March 2008 - Haskell-Cafe

If Version Control Systems were Airlines
by Justin Bailey 06 Mar '08

06 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 06 Mar '08

06 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 05 Mar '08

05 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 05 Mar '08

05 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 05 Mar '08

05 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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