- Haskell-Cafe - Haskell.org

CABAL: conditional executable?
by Magnus Therning 07 Mar '08

07 Mar '08

I'm putting together a package consisting of 2 executables. Only one of them is pure Haskell and thus buildable on all platforms, the other relies on Windows API calls and can only be built on that platform. I found the “if os(...)” conditional in the CABAL docs but I'm having problems getting it to do what I want. if os(mingw32) executable foo ... Results in the error “Section expected”. Swapping the two lines like this executable foo if os(mingw32) ... results in “Setup.hs: Error: No 'Main-Is' field found for executable foo”. Is there a way to get CABAL to do what I want or should I raise a feature request on the CABAL trac? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus．therning＠gmail．com http://therning.org/magnus What if I don't want to obey the laws? Do they throw me in jail with the other bad monads? -- Daveman

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"Hello World" in Haskell for web browser
by Dimitry Golubovsky 07 Mar '08

07 Mar '08

Hi, I have noticed that some people tried to compile a traditional Haskell program using IO monad to show "Hello World" using Yhc Web Service. Thanks for your interest, and here are some clues. The programming paradigm chosen for Haskell Web programming is not based on monads. It is based on CPS-style calls to DOM functions by Haskell code compiled to Javascript. Further on, additional layers may stack up (such as Haskell Web Toolkit) to provide more convenient APIs, but DOM is the basis. So here is an example of "Hello World" program written this way: -- begin pasteable code -- module HelloWorldDOM where import CPS import UnsafeJS import CDOM.Level2.DomUtils import DOM.Level2.Dom import DOM.Level2.Html2 import DOM.Level2.HTMLElement import DOM.Level2.HTMLDivElement main = getHTMLDocument $ \doc -> documentBody doc $ \body -> mkText doc "Hello World" $ \txt -> mkDiv doc $ \dv -> addChild txt dv $ \_ -> addChild dv body $ id -- end pasteable code -- The meaning of this: * get reference to the HTML document node first (it is the parent of everything), * extract the <BODY> tag node, create at text element with "Hello World", * create a <DIV> tag node, * insert the text node into div, * insert div into body Or, same in HTML: <html> <body> <div> Hello World </div> </body> </html> but filled in dynamically. Using Haskell Web Toolkit API, the same may be expressed in more compact fashion: -- begin pasteable code -- module HelloWorldHsWTK where import DOM.Level2.HTMLDivElement import Graphics.UI.HsWTK main = docBodyC (mkDiv |<< textP "Hello World") -- end pasteable code -- Here, docBodyC is roughly equivalent of the first two DOM calls, mkDiv is same as above, |<< means "insert into container", and textP is a wrapper around mkText. Earlier, I posted the link to Haddock-generated documentation which also includes the Haskell DOM API. Here it is again: http://www.golubovsky.org:5984/_utils/yhcws/index.html Hopefully this example along with the documentation provided helps shed some light on Haskell Web programming techniques. Feel free to ask questions. Thanks. -- Dimitry Golubovsky Anywhere on the Web

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Re: [Haskell-cafe] Connection helpers: for people interested in network code
by brad clawsie 07 Mar '08

07 Mar '08

"Gwern Branwen" <gwern0(a)gmail.com> writes: >> i offered some time ago to look at building a cabal package and >> documentation for this. i would offer up excuses as to why this hasn't >> appeared yet, but between kids, work and ski season i just haven't >> allocated the time yet. sorry to all. i still hope to look into this, >> but fair to say i dropped the ball. > > Dunno. I just downloaded the git repo, and I'm not sure what's > stopping anyone from uploading it to Hackage. It builds cleanly and > with essentially no warning on 6.8.2, the Haddock docs build, and so > on. The Cabal file itself is quite good; out of boredom, I tweaked it > a little: awesome! that means someone has indeed recognized my lameness and subbed in to do what i didn't. thanks to whoever you are! sorry again for promising and not delivering [gwern, hope you don't mind if i redirect your personally reply to the list for context preservation]

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If Version Control Systems were Airlines
by Justin Bailey 07 Mar '08

07 Mar '08

Way off topic, but this is the cafe. The below is well worth reading. http://changelog.complete.org/posts/698-If-Version-Control-Systems-were-Air… For the click-impaired, here's Darcs Airlines: "Darcs Airlines: Unlike every other airline, this one uses physicists instead of engineers to design its airplanes. One brilliant Darcs physicist has finally come up with The Theory of Everything, and as such, Darcs knows where you want to go before even you do. Darcs airlines prides itself on customer service, and asks your preference for even the tiniest details about your trip. Each seat pocket features a copy of the Theory of Everything for your reading enjoyment, but nobody actually understands it. Occasionally, you will find that Darcs pilots get into angry conflicts with the control tower in mid-flight. This results in the control tower revoking your permission to land. Legend has it that one Darcs pilot of a plane with exceptionally large fuel tanks actually resolved his conflict with the tower and landed two weeks after taking off. Experienced Darcs users board with several parachutes: one for themselves, and a few more for the newbies. The Darcs physicists claim that the Theory of Everything predicted the pilots would act this way, and that all pilots eventually act this way throughout the entire universe. They toil day and night finding a way to adjust the gravitational constant of the universe, thereby reducing the anger factor of the pilots. Main competitor: OS/2 airlines." Justin

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Re: [Haskell-cafe] Connection helpers: for people interested in network code
by brad clawsie 07 Mar '08

07 Mar '08

> I wonder, though, what happened to the curl bindings for Haskell? i offered some time ago to look at building a cabal package and documentation for this. i would offer up excuses as to why this hasn't appeared yet, but between kids, work and ski season i just haven't allocated the time yet. sorry to all. i still hope to look into this, but fair to say i dropped the ball.

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[GSoC] Project proposal: Hoogle 4
by Neil Mitchell 06 Mar '08

06 Mar '08

Hi I would like to be a Summer of Code student, doing the project "Hoogle 4" (http://haskell.org/hoogle/) PROJECT GOALS: There are two main themes: 1) Make Hoogle more useful to the community, along the same path as it is currently used. 2) Make Hoogle suitable to use as the standard interface to Hackage. This requires adding the following features to Hoogle, to form a Hoogle 4 release: * Faster searching (1000x faster at least, perhaps more like 1000000x for text searching) * Generalised text searching (i.e. searching .cabal fields) * Removal of all bugs :-) * Support for higher-kinded type classes (i.e. Monads) * Support for some Haskell type extensions (i.e. Multi-Parameter type classes) * Support for multiple packages * Generalised interface to all of Cabal Progress on Hoogle 4 has already started, but has currently stalled due to lack of time. I am writing up my PhD currently, and will be available to start at the beginning of the Summer to work on Hoogle. Duncan Coutts has agreed to mentor this project. The position of backup mentor is still available. I do not expect to need a great deal of supervision :-) Please respond with any suggestions, requests for further information or just general comments. Thanks Neil

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Google summer of code
by Malcolm Wallace 06 Mar '08

06 Mar '08

Reply-To: haskell-cafe(a)haskell.org Google Summer of Code As many of you will already know, Google is running its "Summer of Code" project again this year, and haskell.org is once again going to apply to be a mentoring organisation. Are you a student who would like to earn money for hacking in Haskell? Or are you a non-student who has a cool idea for a coding project but no time to do it yourself? Well, our wiki to gather ideas is now up-and-running again: http://hackage.haskell.org/trac/summer-of-code Add yourself to the list of interested people! Especially potential mentors. There are some ideas still there from last year, in the trac tickets. However, due to the amount of spam accumulating there, I suggest that this year, we use the haskell-cafe email list as a place to put out project ideas, solicit feedback on them, and look for interested people. Prefix any message subject line with with [GSoC] to help others find them. Google will start accepting student applications on 24th March, but now is the time to start gathering thoughts and matching up interesting ideas with interested people. The official timeline is as follows: March 12: Mentoring organization application deadline March 17: List of accepted mentoring organizations published March 24: Student application period opens March 31: Student application deadline Interim Period: we review and rank student proposals April 14: List of accepted student applications published Interim Period: Students learn more about their project communities May 26: Students begin coding; Google begins issuing initial payments July 14: Google begins issuing mid-term payments August 11: Suggested end of coding August 18: Definite end of coding Sept 1: Final evaluation deadline; Google begins issuing final payments Sept 3: Students upload code to Google (required) Regards, Malcolm

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SimpleArgs
by Ketil Malde 06 Mar '08

06 Mar '08

Hi, Often when I write small scripts, I find I just want a couple of command line arguments but don't want to go the whole GetOpt route. SimpleArgs is an attempt to make the raw getArgs somewhat less raw, and quick and dirty scripts a bit less dirty without sacrificing the quickness. Since I find this quite useful, I thought I'd advertise the code here, ask what people think, and invite suggestions for improvement. It's at: http://malde.org/~ketil/simpleargs -k -- If I haven't seen further, it is by standing in the footprints of giants

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Functional programmer's intuition for adjunctions?
by Edsko de Vries 06 Mar '08

06 Mar '08

Hi, Is there an intuition that can be used to explain adjunctions to functional programmers, even if the match isn't necessary 100% perfect (like natural transformations and polymorphic functions?). Thanks, Edsko

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Haddock Documentation for Yhc Web Service available online
by Dimitry Golubovsky 06 Mar '08

06 Mar '08

Hi, I have regenerated Haddock documentation for Haskell modules included into the Yhc Web Service. http://www.golubovsky.org:5984/_utils/yhcws/index.html Thanks. -- Dimitry Golubovsky Anywhere on the Web

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Re: [Haskell-cafe] Doubting Haskell
by Cale Gibbard 06 Mar '08

06 Mar '08

(I'm copying the list on this, since my reply contains a tutorial which might be of use to other beginners.) On 19/02/2008, Alan Carter <alangcarter(a)gmail.com> wrote: > Hi Cale, > > On Feb 19, 2008 3:48 PM, Cale Gibbard <cgibbard(a)gmail.com> wrote: > > Just checking up, since you haven't replied on the list. Was my > > information useful? Did I miss any questions you might have had? If > > you'd like, I posted some examples of using catch here: > > Thanks for your enquiry! My experiment continues. I did put a progress > report on the list - your examples together with a similar long an > short pair got me over the file opening problem, and taught me some > things about active whitespace :-) I couldn't get withFile working > (says out of scope, maybe 'cos I'm ghc 6.6 on my Mac) Make sure to put: import System.IO at the top of your source file, if you haven't been. This should import everything documented here: http://www.haskell.org/ghc/docs/latest/html/libraries/base/System-IO.html > but it turned out the line I was looking for (collapsed from the examples) > was: > > text <- readFile "data.txt" `catch` \_ -> return "" > > This ensures the program never loses control, crashing or becoming > unpredictable by attempting to use an invalid resource, by yielding an > empty String if for any reason the file read fails. Then an empty > String makes it very quickly through parsing. I guess that's quite > "functiony" :-) > > Amazing how easy once I knew how. Even stranger that I couldn't find a > "bread and butter" example of it. > > Then I was going very quickly for a while. My file is dumped from a > WordPress MySql table. Well formed lines have 4 tab separated fields > (I'm using pipes for tabs here): > > line id | record id | property | value > > Line IDs are unique and don't matter. All lines with the same record > ID give a value to a property in the same record, similar to this: > > 1|1|name|arthur > 2|1|quest|seek holy grail > 3|1|colour|blue > 4|2|name|robin > 5|2|quest|run away > 6|2|colour|yellow > > Organizing that was a joy. It took minutes: let cutUp = tail (filter (\fields -> (length fields) == 4) (map (\x -> split x '\t') (lines text))) This should almost certainly be a function of text: cutUp text = tail (filter (\fields -> (length fields) == 4) (map (\x -> split x '\t') (lines text))) > I found a split on someone's blog (looking for a library tokenizer), > but I can understand it just fine. I even get to chuck out ill-formed > lines and remove the very first (which contains MySql column names) on > the way through! Sadly, there's no general library function for doing this. We have words and lines (and words would work here, if your fields never have spaces), but nobody's bothered to put anything more general for simple splitting into the base libraries (though I'm sure there's plenty on hackage -- MissingH has a Data.String.Utils module which contains split and a bunch of others, for example). However, for anything more complicated, there are also libraries like Parsec, which are generally really effective, so I highly recommend looking at that at some point. > I then made a record to put things in, and wrote some lines to play > with it (these are the real property names): > > data Entry = Entry > { occupation :: String > , iEnjoyMyJob :: Int > , myJobIsWellDefined :: Int > , myCoworkersAreCooperative :: Int > , myWorkplaceIsStressful :: Int > , myJobIsStressful :: Int > , moraleIsGoodWhereIWork :: Int > , iGetFrustratedAtWork :: Int > } > ... > let e = Entry{occupation = "", iEnjoyMyJob = 0} > let f = e {occupation = "alan"} > let g = f {iEnjoyMyJob = 47} > putStrLn ((occupation g) ++ " " ++ (show (iEnjoyMyJob g))) > > Then I ran into another quagmire. I think I have to use Data.Map to > build a collection of records keyed by record id, and fill them in by > working through the list of 4 item lists called cutUp. As with the > file opening problem I can find a few examples that convert a list of > tuples to a Data.Map, one to one. I found a very complex example that > convinced me a map from Int to a record is possible, but gave me no > understanding of how to do it. I spent a while trying to use foldl > before I decided it can't be appropriate (I need to pass more values). > So I tried a couple of recursive functions, something like: > > type Entries = M.Map Int Entry > ... > let entries = loadEntries cutUp > ... > loadEntries :: [[String]] -> Entries > loadEntries [] = M.empty Entries > loadEntries [x : xs] = loadEntry (loadEntries xs) x -- Possible common beginner error here: [x:xs] means the list with one element which is a list whose first element is x and whose tail is xs. Your type signature and the type of cutUp seems to confirm that this is the right type, but you don't seem to have a case to handle a longer list of lists. If you want just a list with first entry x, and with tail xs, that's just (x:xs). If you want to handle lists of lists recursively, you'll generally need two cases: ([]:xss) and ((x:xs):xss). We'll end up doing something different instead of recursion in a moment. > > loadEntry entries _ rid fld val = entries > > Trying to create an empty map at the bottom of the recursion so later > I can try to fiddle about checking if the key is present and crating a > new record otherwise, then updating the record with a changed one (a > big case would be needed deep in to do each property update). If I'm > on the right track it's not good enough to get better, so now I'm just > throwing bits of forest animals into the pot at random again :-( > > So I certainly would be grateful for a clue! The bits I can do (I got > a non-trivial wxHaskell frame sorted out quite easily, the tokenizing > and record bit were OK) I think show I'm not *totally* stupid at this, > I'm putting loads of time investment in (it's an experiement in > itself) but there do seem to be certain specific things that would be > ubiquitous patterns in any production or scripting environment, which > are not discussed at all and far from obvious. The more I see of > Haskell the more I suspect this issue is the gating one for popular > uptake. > > I couldn't help thinking of this bit, from the Wikipedia entry on the > Cocteau Twins: > > "The band's seventh LP, Four-Calendar Café, was released in late 1993. > It was a departure from the heavily-processed, complex and layered > sounds of Blue Bell Knoll and Heaven or Las Vegas, featuring clearer > and more minimalistic arrangements. This, along with the record's > unusually comprehensible lyrics, led to mixed reviews for the album: > Some critics accused the group of selling out and producing an > 'accessible album,' while others praised the new direction as a > felicitous development worthy of comparison with Heaven or Las Vegas." > > Best wishes, > > Alan I woke up rather early, and haven't much to do, so I'll turn this into a tutorial. :) Okay. The most common ways to build a map are by using the fromList, fromListWith, or fromListWithKey functions. You can see them in the documentation here: http://www.haskell.org/ghc/docs/latest/html/libraries/containers/Data-Map.h… The types are: fromList :: (Ord k) => [(k,a)] -> Map k a fromListWith :: (Ord k) => (a -> a -> a) -> [(k,a)] -> Map k a fromListWithKey :: (Ord k) => (k -> a -> a -> a) -> [(k,a)] -> Map k a They take a list of (key,value) pairs, and build a map from it. Additionally, the fromListWith function takes a function which specifies how the values should be combined if their keys collide. There is also a fromListWithKey function which allows the means of combination to depend on the key as well. At this point we realise something interesting about the way the data is being represented: if there is a field in someone's record with no row in the database, what should the resulting field contain? In C, they often use some integer which is out of range, like -1 for this. How about for a missing occupation field? Well, that's a String, you could use some generic failure string, or an empty string, but I'll show you another possibility that just might be convenient. If t is any type, then the type (Maybe t) consists of the values Nothing, and Just x, whenever x is a value of type t. This is another convenient way to represent the idea that a computation might fail. Let's start by changing your record type so that each field is a Maybe value, that is, either the value Nothing, or the value Just x, where x is the value it would have been. data Entry = Entry { occupation :: Maybe String , iEnjoyMyJob :: Maybe Int , myJobIsWellDefined :: Maybe Int , myCoworkersAreCooperative :: Maybe Int , myWorkplaceIsStressful :: Maybe Int , myJobIsStressful :: Maybe Int , moraleIsGoodWhereIWork :: Maybe Int , iGetFrustratedAtWork :: Maybe Int } There's a very general function in the module Control.Monad which I'd like to use just for the Maybe type here. It's called mplus, and for Maybe, it works like this: mplus (Just x) _ = Just x mplus Nothing y = y So if the first parameter isn't Nothing, that's what you get, otherwise, you get the second parameter. Of course, this operation has an identity element which is Nothing. So this lets you combine partial information expressed by Maybe types, in a left-biased way. It's about to become obvious that record types are less convenient than perhaps they could be in Haskell, and this is absolutely true -- I'd actually probably use a somewhat different representation myself (possibly something involving a Map from Strings (field names) to Int values), but I can't really be sure what you intend with this data, and how much type safety you want. I'll elide the field names just because I can here. It's not necessarily good style. combine :: Entry -> Entry -> Entry combine (Entry a1 a2 a3 a4 a5 a6 a7 a8) (Entry b1 b2 b3 b4 b5 b6 b7 b8) = Entry (a1 `mplus` b1) (a2 `mplus` b2) (a3 `mplus` b3) (a4 `mplus` b4) (a5 `mplus` b5) (a6 `mplus` b6) (a7 `mplus` b7) (a8 `mplus` b8) Even with all the shorthand, this is pretty ugly (and I'll show how I'd represent the data in a moment), but what this does is to combine two partial entries, favouring the information in the first, but filling the holes in the first with data from the second. This operation has an identity element, which is: emptyEntry = Entry Nothing Nothing Nothing Nothing Nothing Nothing Nothing Nothing Let's try a different representation, which is a little more flexible, but expresses less in the type system. data Entry = Entry { occupation :: Maybe String, survey :: M.Map String Int } deriving (Eq, Ord, Show) So now, instead of a bunch of separate Maybe Int fields, we have just one Map from String to Int. If we don't have information for a field, we simply won't have that key in the Map. Of course, this means we'll have to use strings for field labels. If that seems unhappy, you could always define a type like: data SurveyQuestion = IEnjoyMyJob | MyJobIsWellDefined | MyCoworkersAreCooperative | MyWorkplaceIsStressful | MyJobIsStressful | MoraleIsGoodWhereIWork | IGetFrustratedAtWork deriving (Eq, Ord, Show) to be used in place of the String type. Let's see how combine will look now: combine :: Entry -> Entry -> Entry combine (Entry o1 s1) (Entry o2 s2) = Entry (o1 `mplus` o2) (s1 `M.union` s2) Or, using the record syntax more: combine :: Entry -> Entry -> Entry combine e1 e2 = Entry { occupation = (occupation e1 `mplus` occupation e2), survey = (survey e1 `M.union` survey e2) } Again, this new version has an identity with respect to combine, which is: emptyEntry = Entry {occupation = Nothing, survey = (M.empty)} Now, we just need a way to take one of your rows, and turn it into a (key,value) pair, where the value is a partial entry. readRow :: [String] -> (Int, Entry) readRow [n, k, "occupation", v] = (read k, emptyEntry { occupation = Just v }) readRow [n, k, f, v] = (read k, emptyEntry { survey = M.singleton f (read v) }) readRow xs = error "failure case, should never happen!" There is actually a failure case that I'm not handling here, which is what happens when the value or key fails to parse as an Int. For that we'd use reads instead of read, but let's ignore it for now. We can then map this function over our cut up rows, something along the lines of: map readRow (cutUp text) at which point we'll have a list of (Int, Entry) pairs. We then want to fill up our Entries Map with those, and we want to combine them as we go using the combine function: entryMap text = M.fromListWith combine (map readRow (cutUp text)) Some final changes we could consider would be putting more of the error handling into readRow itself: if it was to return a singleton Map rather than an (Int, Entry) pair, then it could return the empty Map on failure, and the results would then be combined using the function M.unionsWith combine. We could move the length 4 test out of cutUp then, and just make it the fall-through case in readRow. I'll also use reads, which returns a list of (parse,rest-of-string) pairs, to handle the failure cases where the numbers don't parse, by just treating those rows as nonexistent: readRow :: [String] -> M.Map Int Entry readRow [n, k, f, v] = case reads k of [] -> M.empty -- the key didn't parse (k',_):_ -> if f == "occupation" then M.singleton k' (emptyEntry { occupation = Just v }) else case reads v of [] -> M.empty -- the value didn't parse (v',_):_ -> M.singleton k' (emptyEntry { survey = M.singleton f v' }) readRow xs = M.empty -- this handles the case when the list is any length but 4 cutUp text = tail (map (\x -> split x '\t') (lines text)) -- which allows cutUp to be simpler entryMap text = M.unionsWith combine (map readRow (cutUp text)) Anyway, I hope this tutorial gives some idea of how things progress, and what sort of thinking is generally involved. Note that the focus here was more on finding the right combining operations, and then using pre-existing higher-order functions to collapse the structure, than it was on recursion. Indeed, the final program there doesn't contain any explicit recursion at all! This is generally something to aim for. Using explicit recursion is generally a means of last resort. Higher order functions on data structures are our control structures. - Cale

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Connection helpers: for people interested in network code
by Adam Langley 06 Mar '08

06 Mar '08

I generally find that I'm wrapping sockets in the same functions a lot and now I'm looking writings code which works with both Sockets and SSL connections. So I wrote a module, presumptuously called Network.Connection, although I'm not actually going to try and take that name (even in Hackage) unless I get a general agreement that this is a good thing. So, any comments on the interface, similar things that I should look at etc? http://www.imperialviolet.org/binary/network-connection/Network-Connection.… I made the BaseConnection an ADT, rather than a class because I wanted to avoid hitting the monomorphism restriction in code. That might have been a mistake, I'm not sure yet. If it doesn't excite anyone enough to reply, I'll change the name and put it in Hackage, mostly as is. Then I'll tie HsOpenSSL into it so that SSL connections work transparently. Cheers, AGL -- Adam Langley agl(a)imperialviolet.org http://www.imperialviolet.org

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Re: [Yhc] Yhc Web Service quietly opened for public testing
by Thomas Hartman 06 Mar '08

06 Mar '08

That is seriously cool. Congratulations Dmitry! thomas. 2008/3/4, Dimitry Golubovsky <golubovsky(a)gmail.com>: > Hi, > > I finally got the Yhc Web Service (web-based front end to the > compiler) running in public testing mode. There hasn't been any > documentation written, and Haddock stuff not brought in order, but if > anyone wants to just get a taste of it, please open this hpaste entry: > > http://hpaste.org/6094 > > and follow the instructions. > > In 10 steps, you'll get Haskell to work in your web browser* ;) No > need to install anything. > > Feel free to edit the source code, and even deliberately make errors > to see how error log looks. At the moment, I do not run automatic > cleanup of failed compilations. > > This is one of my old tests, not showing any GUI interaction. However, > both documents browser and new entry form were written in Haskell > which gives some idea about what can be done. Sources of these forms > are in the Yhc repo. > > Thanks. > > ----------- > * Likely, your results will be better with Firefox than with MSIE. But > MSIE should work as well. > > > -- > Dimitry Golubovsky > > Anywhere on the Web > _______________________________________________ > Yhc mailing list > Yhc(a)haskell.org > http://www.haskell.org/mailman/listinfo/yhc >

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2nd CFP: PLMMS 2008
by Jacques Carette 06 Mar '08

06 Mar '08

SECOND CALL FOR PAPERS * UPDATE: Post-workshop proceedings in Journal of Automated Reasoning * UPDATE: Invited talk by Conor McBride Second Workshop on Programming Languages for Mechanized Mathematics (PLMMS 2008) http://events.cs.bham.ac.uk/cicm08/workshops/plmms/ As part of CICM / Calculemus 2008 Birmingham, UK, 28-29 July 2008 This workshop is focused on the intersection of programming languages (PL) and mechanized mathematics systems (MMS). The latter category subsumes present-day computer algebra systems (CAS), interactive proof assistants (PA), and automated theorem provers (ATP), all heading towards fully integrated mechanized mathematical assistants that are expected to emerge eventually (cf. the objective of Calculemus). The two subjects of PL and MMS meet in the following topics, which are of particular interest to this workshop: * Dedicated input languages for MMS: covers all aspects of languages intended for the user to deploy or extend the system, both algorithmic and declarative ones. Typical examples are tactic definition languages such as Ltac in Coq, mathematical proof languages as in Mizar or Isar, or specialized programming languages built into CA systems. Of particular interest are the semantics of those languages, especially when current ones are untyped. * Mathematical modeling languages used for programming: covers the relation of logical descriptions vs. algorithmic content. For instance the logic of ACL2 extends a version of Lisp, that of Coq is close to Haskell, and some portions of HOL are similar to ML and Haskell, while Maple tries to do both simultaneously. Such mathematical languages offer rich specification capabilities, which are rarely available in regular programming languages. How can programming benefit from mathematical concepts, without limiting mathematics to the computational worldview? * Programming languages with mathematical specifications: covers advanced "mathematical" concepts in programming languages that improve the expressive power of functional specifications, type systems, module systems etc. Programming languages with dependent types are of particular interest here, as is intentionality vs extensionality. * Language elements for program verification: covers specific means built into a language to facilitate correctness proofs using MMS. For example, logical annotations within programs may be turned into verification conditions to be solved in a proof assistant eventually. How need MMS and PL to be improved to make this work conveniently and in a mathematically appealing way? These issues have a very colorful history. Many PL innovations first appeared in either CA or proof systems first, before migrating into more mainstream programming languages. Some examples include type inference, dependent types, generics, term-rewriting, first-class types, first-class expressions, first-class modules, code extraction etc. However, such innovations were never aggressively pursued by builders of MMS, but often reconstructed by programming language researchers. This workshop is an opportunity to present the latest innovations in MMS design that may be relevant to future programming languages, or conversely novel PL principles that improve upon implementation and deployment of MMS. We also want to critically examine what has worked, and what has not. Why are all the languages of mainstream CA systems untyped? Why are the (strongly typed) proof assistants so much harder to use than a typical CAS? What forms of polymorphism exist in mathematics? What forms of dependent types may be used in mathematical modeling? How can MMS regain the upper hand on issues of "genericity" and "modularity"? What are the biggest barriers to using a more mainstream language as a host language for a CAS or PA/ATP? Invited Talk ------------ Conor McBride (Alta Systems, Northern Ireland) will give an invited talk. Submission ---------- Submission works through EasyChair http://www.easychair.org/conferences/?conf=plmms2008 Two kinds of papers will be considered: * Full research papers may be up to 12 pages long. Authors of accepted papers are expected to present their work on the workshop in a regular talk. * Position papers may be up to 4 pages long. The workshop presentation of accepted position papers consists of two parts: a stimulating statement of certain issues or challenges by the author, followed by a discussion in the plenum. Papers should use the usual ENTCS style http://www.entcs.org/prelim.html (11 point version), and will be reviewed by the program committee. Informal workshop proceedings will be circulated as a technical report. Moreover there will be post-workshop proceedings of improved research papers, or position papers that have been completed into full papers, to appear in a special issue of the Journal of Automated Reasoning. There will be a separate submission and review phase for this, where papers from both PLMMS 2007 and 2008 will be considered. Programme Committee ------------------- Jacques Carette (Co-Chair) (McMaster University, Canada) John Harrison (Intel Corporation, USA) Hugo Herbelin (INRIA, Ecole polytechnique, France) James McKinna (Radboud University Nijmegen, Netherlands) Ulf Norell (Chalmers University, Sweden) Bill Page Christophe Raffalli (Universite de Savoie, France) Josef Urban (Charles University, Czech Republic) Stephen Watt (ORCCA, University of Western Ontario, Canada) Makarius Wenzel (Co-Chair) (Technische Universitaet Muenchen, Germany) Freek Wiedijk (Radboud University Nijmegen, Netherlands) Important Dates --------------- * Submission deadline - 5 May 2008 * Notification of acceptance - 6 June 2008 * Final version - 7 July 2008 (approximately) * Workshop - 28-29 July 2008

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Job opening: Use Haskell to save the environment!
by Ketil Malde 05 Mar '08

05 Mar '08

Well - maybe¹. I'm not sure how relevant this is for this list, but I thought I should mention that there is an open position for a 3-year Ph.D. scolarship at IMR where I work. The work is in bioinformatics, and as I'm a happy Haskell user, I'd be happy to see qualified applicants who know Haskell as well. If you are interested, and posess a Master's degree (or equivalent education), a rough translation of the announcement is attached - the official one (in Norwegian) can be found at: http://www.imr.no/aktuelt/ledige_stillinger/stipendiat_bioinformatikk, Please contact me with any questions! -k ¹) The target of the project is the sea louse, which is a large ecological and economical problem. -- If I haven't seen further, it is by standing in the footprints of giants

1 0

Orphan instances
by Henning Thielemann 05 Mar '08

05 Mar '08

Consider the following modules module A class A t module T import A data T a module B import A import T class A t => B t instance B t => A (T t) GHC emits warning about orphan instance A (T t), since neither A nor T are defined in B. However I can't move the instance to A or T since it depends on B. Would it be fine to lift the restriction, such that it is accepted to declare the instance in a module where the type or the class or a superclass (this is new) is defined? In my example class B provides conversion toInteger (that is Integral), class A provides conversion toRational (that is Real) and T is Ratio.

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Re: Exporting Haskell Libraries to C Programmers
by Bruce, Joseph R (Joe) 05 Mar '08

05 Mar '08

Don Stewart <dons <at> galois.com> writes: > > joseph.bruce: > > Hi, > > > > I have a Haskell library that I want to make available via FFI to C > > programmers on my project team. I read this thread > > (http://thread.gmane.org/gmane.comp.lang.haskell.cafe/21447) which had > > some interesting ideas, but they seemed unresolved. Or maybe it answers > > my question but I don't understand it. > > > > Is there a way I can package (ghc-)compiled Haskell code into a > > statically-linked library and not force the C programmers to include > > headers and libraries that they have no knowledge of and undefine a > > seemingly endless list of preprocessor symbols (run ghc with the verbose > > flag and look at the calls to gcc)? Can this process be automated? > > Yes, check the FFI documentation for the main story. > > In short, build the Haskell code with cabal, with your foreign export > Haskell functions in the cbits. That bundle can then be linked against > C code. > > You do need to link your app against libHSrts.a and libHSbase.a (and > other libs you use), but assuming you foreign export, the code > to call will look just like normal C stuff. > Thanks Don. I finally got back to this. I hadn't looked at CABAL before. It's a very useful tool and met all my library-forming needs. That's only half my problem though. I'm trying to make the use of this Haskell code as transparent as possible to the C programmers on the team. Telling them about the Haskell base libraries dependency or distributing those libraries with my code is not too bad, but the list of -u flags that the linker needs is brutal. Is there a way to avoid it, or embed it in the package? Ex (using ghc-6.8.2 -v): ... gcc -v -o primes Primes.o ./Primes_stub.o hs_primes_wrapper.o cprimes.c -Bc:\ghc\ghc-6.8.2\gcc-lib/ -DDONT_WANT_WIN32_DLL_SUPPORT -Lc:/ghc/ghc-6.8.2/lib\base-3.0.1.0 -Lc:/ghc/ghc-6.8.2 -Lc:/ghc/ghc-6.8.2/gcc-lib -lHSbase-3.0.1.0 -lwsock32 -lmsvcrt -lkernel32 -luser32 -lshell32 -lHSrts -lm -lgmp -lwsock32 -u _base_GHCziBase_Izh_static_info -u _base_GHCziBase_Czh_static_info -u _base_GHCziFloat_Fzh_static_info -u _base_GHCziFloat_Dzh_static_info -u _base_GHCziPtr_Ptr_static_info -u _base_GHCziWord_Wzh_static_info -u _base_GHCziInt_I8zh_static_info -u _base_GHCziInt_I16zh_static_info -u _base_GHCziInt_I32zh_static_info -u _base_GHCziInt_I64zh_static_info -u _base_GHCziWord_W8zh_static_info -u _base_GHCziWord_W16zh_static_info -u _base_GHCziWord_W32zh_static_info -u _base_GHCziWord_W64zh_static_info -u _base_GHCziStable_StablePtr_static_info -u _base_GHCziBase_Izh_con_info -u _base_GHCziBase_Czh_con_info -u _base_GHCziFloat_Fzh_con_info -u _base_GHCziFloat_Dzh_con_info -u _base_GHCziPtr_Ptr_con_info -u _base_GHCziPtr_FunPtr_con_info -u _base_GHCziStable_StablePtr_con_info -u _base_GHCziBase_False_closure -u _base_GHCziBase_True_closure -u _base_GHCziPack_unpackCString_closure -u _base_GHCziIOBase_stackOverflow_closure -u _base_GHCziIOBase_heapOverflow_closure -u _base_GHCziIOBase_NonTermination_closure -u _base_GHCziIOBase_BlockedOnDeadMVar_closure -u _base_GHCziIOBase_BlockedIndefinitely_closure -u _base_GHCziIOBase_Deadlock_closure -u _base_GHCziIOBase_NestedAtomically_closure -u _base_GHCziWeak_runFinalizzerBatch_closure -u _base_GHCziConc_ensureIOManagerIsRunning_closure ... This particular example is a build on a windows system and makes use of the .o's directly and not the .a I create, but those are of no consequence as this is ultimately a linking issue, not a compilation issue. I don't know what these -u's are for, but I know that if I take them out, the linker fails. They do change from one system to another (this list fails with the linker on RHEL 5). But right now my solution for the C programmers is a make file with $haskell_undefine_flags = <that mess up there>, and I'd prefer something more elegant and less system dependent. I am willing to accept that there is no good way around it, but perhaps someone has dealt with this issue before. Any help? Thanks. Joe

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ANN: Takusen 0.8
by Alistair Bayley 05 Mar '08

05 Mar '08

Oleg and I are pleased to announce the release of Takusen 0.8. (Don Stewart did an interim 0.7 release for us a few weeks ago, and added us to Hackage. This release is a tidy-up of some loose ends, and some bug fixes. Hence, I've summarise the changes since the 0.6 release.) Changes since 0.6: - ODBC support. This still has a few gaps (and probably bugs and rough edges) but should be fairly usable. - support for reusable/persistent sessions, so you can hang onto the connection object between invocations of withSession (this was in release 0.6 but omitted from the release notes). - improvements to the Cabal Setup scripts, which should give better experiences for ghc-6.4, ghc-6.6, and ghc-6.8. The (eventual) 1.4 release of Cabal should be able to build our haddock docs, too. - improved UTF8 decoder (marshals directly from buffer). The release bundle: http://hackage.haskell.org/packages/archive/Takusen/0.8/Takusen-0.8.tar.gz The latest code: darcs get http://darcs.haskell.org/takusen Docs: http://darcs.haskell.org/takusen/doc/html/index.html A comprehensive description of API usage can be found in the documentation for module Database.Enumerator (look for the Usage section): http://darcs.haskell.org/takusen/doc/html/Database-Enumerator.html Future plans: - FreeTDS backend (Sybase and MS Sql Server) - support for Blobs and Clobs For those of you unfamiliar with Takusen, here is our HCAR blurb: Takusen is a library for accessing DBMS's. Like HSQL, we support arbitrary SQL statements (currently strings, extensible to anything that can be converted to a string). Takusen's `unique-selling-point' is safety and efficiency. We statically ensure all acquired database resources - such as cursors, connection and statement handles - are released, exactly once, at predictable times. Takusen can avoid loading the whole result set in memory, and so can handle queries returning millions of rows in constant space. Takusen also supports automatic marshalling and unmarshalling of results and query parameters. These benefits come from the design of query result processing around a left-fold enumerator. Currently we fully support ODBC, Oracle, Sqlite, and PostgreSQL.

1 1

problem in using wash
by Vikrant 05 Mar '08

05 Mar '08

Hi, I was trying to use wash to learn it. I am using ubuntu and I have ghc6.6.1 installed on my system. I have also installed the package libghc6-wash-dev but in my code when i write "import WASH.CGI" it gives me following error firstCGI.hs:5:7: Could not find module `WASH.CGI': locations searched: WASH/CGI.hs WASH/CGI.lhs Failed, modules loaded: none. can somebody help me in this? Regards, Vikrant

2 2

Haskell & Perl architecture question
by Marty Landman 05 Mar '08

05 Mar '08

I'm looking at a production system running Perl & Haskell in an Apache environment and trying to get a handle on the system architecture. Are there online resources anyone could recommend I start with? Thanks in advance, Marty -- Marty Landman, Face 2 Interface Inc. 845-679-9387 Drupal Development Blog: http://drupal.face2interface.com/ Free Database Search App: http://face2interface.com/Products/FormATable.shtml

2 1

Markup bug in HaskellWiki
by Henning Thielemann 05 Mar '08

05 Mar '08

In the Wiki article http://haskell.org/haskellwiki/Comparison_chain there is the piece unzip">unzip which is improperly formatted.

3 4

Annotating GHC assembler output?
by Justin Bailey 05 Mar '08

05 Mar '08

I'm interested in seeing what kind of assembler my functions turn into. Is there a means of annotating assembler output, similar to the {#- CORE -#} pragma? Is there a trickier way of doing it? Justin

2 3

ID3 tagging
by adekoba 04 Mar '08

04 Mar '08

I was wondering if there are any haskell packages for tagging ID3 frames in mp3 files. Or perhaps if there were any plans for developing them...

2 1

Re: static constants -- ideas?
by Don Stewart 04 Mar '08

04 Mar '08

jason.dusek: > I have an awkward programming problem -- I need to take a > dictionary, parse it, build a bunch of intermediate lists and > then make maps and tries out of the list. A "programming > problem" because it's taken me a fair amount of effort to pull > together the parser and list generator -- and "awkward" > because a 69000 item list, [(String, [(String, String)])], > does not compile under GHC (stack overflow). (It's not likely > to compile under anything else, either!) Here's an example of the approach Bryan outlined, which does seem to work for files as large as gcc can handle: * generate your big Haskell Map * serialise it with Data.Binary, and Codec.Compression.GZip to a file * compile the data into a C const array, and link that into Haskell * decode it on startup, ressurecting the Haskell data. The C source looks like: const uint8_t beowulf[] = { 31, 139, 8, 0, 0, 0, 0, 0, 0, 3, 124, 189, 75, 150, 46, 54, 93, 193, 96, 144, 241, 168, 172, 238, 214, 0, ... http://code.haskell.org/~dons/code/compiled-constants/cbits/constants.c which is the gzip, Data.Binary encoded version of a Map ByteString Int. Then the Haskell code need only access this array as a Ptr Word8, wrap that as a Bytestring, then run Data.Binary over the result to rebuild the Map. As you can see here: http://code.haskell.org/~dons/code/compiled-constants/Constants.hs I've put a couple of examples of how to access C-side serialised Haskell values in a package here: http://code.haskell.org/~dons/code/compiled-constants/ Cheers, Don

1 0

Failing to make `par`
by Jefferson Heard 04 Mar '08

04 Mar '08

I've used Control.Parallel and Control.Parallel.Strategies extensively in the past, and I thought I knew what I was doing. I declared the following function: findSupernodes' :: S.Set Name -> Int -> Tree -> Protein.Tree -> S.Set Name findSupernodes' set size (Node i _ _ s il ir) (Protein.Node _ pl pr _ g) | S.size g > size = (Name (fromIntegral i)) `S.insert` set | otherwise = leftNodes `S.union` rightNodes where leftNodes = (findSupernodes' set size il pl) rightNodes = (findSupernodes' set size il pl) findSupernodes' set size (Leaf _ _) (Protein.Gene _ _ _) = set and then simply wrote the following as an initial stab at parallelizing it, because actually calling this function causes the program to execute an absurd number of string comparisons: findSupernodes = findSupernodes' S.empty findSupernodes' :: S.Set Name -> Int -> Tree -> Protein.Tree -> S.Set Name findSupernodes' set size (Node i _ _ s il ir) (Protein.Node _ pl pr _ g) | S.size g > size = (Name (fromIntegral i)) `S.insert` set | otherwise = leftNodes `par` rightNodes `seq` leftNodes `S.union` rightNodes where leftNodes = (findSupernodes' set size il pl) rightNodes = (findSupernodes' set size il pl) findSupernodes' set size (Leaf _ _) (Protein.Gene _ _ _) = set The thing is, these two functions don't return the same thing. The parallel version is returning an empty set, while the sequential version returns what it should return. Any clue what I did wrong? -- Jeff

1 0

ICFP08 Final CFP
by Matthew Fluet (ICFP Publicity Chair) 03 Mar '08

03 Mar '08

Final Call for Papers ICFP 2008: International Conference on Functional Programming Victoria, BC, Canada, 22-24 September 2008 http://www.icfpconference.org/icfp2008 Submission deadline: 2 April 2008 ICFP 2008 seeks original papers on the art and science of functional programming. Submissions are invited on all topics from principles to practice, from foundations to features, from abstraction to application. The scope includes all languages that encourage functional programming, including both purely applicative and imperative languages, as well as languages with objects and concurrency. Particular topics of interest include * Applications and Domain-Specific Languages * Foundations * Design * Implementation * Transformation and Analysis * Software-development Techniques * Functional Pearls * Practice and Experience Important Dates (at 09:00 Apia time, UTC-11) ~~~~~~~~~~~~~~~ Submission: 2 April 2008 Author response: 21 May 2008 Notification: 16 June 2008 Final papers due: 7 July 2008 Call for Papers (full text) ~~~~~~~~~~~~~~~ http://www.icfpconference.org/icfp2008/cfp/cfp.html The submission URL will be available from the above page shortly. Program Chair ~~~~~~~~~~~~~ Peter Thiemann Albert-Ludwigs-Universität Georges-Köhler-Allee 079 79110 Freiburg, Germany Email: icfp08(a)informatik.uni-freiburg.de Phone: +49 761 203 8051 Fax: +49 761 203 8052 Mail sent to the address above is filtered for spam. If you send mail and do not receive a prompt response, particularly if the deadline is looming, feel free to telephone and reverse the charges.

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STAMP benchmark in Haskell?
by Tom Davies 03 Mar '08

03 Mar '08

I'm experimenting with STM (in CAL[1] rather than Haskell) and want to run the STAMP[2] benchmarks. Is there a Haskell translation available, or can anyone suggest a better/different benchmark suite for STM? Thanks, Tom [1] http://openquark.org/Open_Quark/Welcome.html [2] http://stamp.stanford.edu/

3 2

coerce (safe!)
by Krzysztof Skrzętnicki 03 Mar '08

03 Mar '08

Well, it is simply > coerce :: a -> b > coerce _ = undefined so coerce is simply empty function. But still, it is possible to write a function of type (a->b). Well, possibly I didn't write anything particularly new, but please excuse me for I'm still in sort of a shock after I've discovered it. Yet this function reminds me how little I know so I have a question for you. I didn't took any lectures in Category Theory (for I'm still just a student, and I won't for there are none in my institute), but are there any good (e)books you would recommend for a (future) computer scientist? Regards Christopher Skrzętnicki

6 5

Parody of Darcs patch theory from 1981
by Paul Johnson 03 Mar '08

03 Mar '08

I was looking through my old copy of "The Devil's DP Dictionary" by Stan Kelly-Bootle, and came across the entry for Stepwise Refinement. I thought "I've seen this before: this is a parody of Darcs patch theory". It included the Null patch, chains of patches, inverse patches, and pseudo-inverse patches. But the book was published in 1981. I've got the pages scanned, but I can't upload them to Wikimedia because they are still in copyright. However I'm quite sure that limited distribution would fall into "fair use" (non-commercial, small part of work, no impact on market, academic relevance). The total file size is about 520kBytes. Assuming people would like to see them, does anyone have any ideas beyond "email to requestors"? Paul.

3 3

A mini haskell (in 244 lines of python)
by Tim Newsham 03 Mar '08

03 Mar '08

For education and fun I wrote a small untyped non-strict lambda calculus evaluator in python. One of the goals was to keep it fairly small and simple, and to do most of the work in the implemented scripting language itself. For that reason, I added macros to allow things like "let" and "letrec" to be defined as syntactic sugar easily outside of the evaluation engine itself. The result is 244 lines of python, a small prelude that implements tuples, lists and the state monad, and some small example scripts. The prelude and scripts are heavily influenced by Haskell, the main differences being a much simpler syntax, lack of pattern matching and lack of a type system. Data structures aren't supported but are implemented fairly easily using lambda expressions as described in http://www.cs.nott.ac.uk/~nhn/TFP2006/Papers/03-JansenKoopmanPlasmeijer-Eff… This is all available at http://www.thenewsh.com/%7Enewsham/lambda/ as extracted package and .tgz. The README file has documentation including small examples. Accompanying scripts provide more complete examples, such as: [WITHLIST [LET from2 (iter (add 1) 2) [LET primes (nubBy (\x \y (eq (mod y x) 0)) from2) (traceList Val (take 10 primes)) ]]] which calculates the first 10 primes. Exercise to the reader: I bet the evaluator could all be written much more compactly in Haskell. Tim Newsham http://www.thenewsh.com/~newsham/

1 0

Generating a random list
by Milos Hasan 03 Mar '08

03 Mar '08

Hi, so let's say I want to generate a list of N random floats. The elegant way of doing it would be to create an infinite lazy list of floats and take the first N, but for N = 1,000,000 or more, this overflows the stack. The reason is apparently that the take function is not tail-recursive, and so it uses O(N) stack space.. What is the right way to do this? Sure, I could write my own tail-recursive generator function. But this seems to be an instance of a more general problem - how to avoid algorithms linear in stack space when dealing with large lists. Thanks a lot! Milos

7 12

"Wrong kind" when attempting to build a monad for a circular list of functions
by Aaron Altman 02 Mar '08

02 Mar '08

Hi everybody. I'm working towards a better understanding of Haskell monads the only way I know how: by working through an example. I am working on an AI agent that will perform a finite series of actions before starting the sequence over again. I figured a circular list of functions that shifts as you apply them would be the way to do it. Here is my test code: ----------------------------- import Control.Monad runActionAndIterate :: [a -> a] -> a -> (a, [a -> a]) runActionAndIterate (currentAction:actionList) actionInput = (currentAction actionInput, concat [actionList, [currentAction]]) shiftActionList :: [a -> a] -> [a -> a] shiftActionList (currentHead:rest) = concat [rest, [currentHead]] newtype CircularFuncList funcList arg = CircularFuncList (funcList -> arg -> (arg, funcList)) instance Monad (CircularFuncList funcList arg) where return a = CircularFuncList (\funcList a -> (a, funcList)) CircularFuncList currentFuncList currentArg >>= argTransform = let result = argTransform $ (head currentFuncList) currentArg newFuncList = map argTransform $ shiftActionList currentFuncList in CircularFuncList newFuncList result ----------------------------- I get an error that CircularFuncList funcList arg has kind * while Monad is looking for * -> *. This is a first attempt so it may be I'm a ways off from a monad that implements a circular list of functions. That is the goal though. What advice can you offer? Thanks, Aaron Altman

5 8

.cabal problem
by Galchin Vasili 02 Mar '08

02 Mar '08

Hello, I am trying to install a package on my Linux Ubuntu machine. It chokes build-depends: base, directory because directory dependency is unresolvable. Do I have to specify "extra-libs" so that correct library space is searched? Actually I tried this and it didn't help. ?? Regards, Vasya

2 2

Re: Generating a random list
by Milos Hasan 02 Mar '08

02 Mar '08

OK, you convinced me that sort is not the problem. After all, "last (f 1000000)" overflows too, and last is a very innocent function. I don't know how you found the size (or structure) of the thunks (I'm not aware of a ghci functionality that can tell me that), could you let me know? Anyway, the problem seems to be that the values pulled from the list are not final float values, but instead they're thunks that grow larger and larger as more floats are pulled from the list. That's why last and sort don't work, but print and foldl' do work. Also, an odd number of reverses helps since sort evaluates the elements from the end of the list first. So I forced those values, and it fixed the problem: -- this function returns n random floats (no thunks) f :: Int -> [Float] f n = take n $ randFloats $ mkStdGen 0 where randFloats g = x `seq` x : randFloats g' where (x, g') = random g main = print $ foldl1' (+) $ sort $ f 1000000 This works without overflow, and so does "last (f 1000000)". Would it make sense for System.Random to do this forcing by default, too? Thanks, Milos

1 0

runInteractiveCommand dying uncatchably?
by Evan Martin 02 Mar '08

02 Mar '08

If I run the following program, it never prints "done". If I uncomment the commented line, it does. import Prelude hiding (catch) import Control.Exception import System.Process import System.IO demo = do putStrLn "starting" (inp,out,err,pid) <- runInteractiveCommand "nonesuchcommand" putStrLn "writing to in on bad command" hPutStr inp "whatever" -- putStr "flushing" hFlush inp `catch` \e -> do print e; return () putStrLn "done" main = demo `catch` \e -> do print e; return () On my machine the output is: $ runhaskell test.hs starting writing to in on bad command $ It appears to exit at the hFlush call. (hClose has the same behavior.) I find this surprising -- I'd expect, even if I'm using an API incorrectly, to get an exception.

2 1

Announce: category-extras 0.1
by Dan Doel 01 Mar '08

01 Mar '08

Hello everyone, While fooling about with generalized tries a night or two ago, I found myself once again interacting with fixed points of shape functors and the like, and so decided it was time to finish cabalizing David Menendez' excellent category theory inspired modules.[1] So, it is my pleasure to announce version 0.1 of category-extras. This is an initial import of the code, with only what work it took to get things compiling properly. As such, some of the documentation is a bit scanty, and there is some overlap with other packages (notably, TypeCompose, off the top of my head), but I hope to rectify that later. Notable bits include: * Control.Comonad * Control.Comonad.Context -- the state-in-context monad * Control.Functor.Adjunction -- a class for adjoint functors, and their associated (co)monads * Control.Recursion -- Various generalized recursion operators (cata/zygo/histo/apomorphisms) -- A class for associating fixpoint data types to their shape functors Also included are some Data.* modules with some comonadic data types (infinite trees and streams, for instance), but they are in the as-yet-poorly-documented section of the package. Haddock 2.0 is required to build what documentation there is for Control.Functor.Transform, as it uses type operators. However, the rest should be compatible with earlier versions of haddock. Some modules are portable Haskell98, but many use various extensions (rank-2 types and functional dependencies being the most common). Links: * hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/category-extras-… * darcs: http://code.haskell.org/~dolio/category-extras/ If you identify any issues with the package, or simply have some wild category theoretic abstractions that you think should be made available to Haskell programmers at large, feel free let me know. -- Dan Doel [1] The original home of these modules is here: http://www.eyrie.org/~zednenem/2004/hsce/

1 0

Re: static constants -- ideas?
by Don Stewart 01 Mar '08

01 Mar '08

jay: > Don Stewart dons(a)galois.com: > > >jay: > >> Don Stewart dons(a)galois.com: > >> >jay: > >> >> Don Stewart dons(a)galois.com: > >> >> >jay: > >> >> >> I also have constants that are too large to compile. I am resigned to > >> >> >> loading them from data files--other solutions seem even worse. > >> >> ... > >> >> >> Data.Binary eases the irritation somewhat. > >> >> > > >> >> >Did you try bytestring literals (and maybe parsing them in-memory with > >> >> >Data.Binary)? > >> > >> I finally squeezed enough time to try it, and it didn't work for me. > > > >> > >> -- > >> ghc Overflow.hs > >> [1 of 1] Compiling Overflow ( Overflow.hs, Overflow.o ) > > > >Enable optimisations! Compile with ghc -O2. You need this to avoid > >having a very slow pack call at runtime. > > Yes, I tried basic variations like that. The result is the same with -O1 > or with -O2, and with Data.ByteString or Data.ByteString.Lazy . Ok, hmm, that really shouldn't be the case. Do you have the example available somewhere? It's just a 40M inline bytestring? -- Don

1 0

Re: static constants -- ideas?
by Don Stewart 01 Mar '08

01 Mar '08

jay: > Don Stewart dons(a)galois.com: > >jay: > >> Don Stewart dons(a)galois.com: > >> >jay: > >> >> I also have constants that are too large to compile. I am resigned to > >> >> loading them from data files--other solutions seem even worse. > >> ... > >> >> Data.Binary eases the irritation somewhat. > >> > > >> >Did you try bytestring literals (and maybe parsing them in-memory with > >> >Data.Binary)? > > I finally squeezed enough time to try it, and it didn't work for me. > > -- > ghc Overflow.hs > [1 of 1] Compiling Overflow ( Overflow.hs, Overflow.o ) Enable optimisations! Compile with ghc -O2. You need this to avoid having a very slow pack call at runtime. > Overflow.hs:8:10:stack overflow: use +RTS -K<size> to increase it > -- > > where Overflow.hs is in the vicinity of 40M and looks like > > -- > {-# LANGUAGE OverloadedStrings #-} > > module Overflow where > > import qualified Data.ByteString.Lazy as S > > bigData :: S.ByteString > bigData = "\0\0\0\0\0\5\67\195\0\0\0\0... > -- > > I didn't compress it, because Codec.Compression.GZip didn't compile for > me. It looked like a library change since 6.6 broke it. Probably you don't have the zlib.h header? Or make sure you have the latest version of zlib from hackage -- it does work. > Is there a handy string escaping function in the libraries somewhere? It > only took a minute to write one, and I spent longer than that looking, > so maybe it's the wrong question.... Surely it's in there somewhere, and > I'm just 2 dum 2 c. The show function?

1 0

to stimulate discussion concerning where Haskell is going .. ...
by Galchin Vasili 01 Mar '08

01 Mar '08

1) Berkeley consternation about the inability of current concurrency to keep up with multicor developments 1) http://www.eetimes.com/news/latest/showArticle.jhtml;jsessionid=CWDB5YTIKPC… http://www.eetimes.com/news/latest/showArticle.jhtml;?articleID=206801229 IMO a gauntlet has been thrown down to the Haskell world here. 2) Sun Microsystem's planned support of hardware transactional memory http://www.theregister.co.uk/2007/08/21/sun_transactional_memory_rock/ I have fished around and collected some Sun papers and slides. If any anybody wants I can post the URLs or send to the interested Regards, Vasys

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ANN: Hoogle 3.1
by Dan Weston 01 Mar '08

01 Mar '08

I have a question about the ranking algorithm of Hoogle 3.1: The top match of hoogle "(a -> b) -> b" is the inexact match Control.Monad.State.Class.gets :: MonadState s m => (s -> a) -> m a (which cannot be made to unify with (a -> b) -> b) instead of Control.Monad.Cont.runCont (undefined :: Cont r a) :: (a -> r) -> r which does. I would have thought all exact matches would come first (after considering all partial applications), especially as the increase in complexity is at most a small constant factor (~ 2 or 3) for considering all possible partial applications (type tails) of known library functions. Dan Neil Mitchell wrote: > Hi, > > I am pleased to announce Hoogle 3.1. Hoogle is a search engine for > Haskell functions, that allows the user to search by name or > approximate type signature. Hoogle can be used without installing > anything, by simply visiting the online interface, or can be installed > locally. > > * Online interface: http://www.haskell.org/hoogle/ > * Cabal release: > http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hoogle-3.1 > > The Cabal release contains a command line version of the Hoogle tool, > which you can install with the standard Cabal commands. > > Hoogle 3.1 has a number of new features. > > Command line version: > > * A proper cabal release, including proper installing of the search database. > * cabal-install compatible > * No longer requires haskell-source-extensions (trhsx) to be installed > > Web interface: > > * Better entity escaping in some bits of the web interface > * Keyword links work again > * Lambdabot suggestions work > * Update to the most recent version of base > * Security fixes, specifically possible code injection bugs > (http://en.wikipedia.org/wiki/Code_injection) - none of which were > ever exploited, or could have been exploited (as far as I can tell) > > I intend to write a more detailed account of the security fixes in a > blog post, which will be available from http://planet.haskell.org/ . > Running a custom version of the web interface is not really supported, > but if you are, please upgrade to the latest darcs version. > > There are still several known bugs in the Hoogle 3.1 release, but > these are more difficult to fix, and will probably be solved in a > forthcoming Hoogle 4.0. > > Thanks > > Neil > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell > >

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cabal errors
by Kristofer Buffington 01 Mar '08

01 Mar '08

Hi I installed ghc 6.8 from source and I've been installing packages from hackage. I'm not sure when the problem started, but I've been getting this error trying to install any cabal package.. accept apparently, Cabal itself. Setup.hs:2:0: Warning: Deprecated use of `defaultUserHooks' (imported from Distribution.Simple): Use simpleUserHooks or autoconfUserHooks Setup: /usr/local/lib/Cabal-1.3.6/ghc-6.8.2.20080225/HSCabal-1.3.6.o: unknown symbol `directoryzm1zi0zi0zi0_SystemziDirectory_a9_closure' Setup: exception :: GhcException Any ideas? -- -kb

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Trouble with numbers
by Lloyd Smith 29 Feb '08

29 Feb '08

I have a question about how numeric classes and type checking works. I have two functions that I think should behave the same but don't. -- I want to split n things up using a given list of fractions -- for example >allocate' 100 [1/3,1/3,1/3] -- [33,33,33] allocate' n fs = vs where vs = map (floor . (*n)) fs -- I want to find anything left over eventually I will want to -- return what is unallocated as well but for now allocated -- and unallocated are not used! allocate n fs = vs where vs = map (floor . (*n)) fs allocated = sum vs unallocated = n - allocated When I load these function in the top level everything looks good [1 of 1] Compiling Main ( allocate.hs, interpreted ) Ok, modules loaded: Main.main *Main> allocate' 100 [1/3,1/3,1/3] [33,33,33] *Main> allocate 100 [1/3,1/3,1/3] <interactive>:1:0: Ambiguous type variable `t' in the constraints: `Integral t' arising from a use of `allocate' at <interactive>:1:0-25 `RealFrac t' arising from a use of `allocate' at <interactive>:1:0-25 Probable fix: add a type signature that fixes these type variable(s) *Main> I mixed up my types when finding the allocated and unallocated, but I am not sure why it produces an error when unallocated and allocated are never used? Shouldn't the two functions be compiled down to the same thing? Suggestions on how to do this more elegantly as well as pointers for understanding numeric type classes would be appreciated. TIA Lloyd

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Error I haven't seen before
by Jefferson Heard 29 Feb '08

29 Feb '08

I'm getting this error when I try to render the text contained in a record structure in the Protein module. Does this mean that the thunk that could calculate go_terms is being evaluated for the first time in the program? go_terms is calculated by a Data.Map.! lookup operation. Main: No match in record selector Protein.go_terms

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Wadler's blog post on the expression problem
by Ryan Ingram 29 Feb '08

29 Feb '08

I was reading Philip Wadler's post regarding the expression problem and Wouter Swierstra's paper, "Data Types a la Carte". Philip asks: The instances of the injection function for subtypes are assymmetric. Is there a symmetric solution in Haskell? Here is a solution which works under GHC 6.8.2, heavily inspired by the HList type computation work, available from http://www.okmij.org/ftp/Haskell/types.html#HList There are three keys to this trick, each of which builds on the previous. 1) TypEq: for any two types a, b, there is an instance for TypEq a b HTrue if and only if a == b, and there is an instance for TypEq a b HFalse otherwise. 2) This allows the definition of IsTreeMember which gives HTrue if a type occurs somewhere within a tree and otherwise HFalse. 3) We can then use IsTreeMember as a predicate to determine whether to use the left branch or right branch of a (:+:) constructor. Source code follows: {-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances -fallow-overlapping-instances #-} module Expr where newtype Expr f = In (f (Expr f)) instance Show (f (Expr f)) => Show (Expr f) where showsPrec _ (In x) = showParen True (showString "In " . showsPrec 11 x) out :: Expr f -> f (Expr f) out (In x) = x data (f :+: g) e = Inl (f e) | Inr (g e) deriving Show instance (Functor f, Functor g) => Functor (f :+: g) where fmap h (Inl f) = Inl (fmap h f) fmap h (Inr g) = Inr (fmap h g) class (Functor sub, Functor sup) => (:<:) sub sup where inj :: sub a -> sup a instance TypTree sub sup => (:<:) sub sup where inj = treeInj data Val e = Val Int deriving Show data Add e = Add e e deriving Show data Mul e = Mul e e deriving Show instance Functor Val where fmap f (Val x) = Val x instance Functor Add where fmap f (Add l r) = Add (f l) (f r) instance Functor Mul where fmap f (Mul l r) = Mul (f l) (f r) foldExpr :: Functor f => (f a -> a) -> Expr f -> a foldExpr f (In t) = f (fmap (foldExpr f) t) class Functor f => Eval f where evalA :: f Int -> Int instance Eval Val where evalA (Val x) = x instance Eval Add where evalA (Add x y) = x + y instance Eval Mul where evalA (Mul x y) = x * y instance (Eval f, Eval g) => Eval (f :+: g) where evalA (Inl f) = evalA f evalA (Inr g) = evalA g eval :: Eval f => Expr f -> Int eval expr = foldExpr evalA expr val :: (Val :<: e) => Int -> Expr e val x = In $ inj $ Val x add :: (Add :<: e) => Expr e -> Expr e -> Expr e add x y = In $ inj $ Add x y mul :: (Mul :<: e) => Expr e -> Expr e -> Expr e mul x y = In $ inj $ Mul x y test :: Expr (Val :+: Add) test = In (Inr (Add (val 118) (val 1219))) test2 :: Expr (Add :+: Val) test2 = val 1 test3 :: Expr ((Add :+: Val) :+: Mul) test3 = add (mul (val 1) (val 2)) (val 3) test4 :: Expr (Add :+: (Val :+: Mul)) test4 = add (mul (val 1) (val 2)) (val 3) -- our typtree selection prefers left injection test5 :: Expr ((Val :+: Val) :+: (Val :+: Val)) test5 = val 1 -- -- TypTree. This is basically the same as (:<:). -- I kept it a separate class during development. This also allows the use of -- additional constraints on (:<:) which improve the error messages when you try, -- for example: -- testBroken :: Expr (Mul :+: Add) -- testBroken = val 3 -- class (Functor sub, Functor sup) => TypTree sub sup where treeInj :: sub a -> sup a instance Functor x => TypTree x x where treeInj = id -- -- The magic all happens here -- We use "IsTreeMember" to determine if a type is part of a tree with leaves -- of various types and internal nodes of type (l :+: r). -- class IsTreeMember (sub :: * -> *) (sup :: * -> *) b | sub sup -> b instance TypEq x y b => IsTreeMember x y b instance (IsTreeMember x l bl, IsTreeMember x r br, TypOr bl br b) => IsTreeMember x (l :+: r) b class (Functor sub, Functor l, Functor r) => TypTree' b sub l r where treeInj' :: b -> sub a -> (l :+: r) a -- -- We can then use this result to decide whether to select from the left or the right. -- instance (TypTree x l, Functor r) => TypTree' HTrue x l r where treeInj' _ = Inl . treeInj instance (TypTree x r, Functor l) => TypTree' HFalse x l r where treeInj' _ = Inr . treeInj -- -- Finally, this allows us to select which treeInj' to use based on the -- type passed in. -- instance (IsTreeMember x l b, TypTree' b x l r) => TypTree x (l :+: r) where treeInj = treeInj' (undefined :: b) class TypOr b1 b2 res | b1 b2 -> res instance TypOr HFalse HFalse HFalse instance TypOr HFalse HTrue HTrue instance TypOr HTrue HFalse HTrue instance TypOr HTrue HTrue HTrue -- Type equality, semi-lifted from the hlist paper; this only works in GHC. -- -- You can avoid the reliance on GHC6.8 type equality constraints -- by using TypeCast from the HList library instead. -- -- see http://www.okmij.org/ftp/Haskell/types.html#HList -- for the source of this idea. data HFalse data HTrue class TypEq (x :: * -> *) (y :: * -> *) b | x y -> b instance TypEq x x HTrue instance (b ~ HFalse) => TypEq x y b

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ANNOUNCE: Sessions 2008 2 28 - RFC
by Matthew Sackman 29 Feb '08

29 Feb '08

Hi, So for those of you that watch the hackage what's-new page, you will probably be aware of this. For the last few months, there's been significant work on my implementation of Session Types in Haskell.[0] The reason for this particular announcement is that it should now be at a point where functionality is verging on useful. There are a few things that are still missing which should appear over the next few weeks but if you're at all interested in message-passing as a tool in the box for dealing with concurrency, I'd love to know what you think. To that end, I would *not* encourage you to try to look at the implementation too closely as you may suddenly discover a powerful urge to stick knives in your eyes and/or eat your computer. However, the Tests.hs[1] file should give you a good idea of the kinds of things that are possible and they do cover all the functionality currently implemented. Note that ghci may well take about 5 mins to type check Tests.hs. Or longer if you're still running a BBC B Micro. I would almost certainly not advise anyone to look at the haddock[2]. I really can't quite believe how horrible the type sigs end up looking under haddock. Some of my comments may almost pass the barrier of being useful. Nicely formatted type sigs exist in the source. If however, you are as mad as I, and enjoy pushing the GHC type system to the limit, then you may enjoy looking at the implementation in all its glory. base10 numbers, lists, associative maps and a whole lot of other madness works correctly at the type-level in ghc 6.8.2. Anyway, as the subject says, comments gratefully received. [0] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/sessions-2008.2.… [1] http://wellquite.org/non-blog/sessions-browseable/Control/Concurrent/Sessio… [2] http://wellquite.org/non-blog/sessions-browseable/doc/html/sessions/ [3] http://wellquite.org/non-blog/sessions-browseable/ Matthew -- Matthew Sackman http://www.wellquite.org/

2 1

Best practice for embedding files in a GHC-compiled tool?
by Dave Bayer 29 Feb '08

29 Feb '08

What is the best way to embed an arbitrary file in a Haskell program? I would like to use GHC to compile command-line tools to be used with OS X. I want the tool to be a single file, not a package or a directory, that makes no assumptions about what else is present. For example, it should be able to run as part of an OS X install disk. I want this tool to be "self reproducing" in the sense that one of its options causes it to output its own documentation and source code. I want this data to be stored as a compressed file within the tool binary itself. The distribution model I'm imagining here is where one writes a program anonymously, that isn't hosted anywhere but is passed from user to user because it is useful, and eventually reaches another user who wants to modify the code. Assume that intermediate users will care less about this, and will try to delete anything that they can. That rules out storing the data in a separate file. Think of the M.I.T. game "Core Wars" from the dawn of the computer age. I'm looking for a strategy here that will be evolutionarily successful in a fairly hostile environment. In other words, I want to be able to write in Haskell, without losing the automatic distribution of source code enjoyed by interpreted languages. No one deletes the source code for a Perl script, because by doing so they're killing the program. There must be some library I'm overlooking that would make this very easy. All I can think of is to use Template Haskell to read a compressed tar file into a Haskell variable. Is that what one does, or is there a better way? Thanks in advance, Dave

4 3

ArrowChoice for simulation arrow (Sim)
by Steve Lihn 29 Feb '08

29 Feb '08

Hi, In John Hughes\' paper \"Programming with arrows\" [1] Section 5, the simulation arrow is defined (Sim). All the examples went well. However, when I want to use it beyond circuit simulation and I added a \"if-then-else\" line, GHC is asking to define ArrowChoice for Sim. Does anybody have the ArrowChoice definition for Sim ? Thanks, Steve [1] http://www.cs.chalmers.se/~rjmh/afp-arrows.pdf

1 0

ANNOUNCE: parameterized-data 0.1 - Parameterized data library implementing lightweight dependent types
by Alfonso Acosta 29 Feb '08

29 Feb '08

This library provides an implementation of parameterized types using type-level computations to implement the type parameters and emulate dependent types. Right now only fixed-sized vectors are provided (based on Oleg's "Number-parameterized types" [1] and Frederik Eaton's Vectro library [2]) HackageDB page: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/parameterized-da… Darcs repository: http://code.haskell.org/parameterized-data/ Again, big thanks to Oleg and Wolfgang for their help and suggestions. [1] http://okmij.org/ftp/Haskell/number-parameterized-types.html [2] http://ofb.net/~frederik/vectro/

1 0

Re: [Haskell-cafe] Bay Area talk tomorrow, on concurrent/parallel Haskell
by PR Stanley 28 Feb '08

28 Feb '08

At 13:23 28/02/2008, you wrote: >Hello, > >I wish I could be there, but I'm in Kentucky. ;) >It would be great if someone could have this talk recorded and >posted on youtube, or something similar. > >Thank you. >Now, there's an idea which could be extended to all Haskell related >gatherings. >__ >Donnie > >On 2/27/08, Bryan O'Sullivan ><<mailto:bos@serpentine.com>bos(a)serpentine.com> wrote: >Satnam Singh of Microsoft Research will be speaking about concurrent >and parallel programming at Stanford tomorrow. > >Details here: > ><http://www.realworldhaskell.org/blog/2008/02/28/stanford-haskell-talk-2008-…>http://www.realworldhaskell.org/blog/2008/02/28/stanford-haskell-talk-2008-… > > <b >_______________________________________________ >Haskell-Cafe mailing list ><mailto:Haskell-Cafe@haskell.org>Haskell-Cafe(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell-cafe > > >_______________________________________________ >Haskell-Cafe mailing list >Haskell-Cafe(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell-cafe

1 0

Bay Area talk tomorrow, on concurrent/parallel Haskell
by Bryan O'Sullivan 28 Feb '08

28 Feb '08

Satnam Singh of Microsoft Research will be speaking about concurrent and parallel programming at Stanford tomorrow. Details here: http://www.realworldhaskell.org/blog/2008/02/28/stanford-haskell-talk-2008-… <b

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