random access the list by index. The algorithm mentioned in the book uses suffixes for sorting instead of rotations. The performance are same in terms of big-O. I wrote the following program accordingly. BWT: sort S along with the index to get a new order of IDs, and return a permutation of S based on IDs. -- O( n^2 \lg n) if (!!) takes O(n) time bwt' :: (Ord a)=> [a] -> ([a], Int) bwt' s = (l, i) where l = map (\i->s !! ((i-1) `mod` n)) ids (Just i) = elemIndex 0 ids ids = map snd $ sortBy (compare `on` fst) $ zip rots [0..] rots = init $ zipWith (++) (tails s) (inits s) -- only non-empties n = length s IBWT: Sort the input L along with index to get a Transform vector, T [1], then permute L iteratively on T start from row I. -- O( n^2 ) if (!!) takes O(n) time ibwt' :: (Ord a) => ([a], Int) -> [a] ibwt' (r, i) = fst $ iterate f ([], i) !! n where t = map snd $ sortBy (compare `on` fst) $ zip r [0..] f (s, j) = let j' = t !! j in (s++[r !! j'], j') n = length r However, As I commented, the (!!) takes time proportion to the length of the list, Although it can be turned into real Haskell Array by listArray (0, n-1) xs. I wonder if there are any purely functional implementations of BWT/IBWT, which don't base on random access idea nor in brute-force way. [Reference] [1], Mark Nelson. `Data Compression with the Burrows-Wheeler Transform'. http://marknelson.us/1996/09/01/bwt/ -- Larry, LIU Xinyu https://github.com/liuxinyu95/AlgoXY

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Re: [Haskell-cafe] Category theory as a design tool
by wren ng thornton 24 Jun '11

24 Jun '11

On 6/23/11 4:30 PM, Alexander Solla wrote: > Please read "Fast and Loose Reasoning is Morally Correct". > http://www.comlab.ox.ac.uk/people/jeremy.gibbons/publications/fast+loose.pdf > > As I have told you before, it is perfectly appropriate to ignore > bottom-the-type and bottoms-the-inexpressible proto-values to recover the > categorical product semantics of (,). It's not even hard. It will make > your life easier. All you have to do is apply an equivalence relation that > considers all bottom-proto-values as equivalent. Domain theory is an > unnecessary complication. All bottoms have the same semantics anyway -- > they all diverge. I've read it a few times, even before the first time you suggested it. It's an excellent paper. And yet, there are times when fast and loose reasoning is not factually correct, morals aside. For one, not all bottoms do diverge, thanks to extensible exceptions. The fact that tuples are not products does have real, moral(!) ramifications--- even if we fallaciously assume that all bottoms do diverge. For example, it dictates a particular implementation for Arrow (->), an implementation which can cause performance issues due to excessive laziness. There are plenty of other examples where the choice of implementation is forced by the desire to be lazy, even when the laziness is not necessary or helpful in all contexts. I'm a big fan of laziness, but it's a two edged sword. Forcing people to grip one edge or to write duplicate code for different circumstances is not an agenda I can support. Another example that's near and dear to many folks on this list is the fact that the existence of bottoms means Haskell as a logic is inconsistent and therefore lacks many properties we would like for provably correct programming. Regardless of how we reason about bottom, the fact that we cannot choose to *not* reason about bottom means that the game is already fixed. Clearly you're not interested in the goals of total functional programming, but many people on this list are, and your continuing argument that those goals are irrelevant (or irreconcilable with laziness) is not endearing. Arnaud was asking how CT could be used when designing systems. I gave an example. That you do not like my example is neither here nor there. Besides, domain theory isn't all that complicated; especially not when comparing it to the whole of category theory. -- Live well, ~wren

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ANN: Two new random-fu releases
by James Cook 24 Jun '11

24 Jun '11

Announcing the Hackage release of two new versions[1,2] of the random- fu package: Version 0.1.4 is a build-fix release that extends support of the 0.1 API to GHC 7. Version 0.2 is a significantly restructured version of the package. Its API is mostly the same as the old for end users, but its back-end has been redesigned and separated into two new packages: "random- source" provides an abstract interface for sources of entropy, and "rvar" provides an abstract interface for consumers of entropy. The parts remaining in "random-fu" provide a more concrete interface for representing and sampling specific distributions. -- James [1] http://hackage.haskell.org/package/random-fu-0.1.4 [2] http://hackage.haskell.org/package/random-fu-0.2

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Haskell Weekly News: Issue 187
by Daniel Santa Cruz 24 Jun '11

24 Jun '11

Welcome to issue 187 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of June 12 to 18, 2011. Announcements Ian Lynagh announced a new patchlevel release of GHC (7.0.4). "This release contains a handful of bugfixes relative to 7.0.3, so we recommend upgrading." http://goo.gl/hOvdA Nicolas Wu released the third instance fo Haskell Parallel Digest. Many thanks to Nick and Eric for putting this together. http://goo.gl/s5muy Jeroen Janssen invited us to the 8th Ghent Functional Programming Group Meeting, to be held on Thursday, the 30th of June, in the Technicum building of Ghent University. http://goo.gl/G081Q Quotes of the Week * monochrom: if I were dying to learn covariance, I would not be looking for entertainment, like, I'm dying, I only have 5 minutes to learn covariance, thank you very much * hpc: the categorical dual of a hippomorphism should be a giraffomorphism * ksf: but, as, maybe indeed or not apparently, english, in, or especially in, punctuation matters is an utter mess. * 00:56:03 <benmachine> @faq can haskell tell if I am lagging [...] 00:57:18 <lambdabot> The answer is: Yes! Haskell can do that. Top Reddit Stories * Haskell: the Craft of Functional Programming, 3rd edition is out! Domain: haskellcraft.com, Score: 44, Comments: 10 On Reddit: http://goo.gl/6ZInj Original: http://goo.gl/t0GAX * GHC 7.0.4 is out Domain: haskell.org, Score: 42, Comments: 2 On Reddit: http://goo.gl/oxSJH Original: http://goo.gl/EzWUP * SafeHaskell pushed into GHC Domain: haskell.org, Score: 39, Comments: 5 On Reddit: http://goo.gl/sOrjr Original: http://goo.gl/2Rw56 * Minimum footprint for a GHC program: or, think about your TSOs Domain: stackoverflow.com, Score: 23, Comments: 0 On Reddit: http://goo.gl/eIF0o Original: http://goo.gl/dRgLG * Pieces of Yesod: Inverting a Haskell Function Domain: chplib.wordpress.com, Score: 21, Comments: 8 On Reddit: http://goo.gl/NFJNH Original: http://goo.gl/x2cz2 * The 2011 ICFP contest is starting in just 6 hours! Domain: icfpcontest.org, Score: 20, Comments: 15 On Reddit: http://goo.gl/BS7XI Original: http://goo.gl/oqj0Y * A pattern for avoiding allocation : Inside T5 Domain: blog.ezyang.com, Score: 19, Comments: 12 On Reddit: http://goo.gl/thuRP Original: http://goo.gl/IZCIh * Package of the Day: an improved runghc for fast repeated runs Domain: hackage.haskell.org, Score: 16, Comments: 9 On Reddit: http://goo.gl/k6mQK Original: http://goo.gl/a0DAC * The Supero Supercompiler Domain: community.haskell.org, Score: 16, Comments: 2 On Reddit: http://goo.gl/vBeFO Original: http://goo.gl/3QsmC * Galois Video: Building an Open-Source Autonomous Quad-Copter Domain: corp.galois.com, Score: 15, Comments: 3 On Reddit: http://goo.gl/HZghO Original: http://goo.gl/6AcMw Top StackOverflow Questions * Why does performGC fail to release all memory? votes: 17, answers: 2 Read on SO: http://goo.gl/dRgLG * OSX, ghci, dylib, what is the correct way? votes: 17, answers: 1 Read on SO: http://goo.gl/0s9Tu * Why does Haskell's `head` crash on an empty list (or why *doesn't* it return an empty list)? (Language philosophy) votes: 15, answers: 6 Read on SO: http://goo.gl/tghQR * Towards understanding CodeGen* in the Haskell LLVM bindings votes: 13, answers: 1 Read on SO: http://goo.gl/LPZ2F * What happens to you if you break the monad laws? votes: 13, answers: 3 Read on SO: http://goo.gl/Trvny * Is anyone using delimited continuations to do web development in Haskell? votes: 12, answers: 3 Read on SO: http://goo.gl/hGjuB * The concept of Bottom in Haskell votes: 12, answers: 1 Read on SO: http://goo.gl/9b1ZJ About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz

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Iteratee IO examples
by David Place 24 Jun '11

24 Jun '11

Hi, I've been trying to learn Iteratee IO. I've made some progress by studying John Millikin's examples in the source of the enumerator package. I was surprised how confusing I found the tutorials that are available. I think that it is primarily because of the lack of concrete examples. It would be great if we could accumulate a collection of small concrete programs like John's wc.hs which show various uses of Data.Enumerator. Here's a little program I wrote to find the longest run of same characters in a file. > http://hpaste.org/48255 Does anyone else have little examples like this that use Iteratee IO? Cheers, David ____________________ David Place Owner, Panpipes Ho! LLC http://panpipesho.com d(a)vidplace.com

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Weird multi-threading runtime behaviour of single-threaded program with GHC-7.0.3
by Herbert Valerio Riedel 24 Jun '11

24 Jun '11

Hello, I'm experiencing an unexpected behaviour with GHC-7.0.3/x86_64's multithreading runtime when running some more demanding single-thread computations. I've isolated the part of the larger application which exposes the problem down to the parsing of a ~8MiB JSON file which causes quite a bit of memory allocations (source at end of email). The program is compiled with ghc --make -O2 -threaded -eventlog -rtsopts parse_aeson.hs and I ran it with and w/o the -N12 option on a 12-core machine (the problem is visible on a 4-core machine as well): $ time ./parse_aeson test.json 0.000392s after readFile 0.001016s parse was a success... 6.050836s NFed parse-tree... 6.050968s NFed parse-tree... no exception real 0m6.100s user 0m5.680s sys 0m0.419s $ time ./parse_aeson test.json +RTS -N12 0.001249s after readFile 0.001777s parse was a success... 10.048607s NFed parse-tree... 10.048738s NFed parse-tree... no exception real 0m10.125s user 0m57.496s sys 0m39.927s When observing top(1)'s output, I saw that all HECs consumed a substantial amount of cpu cycle, so I loaded up threadscope to see what the remaining 11 HECs were doing all the time. I've put the resulting eventlog files and screenshots of threadscope's visualization online at http://www.narf.at/~hvr/Haskell/parse_aeson.N0.eventlog.png http://www.narf.at/~hvr/Haskell/parse_aeson.N0.eventlog.xz http://www.narf.at/~hvr/Haskell/parse_aeson.N12.eventlog.png http://www.narf.at/~hvr/Haskell/parse_aeson.N12.eventlog.xz What's happening there? The actual processing work seems to be done in a single HEC... but what are the remaining 11 HECs doing exactly? Am I doing something wrong? What does it mean when the GC-bars in threadscope becomes green as in the middle of the screenshot below? http://www.narf.at/~hvr/Haskell/parse_aeson.N4.eventlog.pdf ------------------------------------------------------------------------- -- also available at http://www.narf.at/~hvr/Haskell/parse_aeson.hs module Main where import Control.DeepSeq import Control.Exception import Data.Aeson import Data.Time import System.Environment import qualified Data.Attoparsec.Lazy as APL import qualified Data.ByteString.Lazy.Char8 as LB parseIt :: FilePath -> IO () parseIt fn = do t0 <- getCurrentTime let putStrLn' !s = do t0' <- getCurrentTime putStrLn $ show (t0' `diffUTCTime` t0) ++ " " ++ s json_text <- LB.readFile fn putStrLn' "after readFile" let Success p = decode json_text :: Result Value putStrLn' "parse was a success..." putStrLn' $ p `deepseq` "NFed parse-tree..." putStrLn' $ p `deepseq` "NFed parse-tree..." decode :: FromJSON a => LB.ByteString -> Result a decode = either Error fromJSON . APL.eitherResult . APL.parse json main :: IO () main = do fns <- getArgs case fns of [fn] -> do res <- try $ parseIt fn case res of Left e -> putStrLn $ "caught: " ++ show (e :: SomeException) Right () -> putStrLn "no exception" _ -> putStrLn "usage: parse_aeson <json-file>"

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Re: [Haskell-cafe] Haskell Weekly News: Issue 187
by Jack Henahan 24 Jun '11

24 Jun '11

You could always just subscribe to the HWN feed on Contemplating Code. I just load up my reader when I don't want to read the text dispatches. Try http://contemplatecode.blogspot.com/feeds/posts/default?alt=rss That's the feed URL I use. Then the one on the mailing list is your plaintext fallback. :D On Jun 23, 2011, at 8:48 PM, Conrad Parker wrote: > On 24 June 2011 02:24, Rogan Creswick <creswick(a)gmail.com> wrote: >> On Thu, Jun 23, 2011 at 11:16 AM, Simon Michael <simon(a)joyful.com> wrote: >>> On 6/23/11 10:49 AM, Iustin Pop wrote: >>>> >>>> FYI, a regular link (though longer) seems more appropriate to me. >>>> Don't know if other people feel the same though. >> >> I prefer the short links, since it is much easier to keep track of >> what's going on when reading on a small screen (much of my email >> reading these days is done on my phone.) > > I'd prefer to just read an HTML version of HWN on my phone. How about > simply sending an HTML email with a plaintext fallback? > > In the plaintext email I'd prefer full URLs to cut/copy on my computer terminal. > > Conrad. > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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Data.Enumerator question
by David Place 24 Jun '11

24 Jun '11

I can see that an Enumeratee can produce fewer outputs than it receives as inputs. For example, this GroupBy Enumeratee. > http://hpaste.org/47916 Can an Enumeratee produce more outputs than inputs? For instance, take a Data.Text and send each character to the inner Iteratee individually. I feels this can be done, but don't see how to do it. Thanks. ____________________ David Place Owner, Panpipes Ho! LLC http://panpipesho.com d(a)vidplace.com

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Re: [Haskell-cafe] Category theory as a design tool
by wren ng thornton 23 Jun '11

23 Jun '11

On 6/23/11 1:39 AM, Arnaud Bailly wrote: > Of course, we can always say that each > system is a language of its own (rather than *has* a language...) which is > what Eric Evans coined with its "Ubiquitous language" term. But I find it > difficult to connect that particular dots. That's certainly my take on things (which I argued for at a workshop last week). Then again, I'm a linguist and type theorist, so viewing systems as languages[1] comes naturally. Do you have a link to Evans' coinage and argument? [1] Or rather, APIs as languages, their implementations as (defining their) semantics, and their execution as parsing. >> As an example of this approach, see my comments in the thread about >> anonymous sum types. For something to be a product means that a particular >> diagram commutes: namely, a product is the limit of the two point category >> (and coproducts are the colimit). One of the many things this tells us is >> that if we posit that A*A ~ A in a category that "has products", we must >> infer that the category is a meet-(pre)semilattice[1]. > > I certainly won't argue with you on this given my limited knowledge of what > a meet-(pre)semilattice could be :-) A preorder is just a generalization of a partial order, which we can gloss over here. A meet-semilattice is just a partial order with a "meet" operation, i.e. every pair of elements has a greatest lower bound. (A join-semilattice is the dual, with LUB instead of GLB.) > But then, if I happen to understand > what it is, how could that help me design and code "better" systems? I have > the intuition it could help me but again I do not (yet) see how. Well, for this example, there was the proposed idea to have anonymous union types with the injectors given by the types of the components (e.g., for the type A|B we'd have data constructors A :: A -> A|B and B :: B -> A|B). But this raises the issue of how to deal with A|A. Someone suggested that we could just say that A|A is the same as A. However, doing so means that the (|) type operator cannot be a coproduct. That's not a game-killer per se ---afterall, Either isn't a corpoduct--- but it does suggest that something fishy is going on. In this case, the better system is the one we already have, where we use disjoint unions (Either) instead of non-disjoint unions (|). To put a different spin on it, in the dual case we can show that Haskell's (,) is not a categorical product. There's a good deal of historical debate about why it works the way it does, but if we're looking to make a better system then the fact that tuples are not well-behaved suggests a place for improvement. The current (,) is a compromise between two different notions of product in domain theory. On the one hand we have domain products (which do not produce domains) and on the other hand we have smash products (which can "lose" information). I've had in mind for a while to make a Haskell-like language with a total functional core. In a total language (even a lazy one), we don't have bottom so we don't need domain theory to reason about the language. So in a language which is only partially total, it's reasonable to have two different kinds of products--- which means we can get away from Haskell's compromise, but do so in a way that's harmonious with the rest of the language. Would this new system be "better"? I dunno. It would behave in a more lawful manner, so it should be easier to reason about; but then we're making programmers keep that reasoning in mind, and maybe that's too much work. A decent system has to be both correct but also usable, and human factors are messy and hard to predict. -- Live well, ~wren

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Category theory as a design tool
by Arnaud Bailly 23 Jun '11

23 Jun '11

(2nd try, took my gloves off...) Hello Café, I have been fascinated by Cat. theory for quite a few years now, as most people who get close to it I think. I am a developer, working mostly in Java for my living and dabbling with haskell and scala in my spare time and assuming the frustration of having to live in an imperative word. More often than not, I find myself trying to use constructs from FP in my code, mostly simple closures and typical data types (eg. Maybe, Either...). I have read with a lot of interest FPS (http://homepages.mcs.vuw.ac.nz/~tk/fps/) which exposes a number of OO patterns inspired by FP. Are there works/thesis/books/articles/blogs that try to use Cat. theory explicitly as a tool/language for designing software (not as an underlying formalisation or semantics)? Is the question even meaningful? Thanks in advance, Arnaud

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Re: [Haskell-cafe] Problem with GLUT - no visuals found
by Jason Dagit 23 Jun '11

23 Jun '11

On Thu, Jun 23, 2011 at 11:57 AM, Chris Smith <cdsmith(a)gmail.com> wrote: > On Thu, 2011-06-23 at 11:50 -0700, Jason Dagit wrote: >> What is your deadline for making sure you have a working library? >> > > I don't know... August? In any case, I do now have a working library, > so long as I remember not to use GHCi for it! > > While GLFW would certainly be far preferable to GLUT in the long run, > the fact is that Gloss is already implemented using GLUT, and I can make > GLUT work. So I'll plan on using that unless someone else volunteers to > port Gloss to GLFW. > > (To be honest, I did consider it when I first ran into this problem a > week ago; but after a few hours of hacking at it, it became clear that > it's too large a job, and I'm far too unfamiliar with GLUT, GLFW, or > OpenGL, for that to be feasible as yet another side project for me.) Yes, I've suggested to the author of Gloss that he should switch to GLFW-b but he basically said what you said. He considered it when I mentioned it, but since Gloss already works with GLUT he wouldn't do it unless someone sent him the patches. Which is fair from a maintenance point of view. The downside is that all the windows users of Gloss have to get/install the glut32.dll from somewhere before Gloss will work for them. GLFW-b doesn't have that restriction. Also, when I first looked at GLFW I thought it didn't call atexit() (GLUT uses atexit(), which I think is kind of an evil C function, without having a specific gripe other than it being global mutable state), but it does actually use it. This line of thought always leads me to think that we should have a purely Haskell implementation of GLFW/GLUT on top of the platform specific gui bindings. At any rate, I'm glad to hear you have what you need for your class. Jason

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Problem with GLUT - no visuals found
by Chris Smith 23 Jun '11

23 Jun '11

So I'm running into an embarrassing problem trying to prepare for the computer programming class that I intend to 12-13 year olds in Haskell this coming fall. I had decided to use gloss, an excellent package by Ben Lippmeier that makes it really easy to build images, animations, simulations, and games in a straight-forward and functional way. Excellent... but suddenly broken on my system since installing Ubuntu 11.4, and it looks like the cause traces back to GLUT. Prelude Graphics.Gloss> displayInWindow "Test" (100,100) (100,100) white (Circle 80) freeglut (<interactive>): ERROR: Internal error <Visual with necessary capabilities not found> in function fgOpenWindow X Error of failed request: BadWindow (invalid Window parameter) Major opcode of failed request: 4 (X_DestroyWindow) Resource id in failed request: 0x0 Serial number of failed request: 25 Current serial number in output stream: 28 and Prelude Graphics.UI.GLUT> initialize "foo" [] [] Prelude Graphics.UI.GLUT> get displayModePossible False Nothing I can find to change in GLUT causes displayModePossible to be True, and I'm at the end of my knowledge here. A quick web search turns up the occasional person complaining about the freeglut error, but in all cases they have trouble running any GLUT applications, and it turns out their configuration doesn't do direct rendering. In my case, glxgears and any other non-Haskell GLUT applications I can find work great, but Haskell's GLUT bindings are giving me no hint of success. Any ideas? -- Chris Smith

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OSX, ghci, dylib, what is the correct way?
by Jason Dagit 23 Jun '11

23 Jun '11

Hello, I also asked this question on SO: http://stackoverflow.com/questions/6323755/osx-ghci-dylib-what-is-the-corre… I wonder where I'll get more/better answers? Read on for the question. Thanks! Jason I need to build some C code and then reference that C code via the FFI. I would like to use my binding from inside ghci on osx. On of my constraints is that I cannot just hand the C sources to ghc in the .cabal file. This is due to a limitation with ghc/cabal that may be fixed in the next release of ghc (but I want my code to work now and on older releases). See this [bug for details][1]. The gist of that bug is that the C code needs to be compiled with some Objective-C modules and ghc misinterprets those as linker scripts. I've tried many things and building the files myself with a makefile is the only thing that has worked. Really, this shouldn't be an issue though because it should be the same as if I decided to use a external C library that I didn't build myself. For the sake of this issue, let's pretend it's a separate C library that I can easily rebuild with different options. If I build the C library as a .a, then ghci comlains that it cannot open the .dylib. My first question is: Why does ghci need a .dylib and does it really use it? When I build a dylib I get a [segfault when loading the code into ghci][2]. Keep in mind, this binding works already on other platforms, both linux and windows, and the binding works fine on osx when I'm compiling instead of using ghci. This problem specific to the osx/ghci combo. In that trace above, I'm using gdb but it crashes regardless of whether I use gdb. I tracked it down to the lines that cause the crash: void _glfwClearWindowHints( void ) { memset( &_glfwLibrary.hints, 0, sizeof( _glfwLibrary.hints ) ); } The trouble maker is that memset line, well actually the problem is that when running inside ghci writing to the hints structure of `_glfwLibrary` is a memory access violation. The hints struct is simply a bunch of ints. It's very flat and simple, and thus I think the problem is an issue either with how I'm linking things or with the way ghci is loading the code. Here are the bits of my makefile that I use to build the dylib and the .a: GCCFLAGS := $(shell ghc --info | ghc -e "fmap read getContents >>= \ putStrLn . unwords . read . Data.Maybe.fromJust . lookup \ \"Gcc Linker flags\"") FRAMEWORK := -framework Cocoa -framework OpenGL GLFW_FLAG := $(GCCFLAGS) -O2 -fno-common -Iglfw/include -Iglfw/lib \ -Iglfw/lib/cocoa $(CFLAGS) all: $(BUILD_DIR)/static/libglfw.a $(BUILD_DIR)/dynamic/libglfw.dylib $(BUILD_DIR)/dynamic/libglfw.dylib: $(OBJS) $(CC) -dynamiclib -Wl,-single_module -compatibility_version 1 \ -current_version 1 \ $(GLFW_FLAG) -o $@ $(OBJS) $(GLFW_SRC) $(FRAMEWORK) $(BUILD_DIR)/static/libglfw.a: $(OBJS) ar -rcs $@ $(OBJS) Most of the flags are taken straight from the GLFW Makefile so I think they should be correct for that library. The first line looks a bit weird but it's the solution I used for this [problem][3]. Platform details: - OSX 10.6.6 - x86_64 - 4 cores - GHC version 7.0.3 installed via Haskell Platform installer - Source repo: https://github.com/dagit/GLFW-b [1]: http://hackage.haskell.org/trac/ghc/ticket/5025 [2]: http://hpaste.org/47457/segfault_in_ghci [3]: http://stackoverflow.com/questions/6217406/how-can-i-detect-if-ghc-is-set-t…

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Darcs 2.8 beta 1 is available
by Florent Becker 23 Jun '11

23 Jun '11

Hi cafe-dwellers, On behalf of the darcs team, I'm pleased to announce the release of darcs 2.8 beta 1. This new darcs is just one cabal install away from you, as you can install it by: -cabal update -cabal install darcs-beta (note this will create an executable named "darcs", not darcs-beta). You can then come back to your usual darcs if you needby -cabal install --reinstall darcs Alternatively, you can get the source tarball from here: http://hackage.haskell.org/packages/archive/darcs-beta/2.7.98.1/darcs-beta-…. More information on the release is available at http://wiki.darcs.net/Releases/2.8 This is a beta, so bug reports are very welcome. Highlights of this new version of darcs are: - a faster and much more readable annotate. - you can use darcs optimize --http to make darcs get from your repository over http much faster - you can now use darcs obliterate -o bundle.dpatch to save some patches into a bundle while unpulling them. This allows to stash these patches away while you do something else. - rollback now has a --no-record option which allows to undo some changes from a past patch without recording that as a new patch straight away. - end of write support of "old-fashionned" repositories. These repositories are the one darcs-1 used to create, and they have been replaced by the new "hashed" repositories since darcs-2. You can still use get, pull and so on from these repositories, but darcs 2.8 will refuse to write to them. Enjoy your new darcs, The darcs team

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Quickcheck: Help on non trivial test
by jean-christophe mincke 23 Jun '11

23 Jun '11

Hello Café, I am using quicheck for some kind of non trivial tests. The general form of these tesst is summarized by the following code. -- Function to be tested. Given a list of splitting functions, split the given list process :: [a] -> [[a] -> [a]] -> [a] process l splitFuns = List.foldl proc l splitFuns where proc l splitFun = splitFun l -- a split function generator splitFunGen :: Gen ([a] -> [a]) splitFunGen = return $ proc where proc l = let splitPos = List.length l `div` 2 -- Problem I would like splitPos = some random value between [0 and length of l] in fst $ List.splitAt splitPos l splitFunsGen :: Gen [[a] -> [a]] splitFunsGen = vectorOf 20 splitFunGen instance Show a => Show ([a] -> [a]) where show _ = " a split fun " r = quickCheck $ forAll splitFunsGen prop where prop splitFuns = let l = process [1..100] splitFuns in List.length l >= 0 -- dummy test here for the sake of example The process to be tested takes a list that is randomly perturbated (spitFuns). The result of each perturbation is fed into the next perturbation. Ideally I would like the perturbating function to depend on the previous perturbated list (see computation of splitPos in splitFunGen). I am not sure how I could use quickcheck in this case. Has anyone a better idea? Thanks J-C

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Re: [Haskell-cafe] Category theory as a design tool
by wren ng thornton 23 Jun '11

23 Jun '11

On 6/22/11 3:59 PM, Arnaud Bailly wrote: > Hello Greg and Alexander, > Thanks for your replies. Funnily, I happen to own the 3 books you mentionned > :-) My interest in category theory is a long standing affair... > > Note that owning a book, having read (most of) it and knowing a theory (or > at least its principles and main concepts) is really quite a different thing > from being able to apply it. One of the big benefits I see to using category theory for dealing with programming languages comes from using CT as a generalized logic for equational reasoning. In particular, making use of the ideas of (co)limits and adjunctions in order to prove that something must behave in a particular way. This requires a bit of work to contort your thinking into the patterns of CT, but it means that once you can prove that something is a foo, then all the theorems about foos will apply. Just having a diagram and knowing it commutes doesn't tell you a whole lot, it just says that one particular equation holds. The real power comes from looking at diagrams that are parametric (since then proving it holds will mean it holds for all parameter valuations), or looking at diagrams where one of the arrows is (essentially) unique (since then you can show things that look like foos actually are foos). That is, the real power of diagrams comes from the quantifiers, which are explained in the text rather than shown in the diagram itself. As an example of this approach, see my comments in the thread about anonymous sum types. For something to be a product means that a particular diagram commutes: namely, a product is the limit of the two point category (and coproducts are the colimit). One of the many things this tells us is that if we posit that A*A ~ A in a category that "has products", we must infer that the category is a meet-(pre)semilattice[1]. [1] Or a really goofy structure where for every A and B there are at most two A->B, with a canonical way of distinguishing them. Of course, for this to be a category we need to define a composition operator, which will take the four compositions into the two possible arrows, which is where it gets strange. -- Live well, ~wren

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Re: [Haskell-cafe] Why aren't there anonymous sum types in Haskell?
by wren ng thornton 23 Jun '11

23 Jun '11

On 6/22/11 6:00 PM, Casey McCann wrote: > On Wed, Jun 22, 2011 at 2:19 PM, wren ng thornton<wren(a)freegeek.org> wrote: >>>> [1] modulo the A:+:A ~ A issue. >> >> Oops, I should've said A:*:A there. >> >>> That issue is exactly my concern, though, and it seems a bit too >>> thorny to handwave aside. >> >> Indeed. If we have A:*:A ~ A, then A:*:A is not a categorical product. > (...) > > Thank you, that clarified my intuitive sense of why it didn't seem to > work. The same argument, with arrows flipped appropriately, would > apply to a hypothetical coproduct where A :+: A ~ A, wouldn't it? It should. But let's make sure... A coproduct of A and B is an object A+B along with morphisms inl :: A -> A+B and inr :: B -> A+B, such that for any object C and morphisms c1 :: A -> C and c2 :: B -> C, there exists a unique c1|||c2 :: A+B -> C such that c1|||c2 . inl = c1 and c1|||c2 . inr = c2. All of this is the straightforward dualizing of products. So, lets assume A ~ A+A. And lets assume we have some functions inl and inr. Now, for every pair of functions f, g :: A -> C we must come up with a unique function h :: A+A -> C such that h . inl = f and h . inr = g. By isomorphism this is the same as coming up with a unique function h' :: A -> C such that h' . iso . inl = f and h' . iso . inr = g. If inl = inr, then it follows that f = g. Consequently there can only be one morphism between any two objects, so the category is a preorder. If the category has all coproducts then it's a join-semilattice--- exactly dual to the meet-semilattice case for products. If we want a category more interesting than a preorder, then we must have that inl and inr are different. We must find two injection functions which keep their images in A+A apart, so that we can define partial functions -|||g and f|||- which don't conflict with each other (so that they can be combined into the total function f|||g). The obvious way to do that is to use tagged/disjoint unions, but we can't do that since we've assumed A ~ A+A. Basically, the type A+A isn't big enough (whereas before, the values of type A*A weren't big/informative enough). I'm playing a bit fast and loose with the notion of "big/informative enough". Perhaps someone more familiar with large ordinals can come up with a counter example. But if they do, I'm sure it wouldn't be a very intuitive place to program in. -- Live well, ~wren

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Re: [Haskell-cafe] Why aren't there anonymous sum types in Haskell?
by wren ng thornton 23 Jun '11

23 Jun '11

On 6/21/11 10:48 PM, Casey McCann wrote: > On Tue, Jun 21, 2011 at 9:51 PM, wren ng thornton<wren(a)freegeek.org> wrote: >> I don't think there are any problems[1]. > (...) >> [1] modulo the A:+:A ~ A issue. Oops, I should've said A:*:A there. > That issue is exactly my concern, though, and it seems a bit too > thorny to handwave aside. Indeed. If we have A:*:A ~ A, then A:*:A is not a categorical product. The reasoning is as follows. An object A*B with morphisms fst :: A*B->A and snd :: A*B->B is a product iff for all objects P with morphisms p1 :: P -> A and p2 :: P -> B, there is a unique morphism p1&&&p2 :: P -> A*B, such that p1 = fst . p1&&&p2 and p2 = snd . p1&&&p2. Given the definition, if we further say that A*A ~ A, then it's clear that we can't come up with any satisfactory definitions of fst and snd which obey the laws--- unless there can be only one morphism between any two objects, in which case we know that p1 = p2 and so it's okay for fst = snd; or unless there can be only two morphisms between any two objects and we have a canonical property for distinguishing them so that we can correlate p1 with fst and p2 with snd. In all other cases, there are simply too many possible choices of p1 and p2, and there's no way for fst and snd to carry enough information to make the equations hold. What that means then is that the category which tries to use :*: to make products would only have products in the cases where the first and second component are different; which would be a strange category indeed. Alternatively, it could always have products but would only allow one morphism between objects; in which case the category is just a meet-semilattice (actually, the preorder generalization thereof). > In contrast, ordered pairs and disjoint unions are tidy, simple, and obvious. Disjoint pairs are sufficient; they needn't be ordered. All we need is that they are "tagged" in the same way that disjoint unions are, so that we can distinguish the components of A*A. -- Live well, ~wren

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Graph diagram tools?
by Dmitri O.Kondratiev 23 Jun '11

23 Jun '11

Hi, I am looking for Haskell library to create graph diagrams in png or similar formats. Thanks!

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Re: [Haskell-cafe] category theory as a design tool
by Brett Giles 23 Jun '11

23 Jun '11

Hi Arnaud, you may want to look at http://mathcs.mta.ca/research/rosebrugh/Easik/ This is a java tool which allows entity design using various cat theory constraints such as sum, product and pullbacks. Brett Giles Sent from my iPhone > Message: 14 > Date: Wed, 22 Jun 2011 14:06:21 -0500 > From: Gregg Reynolds <dev(a)mobileink.com> > Subject: Re: [Haskell-cafe] Category theory as a design tool > To: Arnaud Bailly <arnaud.oqube(a)gmail.com> > Cc: Haskell Cafe <haskell-cafe(a)haskell.org> > Message-ID: <BANLkTingApLo7ysYGgg1DUxbW5iK1kLmrQ(a)mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > On Tue, Jun 21, 2011 at 11:30 PM, Arnaud Bailly <arnaud.oqube(a)gmail.com> > wrote: >> (2nd try, took my gloves off...) >> Hello Caf?, >> I have been fascinated by Cat. theory for quite a few years now, as >> most people who get close to it I think. >> >> I am a developer, working mostly in Java for my living and dabbling >> with haskell and scala in my spare time and assuming the frustration >> of having to live in an imperative word. More often than not, I find >> myself trying to use constructs from FP in my code, mostly simple >> closures and typical data types (eg. Maybe, Either...). I have read >> with a lot of interest FPS (http://homepages.mcs.vuw.ac.nz/~tk/fps/) >> which exposes a number of OO patterns inspired by FP. >> >> Are there works/thesis/books/articles/blogs that try to use Cat. >> theory explicitly as a tool/language for designing software (not as an >> underlying formalisation or semantics)? Is the question even >> meaningful? > > You might try: Category Theory for Computing > Science<http://www.cwru.edu/artsci/math/wells/pub/ctcs.html> (Barr > and Wells) > > and Conceptual Mathematics: a first introduction to > categories<http://books.google.com/books/about/Conceptual_mathematics.html?id=o1tHw4W5…> > (Lawvere) > > "Kinship and Mathematical Categories" (by Lawvere) is also interesting. > > -Gregg > >> >> Thanks in advance, >> Arnaud >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >

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Homework help - calculator function
by SM Design 22 Jun '11

22 Jun '11

I have a homework which is very important to be done but I can't complete the task at all. The program i should write is: Make calculator- function in Haskell. The function argument is a list of strings and also form such list, as each string of the argument made definite action. • If the string has the form of an arithmetic expression - calculate this expression. The string result becomes part of the list- result. If the expression contains a variable which is not assigned value, the result is displayed "undefined". • If the string has the form Name = value calculated from the last expression is assigned to the variable with the corresponding name in the list, and in the result list is formed a string with type name = ... where the site stands blank corresponding value. If there is not a calculated expression to be assigned to form a string "no value". • If the string is non-blank, but there is a species different from the above two case, form the string "error". • If the string is empty, incl. when it contains only spaces, in the result there is not form a string. Expressions consist of integers without sign variables, operations + (Addition), - (subtraction), * (multiplication) and / (divide) and parentheses. Where no brackets, the operations are performed from left to right, but * and / precede the + and -. Implementation of any operation gives integer; in the division rejected the fractional part , if any . Variables have names of one letter - from the Latin small letter . In the beginning, end or between the elements of each row can have spaces - they are irrelevant to its correctness. Example: the list-argument ["3 +7 / 2" "2 + x", "= s", "2 * s +4", "", "2 + +4 / 5] function should provide a result-list ["6", "undefined", "s = 6", "16", "error"]. Thank you an advance!

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Verifying Haskell programs vs. generating verified Haskell programs
by Uli Kastlunger 22 Jun '11

22 Jun '11

Hello Haskell fellows, recently there has been a huge progress in generating real programs by specifying them in interactive theorems prover like Isabelle or Coq, in particular a verified C Compiler has been generated out of a specification in Coq [0]. Furthermore it is often stated, that it is easier to reason about functional languages. So in my opinion there are two approaches building verified software (e.g. in case of compilers: verify that the semantics of a high-level language maps correctly to the semantics of a low-level language). (0) Programming in any language and then verifying it with an external theorem prover (1) Specifying the program in a proof language and then generating program code out of it (like in the CompCert project) I think both ideas have their (dis)advantages. The question is, which concept will be used more frequently? In particular how hard would it be to build something like CompCert the other way around (in this case writing a C compiler in SML and proving its correctness?) Best regards, Uli Kastlunger [0] http://compcert.inria.fr/

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Re: [Haskell-cafe] Getting library documentation for installed packages on Windows
by John Ky 22 Jun '11

22 Jun '11

Hi Svein, Where can I find this file on Windows 7 or Windows generally if its all the same? Cheers, -John On 22 June 2011 10:15, Svein Ove Aas <sveina(a)gmail.com> wrote: > Yes, cabal does this if you have haddock installed. > > You'll probably have to edit .cabal/config, though. The relevant options > should be fairly obvious in there. > On Jun 22, 2011 12:52 AM, "John Ky" <newhoggy(a)gmail.com> wrote: > > Hi all, > > > > Lately I've been finding the Network module missing from the docs I > download > > from the GHC website, which brings me to the question: > > > > Is there any way I could generate libraries for this, the core libraries > and > > all installed cabal packages into one reference so I can work offline? > > > > Cheers, > > > > -John >

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Re: [Haskell-cafe] PhD studentship in Functional Programming
by Graham Hutton 22 Jun '11

22 Jun '11

Dear all, Just a quick reminder that the deadline for receiving applications for this PhD studentship position is this Friday (24th June.) Best wishes, Graham > +------------------------------------------------------------------+ > > PhD Studentship in Functional Programming > > School of Computer Science > University of Nottingham, UK > > Applications are invited for a fully-funded PhD studentship in > the Functional Programming Lab in the School of Computer Science > at the University of Nottingham, starting on 1st October 2011, > under the supervision of Prof Graham Hutton. > > The suggested topic for the studentship is to develop a practical > implementation of the worker/wrapper transformation, a simple but > powerful unifying paradigm for program optimisation, but other > ideas for possible topics are welcome too. The studentship is > for three and a half years, and includes a maintenance grant > of 13,590 UK pounds per year, and tuition fees. > > Applicants for the position will require a first-class Honours > degree (or equivalent) in Computer Science and/or Mathematics, > some experience in functional programming, and an aptitude for > mathematical subjects. Additional desirable attributes include > a Masters degree, and/or some experience in programming language > semantics, implementation and optimisation. Due to the nature > of the funding, the position is only open to UK citizens, or EU > citizens who have resided in the UK for the last three years. > > The successful applicant will work under the supervision of Prof > Graham Hutton in the FP Lab in Nottingham, a leading centre for > research on functional programming. The group currently comprises > 5 academic staff, 1 research fellow, and 10 PhD students. > > In order to apply, please submit the following to Prof Graham > Hutton (graham.hutton(a)nottingham.ac.uk) by 24th June 2011: an > up-to-date copy of your CV (including the results of all your > University examinations to date) along with a brief covering > letter that describes your experience in functional programming, > your reasons for wishing to pursue a PhD in this area, and any > ideas you have regarding possible research directions. > > Closing date for applications: 24th June 2011. > > +------------------------------------------------------------------+ > > -- > Graham Hutton > Functional Programming Lab > School of Computer Science > University of Nottingham, UK > http://www.cs.nott.ac.uk/~gmh This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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Fwd: Re: Hackage Server not reachable
by Steffen Schuldenzucker 22 Jun '11

22 Jun '11

Forwarding to -cafe. -------- Original Message -------- Subject: Re: [Haskell-cafe] Hackage Server not reachable Date: Wed, 22 Jun 2011 20:43:59 +1000 From: Stuart Coyle <stuart.coyle(a)gmail.com> To: Steffen Schuldenzucker <sschuldenzucker(a)uni-bonn.de> Cabal fails with a timeout like this: stuart@Panforte:~/Code/gift-parser# cabal update Downloading the latest package list from hackage.haskell.org <http://hackage.haskell.org> cabal: timeout From traceroute it seems that I'm having routing problems. traceroute to hackage.haskell.org <http://hackage.haskell.org> (69.30.63.204), 64 hops max, 52 byte packets 1 moodle-server (192.168.1.1) 1.834 ms 0.923 ms 0.904 ms 2 10.67.0.1 (10.67.0.1) 18.781 ms 10.031 ms 14.738 ms 3 rdl2-ge2.gw.optusnet.com.au <http://rdl2-ge2.gw.optusnet.com.au> (198.142.160.5) 13.442 ms 11.920 ms 8.767 ms 4 mas2-ge10-1-0-904.gw.optusnet.com.au <http://mas2-ge10-1-0-904.gw.optusnet.com.au> (211.29.125.138) 55.286 ms 56.473 ms 58.150 ms 5 mas3-ge11-0.gw.optusnet.com.au <http://mas3-ge11-0.gw.optusnet.com.au> (211.29.125.241) 61.473 ms 58.477 ms 56.198 ms 6 203.208.192.169 (203.208.192.169) 202.737 ms 207.018 ms 209.159 ms 7 lap-brdr-03.inet.qwest.net <http://lap-brdr-03.inet.qwest.net> (63.146.26.145) 173.527 ms 174.207 ms lap-brdr-03.inet.qwest.net <http://lap-brdr-03.inet.qwest.net> (63.146.26.149) 172.122 ms 8 tuk-edge-13.inet.qwest.net <http://tuk-edge-13.inet.qwest.net> (67.14.4.206) 235.820 ms 237.548 ms 246.359 ms 9 206.81.193.22 (206.81.193.22) 334.748 ms 511.734 ms 303.460 ms 10 209.162.220.81 (209.162.220.81) 310.886 ms 244.478 ms 241.544 ms 11 * * * 12 * * * 13 * * * continues like this.... I'll get a fix from this end. Time to call the ISP. I wonder if any others in Australia on Optus' network are having the same issue? Thanks all. On Wed, Jun 22, 2011 at 7:16 PM, Steffen Schuldenzucker <sschuldenzucker(a)uni-bonn.de <mailto:sschuldenzucker@uni-bonn.de>> wrote: On 06/22/2011 11:02 AM, Stuart Coyle wrote: I cannot reach the hackage server so cabal can't download packages. Have I the correct address? http://hackage.haskell.org Yes. stuart@rumbaba:~# resolveip hackage.haskell.org <http://hackage.haskell.org> <http://hackage.haskell.org> IP address of hackage.haskell.org <http://hackage.haskell.org> <http://hackage.haskell.org> is 69.30.63.204 Same result for me. I also cannot access any of the Hackage web pages. No problem here. What is the error for you? hackage.haskell.org <http://hackage.haskell.org> doesn't seem to answer pings btw. -- Steffen -- Stuart Coyle stuart dot coyle at gmail dot com

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Hackage Server not reachable
by Stuart Coyle 22 Jun '11

22 Jun '11

I cannot reach the hackage server so cabal can't download packages. Have I the correct address? http://hackage.haskell.org stuart@rumbaba:~# resolveip hackage.haskell.org IP address of hackage.haskell.org is 69.30.63.204 I also cannot access any of the Hackage web pages. I suspect that my ISP may be filtering or that their DNS is giving me the wrong info. -- Stuart Coyle stuart dot coyle at gmail dot com

3 2

yi + cabal-dev
by Alex Rozenshteyn 22 Jun '11

22 Jun '11

I'm trying to run yi. More precisely, I'm trying to run yi in its own sandbox, created by cabal-dev. yi uses dyre to recompile its config file. Unsurprisingly, this fails, since ghc doesn't know anything about the yi install unless pointed to a separate package database. Has anyone gotten a similar setup to work, or does anyone have any suggestions? -- Alex R

3 5

Category theory as a design tool
by Arnaud Bailly 22 Jun '11

22 Jun '11

Hello, I ha

5 5

Using cmake with haskell
by Robert Clausecker 22 Jun '11

22 Jun '11

Hello readers of haskell-cafe! I asked this question on Stackoverflow: http://stackoverflow.com/q/5969616/417501 Because I want to get a good answer, I cross post it here. It may be interesting: Is it possible to use cmake for Haskell projects? I am planning a project written in Haskell, maybe there are some parts in C as well. For the buildsystem I decided against the common choice for Haskell programs cabal, mainly because I want to learn how building programs in other languages works. I heard about CMake and I think it is a pretty cool product. Although I have no knowledge in how to use it, I want use CMake for that project, just to find out how it works. Googleing did not reveal any facts about how to use cmake with haskell, and all tutorials I read were rather confusing. Is it possible, and if yes, how is it possible, to compile a projectwritten in Haskell using CMake? Robert Clausecker

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Re: [Haskell-cafe] Why aren't there anonymous sum types in Haskell?
by wren ng thornton 22 Jun '11

22 Jun '11

On 6/21/11 6:54 PM, Casey McCann wrote: > That said, I don't think either retains the tidy algebraic properties > that disjoint unions and tuples have, so I'm not sure if calling them > sums and products is actually correct. I don't think there are any problems[1]. Categorically speaking, a product of A and B is simply a thing, which we can call Foo[2], along with two projections, f :: Foo -> A and oo :: Foo -> B[3], such that the Foo is the "closest" thing to A and B[4]. I chose the funny names intentionally. Nothing in the definition of products says that there has to be any kind of notion of order between the components, only that the composite has projections for each component and that it has no extra junk. Consequently A*B is isomorphic to B*A, which means they are essentially the same thing[5]. The choice of order is just a convention. For a slightly stronger notion of products we can look to monoidal categories which have that A*(B*C) is isomorphic to (A*B)*C, again meaning that they're essentially the same thing. Outside of category theory, we can look to domain theory (or the associated CPO categories), but domain products and smash products both have the same order independence as categorical products. For coproducts, dually. [1] modulo the A:+:A ~ A issue. [2] More conventionally called A*B, but remember: that's just a name. [3] More traditionally, pi_1 and pi_2, or fst and snd, or... [4] That is, for any other Bar with b :: Bar -> A and ar :: Bar -> B, there is a unique function from Bar to Foo such that everything commutes. [5] The clever reader will notice that they are not *exactly* the same thing, not identical; however, category theorists rarely care about the differences between isomorphic things unless it's mucking up a proof. -- Live well, ~wren

2 1

Getting library documentation for installed packages on Windows
by John Ky 22 Jun '11

22 Jun '11

Hi all, Lately I've been finding the Network module missing from the docs I download from the GHC website, which brings me to the question: Is there any way I could generate libraries for this, the core libraries and all installed cabal packages into one reference so I can work offline? Cheers, -John

1 0

Best Linux Distro for >>=
by Tom Murphy 22 Jun '11

22 Jun '11

Hi List, If my choice of Lunix distro depended 100% on its solidness as a Haskell devel platform (I am), what would you all recommend? Thanks for your time, Tom

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Re: [Haskell-cafe] Choice of Linux distro redux
by John Wagner 22 Jun '11

22 Jun '11

Friends: When you say that support for GHCi is in general best among the Linux distros (unordered) "Gentoo, Arch, Fedora and Debian (I think Testing)", is Ubuntu to be inferred from Debian, or is there some scandalous and well-known to all (but me) malady that lingers around the bellies of Ubuntu distros that interferes with the purity of Glasgow, producing side-effects? My grandfather was a Scot, so I can be trusted: tell me if I should cast off Canonical ways. Thanks, John Wagner -- John Wagner Alhambra and Los Angeles

3 2

Re: [Haskell-cafe] [Haskell-beginners] boomBangs xs = [ if x < 10 then "BOOM!" else "BANG!" | x <- xs, odd x]
by Jack Henahan 21 Jun '11

21 Jun '11

Hi Anthony. What you've got there is a list comprehension. I'll try to break down what it says for you. The basic structure of a list comprehension is function <arguments> = [ description of the list ] In this function, the argument 'xs' is a Haskell idiom. We use 'xs' to mean "a list of x's". 'xs' doesn't *have* to be a list (since you can call anything 'xs'), but conventionally it is. So 'boomBang xs' simply means "the function boomBangs takes a list xs as an argument." Now the odd part, the list comprehension itself. The first part describes what will be in the list, and the second part limits it. What this comprehension does is construct a list of all the odd members of the list 'xs' (in this case, they're numbers) and map "BOOM!" to values under 10, and "BANG!" to anything above 10. So what is says is "if the nth number in my list is less than 10, I want you to make the nth element of the new list "BOOM!", otherwise I want you to make it "BANG!". Furthermore, I only want you to consider odd values of x from my list." So testList xs = [ x | x <- xs, odd x ] called with the list [1,4,8,9,12,15] would return [1,9,15] -- That is, all the odd members of the list xs Now, if 'x' is the expression 'if x < 10 then "BOOM!" else "BANG!"', the list returned is instead ["BOOM!","BOOM!","BANG!"] Decent explanation? Or have I just muddled the issue? On Jun 21, 2011, at 3:58 PM, anthony niro wrote: > Hello, > > My name is Anthony i have few question about Haskell. > > I was reading a tutorial everything goes well. but now i have few things that i dont understang. > > HERE: > > boomBangs xs = [ if x < 10 then "BOOM!" else "BANG!" | x <- xs, odd x] > > > > I dont understand this line except the "if" "then" "else". > What is xs? > what is the | ? > and why doing this " | x <- xs, odd x]" > > why x <- xs????? what is that > > and what is odd x? > > thx everyone for answer me. > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners

1 0

[ANN] mysql-simple - your go-to package for talking to MySQL
by Bryan O'Sullivan 21 Jun '11

21 Jun '11

Hi, folks - Over the past few days, I've released two MySQL-related packages on Hackage that I think should be pretty useful. The first is mysql-simple: http://hackage.haskell.org/package/mysql-simple This is a mid-level binding to the MySQL client API. I aimed it squarely at being both fast and easy to use, and I'm very pleased with the results so far. - Performance: compared to HDBC-mysql, mysql-simple yields a 60% performance improvement on my real-world application. - Ease of use: compared again to HDBC, my real-world application's DB-specific code shrunk by 50% while achieving the above performance increase. - Type safety: it's intentionally hard to construct SQL queries by string concatenation, but of course I provide some nice safe APIs for formatting queries and converting results. That safety does not come at the expense of performance or expressive bloat, as the above results indicate. Because I know that some people favour interacting with their databases via a model such as iteratees, the mysql-simple library is built on top of a very lightweight library. That lower-level library is named mysql: http://hackage.haskell.org/package/mysql This is a low-level binding to the MySQL client API. It is aimed at high performance and simplicity, but more specifically for consumption by authors of higher-level database libraries. It is bare enough of features that it doesn't even perform conversion between Haskell and SQL types, but at the same time it uses bytestrings sensibly, cheaply avoids some nasty signal interruption problems with the MySQL client library, and abstracts some of the tiresome details of memory management. It gives a higher-level library complete control over result conversion, how to fetch results, and all that, so you can focus purely on building iteratees and not the lower-level gunk. Enjoy! B.

5 11

Opaque types vs automated instance deriving
by Alexey Karakulov 21 Jun '11

21 Jun '11

Hi all, I encountered a problem when trying to > derive makeBinary ''DiffTime with help of *derive* package. The error was: Not in scope: data constructor `MkDiffTime' Which makes a sense, since it's not exported in Data.Time.Clock. I bypassed the problem (yes, I'm too lazy to write instances by hands) with > instance Binary UTCTime where > put = putGeneric > get = getGeneric But it must be less efficient (and more verbose) than compile-time deriving. If there was such a module like Data.Time.Clock.Internal, I could import it to get hidden constructors (and maybe I'll have to fork the library for this purpose). OTOH, is it possible to change the derive TH function so it can bypass module encapsulation mechanism and access un-exported things?

2 1

Haskell on iPad? (Scheme and Ocaml are there)
by John Velman 21 Jun '11

21 Jun '11

There are (at least) two Scheme interpreters for iPad at the iTunes store: PixieScheme and GambitREPL. Both allow entry of scripts, by typing or pasting. The Gambit community is very busy trying to expand the usefulness of their interpreter. Both have pretty good interfaces. There is also an Ocaml app, but I don't know or want to know Ocaml, and the interface looks very unfriendly. I'd really like to have something like this in Haskell, in the "education" pot, as is the GambitREPL. Hugs is written in C, if I recall correctly. Would it be possible to compile Hugs for the iPad processor, taking out enough system calls to make it acceptable? John Velman

8 14

Job position for haskell/clojure dev. in San Dimas, CA
by vagif.verdi＠gmail.com 21 Jun '11

21 Jun '11

Anyone interested in full time employment working with haskell and clojure in San Dimas, CA (local job only, NO telecommute) please let me know. Regards, Vagif Verdi

2 1

AusHac2011 - The Australasian Haskell Hackathon - July 8-10
by Alex Mason 21 Jun '11

21 Jun '11

It's almost that time of year again! Next month is the second annual AusHac event, at UNSW in Sydney, and you are invited! But if you want to come, we need to hear from you soon, as we need to organise access to the university network for you. If you're an Australian Haskell hacker, enthusiast, newbie or professional, we would love to see you there. Below is the announcement, if you have any questions, please just hit reply, and Ivan or I will get back to you. We would absolutely love to see you there if you can make it, we'd love for this year to be even more awesome than last year. -- Alex Mason ------------------------------------------------------------------------------ ==== Attention all Australasian Haskellers! ==== After last year’s fantastic turnout for the first Australian Haskell Hackathon - AusHac 2010 - we’ve decided to organise another. As it’s rather boring to have a Hackathon with only two people, we encourage and welcome anyone interested in Haskell in Australia, New Zealand and surrounding areas (or further afield if you want!) to come and join us. Last year we had more than 25 Haskellers come from Brisbane, Canberra, Melbourne, Sydney and even two from as far away as New Zealand. AusHac 2011 will be held from Friday July 8th until Sunday July 10th, again at UNSW’s Computer Science building in Sydney. There’s no need to come for the entire weekend, just for whenever you’re able to. Last year, people worked on several projects including: • Accelerate: a high performance library for array computations using various backends (CUDA, LLVM, etc.) being written at UNSW. • Leksah: the Haskell IDE written in Haskell • DDC: the compiler for the Haskell-like language Disciple, which has strict evaluation by default, region, effect and closure typing and other interesting language features. • Hubris: the Haskell/Ruby bridge for calling Haskell from Ruby and vice-versa. • haskell-mpi: bindings to MPI, the message passing interface used frequently on high performance super-computers to allow programs to run on many nodes in parallel with easy communication. • There was even some work on LLVM to produce HTML output showing register allocations (as an aid to those writing new code generators etc.), which has now been included upstream. This work did not use Haskell, but it did happen at AusHac 2010. • And much more! This is a great chance to collaborate with others and get feedback on your work, while having heaps of fun and meeting other Haskellers, to verify they do actually exist! And don’t worry if you can’t think of a project to work on; just come along and pitch in with whatever strikes your fancy! If you’re interested in coming and have a project you’d like to work on, check out the wikipage http://www.haskell.org/haskellwiki/AusHac2011 where you can add your project ideas, or just take a look and see what others will be working on that you can help out with. People of all skill levels are welcome, from complete beginners to Oleg-level sage masters. We’re more than happy to help you learn Haskell if you’re just starting out or help you solve that little niggly bit in your code. If you’re interested in coming along, fill out our registration form http://axman6.wufoo.com/forms/aushac-2011-sign-up/. Registration is required so we know numbers and for you to get access to the university Wifi. Even if you’re not sure, fill it in to express your interest, obligation free. We’d rather know we have room for too many people than not enough. So come along! And if for some reason you don’t enjoy yourself, registration comes with a money-back guarantee! Hope to see you there, -- Alex Mason and Ivan Miljenovic, AusHac organisers ------------------------------------------------------------------------------

1 0

Text report tools?
by Dmitri O.Kondratiev 21 Jun '11

21 Jun '11

Hi, I am looking for an easy way to generate text reports. For starters I need a very simple report that may contain: - Some headers - Lists of text strings where each string can include instances of basic Haskell types. Each string should be printed on a separate line (terminated with LF). Do I understand this right that for such simple reports (as above) Text.Printf is the main tool? As a next step, a simple report engine controlled by templates or DSL would help. Thanks!

3 2

Probably type checker error.
by Serguey Zefirov 20 Jun '11

20 Jun '11

Right now I write a quite heavy transformation of Haskell source code and found some strange behaviour of typechecker. Some prerequisites: -- dummy class. My own class is much bigger, but I -- could reproduce that behaviour with that class. class ToWires a -- a type with phantom type arguments. data E ins outs = E -- a function that relates E and its inputs. projectInsType :: E ins outs -> ins projectInsType = error "projectInsType gets evaluated." -- "register" function. register :: a -> a -> a register def a = def -- a simple addition. add2 :: (ToWires a, Num a) => (a,a) -> a add2 (a,b) = a+b First I have a function: func :: (ToWires a, Num a) => Maybe a -> a func mbA = currentSum where x = case mbA of Just a -> a Nothing -> 0 nextSum = add2 (x,currentSum) currentSum = register 0 nextSum It typechecks and works just fine after some transformation. The transformation I work on transform code into something like that: func_E :: (ToWires a, Num a) => E (Maybe a) a func_E = r where r = E -- here we relate mbA and r. mbA = projectInsType r x = case mbA of Just a -> a Nothing -> 0 nextSum = add2 (x,currentSum) currentSum = register 0 nextSum Note the absence of input of func in transformed func_E. I relate mbA with it's proper type using binding "mbA = projectInsType r". Then suddently ghc loses all of the context associated with mbA. And find type error at the calling of add2. If I drop ToWires from contexts of func_E and add2 types, all works fine. If I change add2 to simple addition (x + currentSum), all works fine. Full source code is in attachment. I found it using ghc 6.12.1. I asked colleagues, they told me that the same error manifests itself in ghc 7.0.3. Should I fill a bug report or maybe I misunderstood something?

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ANNOUNCEMENT: TinyLaunchbury 1.0.1
by Elliot Stern 20 Jun '11

20 Jun '11

Sankel Software is pleased to announce the release of TinyLaunchbury 1.0.1 [1], a small implementation of John Launchbury's natural semantics for call-by-need. We hope that is is useful in helping to understand how call-by-need works. There is also a new blog post[2], explaining the semantics, the code and why the semantics are useful to understand. Elliot Stern Sankel Software [1] http://hackage.haskell.org/package/TinyLaunchbury [2] http://netsuperbrain.com/blog/posts/haskells-evaluation-isnt-magic/

1 0

Conditional IO ?
by Dmitri O.Kondratiev 20 Jun '11

20 Jun '11

Hi, What is right way to do conditional IO? For example, I need to write to file errors only in case they exist, otherwise my function should do nothing: handleParseErrors errors | (not . null) errors = writeFile "parse-errors.txt" (show errors) | otherwise = ? What should be an 'otherwise' case ? Thanks!

6 6

Software patents covered in GHC?
by Shakthi Kannan 20 Jun '11

20 Jun '11

Hi, I would like to know if there are any software algorithms used in the Glasgow Haskell Compiler that have been patented? If yes, in which countries do they apply? Just curious to know. SK -- Shakthi Kannan http://www.shakthimaan.com

1 0