October 2013 - Beginners

Constrained polymorphic functions as natural transformations
by Matt R 05 Jan '14

05 Jan '14

I read this excellent blog article on natural transformations[1], which helped me gain one intuition as to what they are: "functions that don't make too many assumptions about their arguments". In the article, natural transformations make "no assumptions at all" about their arguments. However, it's often the case that a Haskell function will require a type argument constrained by a type class, e.g. show :: Show a => a -> String. I wondered if it was possible to characterise these functions using category theory too. Below is my attempt, and I'm hoping people can tell me whether I'm in the right ballpark (or point me in the direction of a better ball-park...) For a particular type class T, we can form a subcategory Hask_T of Hask consisting of all the instances of the type class as objects, with the arrows just those functions that "commute" with the signature of the type class. For example, in Hask_Show, the arrows would be all the functions f: A -> B such that: show == show . f Or for Data.Monoid, all the monoid homomorphisms f: mappend (f x) (f y) == f (mappend x y) mempty == f mempty In general, for any type class, we can formulate two functors f and g and a function sig :: f a -> g a that captures the operations in the type class, and then the condition is that: fmap f . sig == sig . fmap f. Then we have that the polymorphic functions with a type class constraint of T are just the same thing as natural transformations in this subcategory Hask_T. Is the above correct / along the right lines? Thanks, -- Matt [1] "You Could Have Defined Natural Transformations", http://blog.sigfpe.com/2008/05/you-could-have-defined-natural.html

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Raumzeit formulae in Haskell
by Roman Czyborra 30 Oct '13

30 Oct '13

Dear fellow beginning Haskellers, I am delurking today because I think this might be of interest to you and you might be of help to get more proofs out of the pudding since information wants to be free and life wants to live and I would love to have someone helping me here to transform this formulation into a time-travel movie generating bioscope. ---------- Forwarded message ---------- Date: Wed, 30 Oct 2013 11:05:54 From: Roman Czyborra <czyborra(a)campus.tu-berlin.de> To: Prof Dr Magdalena Bushart <magdalena.bushart(a)tu-berlin.de>, Sigrid Krajewski <krajewski(a)fakii.tu-berlin.de>, Aris Dreismann <dreismann(a)chem.tu-berlin.de>, Prof Dr Debora Weber-Wulff <weberwu(a)htw-berlin.de>, Stephan Voelker <stephan.voelker(a)tu-berlin.de>, Sabine Glesner <sabine.glesner(a)tu-berlin.de>, Peter Pepper <pepper(a)cs.tu-berlin.de>, Horst Voelz <h.voelz(a)online.de>, Marie Neubert <marie.neubert(a)gmail.com>, Barbara Jochum <barbara.jochum(a)tu-berlin.de>, Prof Dr Dr Beier <klaus.beier(a)charite.de>, Universiteit van Amsterdam <g.hekma(a)uva.nl> Cc: Philippe Verdy <verdy_p(a)wanadoo.fr>, Doug Ewell <doug(a)ewellic.org>, Paul Hardy <hardy_paul(a)hotmail.com>, Donald Buczek <buczek(a)molgen.mpg.de>, /dev/fd/0 <fd(a)taz.de>, Di Benedikt Homann <bene(a)berlin.de>, Dr Juergen Hoehn <juergen(a)z-bi.de>, Drs ℝ⁴ Ilja Schmelzer <iljaschmelzer(a)gmail.com>, Marsha Linehan <linehan(a)u.washington.edu>, Dbiol Dr med MSc PhD Mark Benecke <mark(a)benecke.com>, Die PARTEI <mail(a)die-partei.de>, Wolfgang Baitz <wolfgang.baitz(a)web.de>, Andreschka Grossmann <a(a)videapolis.de>, Reinhard Knoppka <r.knoppka(a)gmx.de>, Db Lieselotte Juergensen <lieselottejuerg(a)aol.com>, Daniela Czyborra Pacolli <danielapacolli(a)gmail.com>, bipolaris eV <bipolaris(a)bipolaris.de>, Monika Schaefer-Ligustro <schaefer.ligustro(a)yahoo.de>, Robert Sengl <robert(a)sengl.de>, Werner Fuss <werner-fuss(a)gmx.de>, Behindertenbeauftragte <mechthild.rolfes(a)tu-berlin.de>, Angela Merkel <angela.merkel(a)bundestag.de>, Ulrike Pohl <upohl32(a)gmail.com>, Cornelia Otto <cornelia.otto(a)berlin.piratenpartei.de>, Kandidat Rhotep <andreas.pittrich(a)posteo.de>, Sven Guckes <sven(a)guckes.net>, Felix von Leitner <fefe(a)fefe.de>, OS X Survivors <machackers(a)lists.ccc.de>, Ulf Kasch <ulf.kasch(a)fourlife.de>, Marek Czyborra <marekczyb(a)hotmail.com>, Konrad Opitz <ko(a)sauerbruchhutton.de>, Carbo Kuo <byvoid1(a)gmail.com>, Richard Stallman <rms(a)gnu.org>, Jacob Appelbaum <ioerror(a)appelbaum.net>, Claudia Schreck <claudia.schreck(a)bezirksamt-neukoelln.de>, Kurt Hartmann <mf.buchholz(a)yahoo.de>, Mayra Hartmann <mayra.hartmann(a)facebook.com> Subject: Antrag auf Promotion zum Doktor der Naturwissenschaft (Dr sci ℕ⁴ Roma Czyborra) Verehrte Professor(inn)en, als Träger des durch Ihr http://czyborra.com/diplom/diplom.pdf verliehenen akademischen Grades, welchen ich bereits 2001 wieder zurückzugeben müssen glaubte, als mich nach dem Einbruch des Werbekommerzes ins Internet die Welt nicht mehr verstand, beantrage ich hiermit, schnellstmöglich zur wissenschaftlichen Verteidigung meiner in http://czyborra.com/diplom/plank.hs ausformulierten Dissertationsthese (für Sehbehinderungsfetischisten auch als http://czyborra.com/diplom/plank.hs.unifont.pdf gecryptet verfügbar) geladen zu werden, um noch 2013 die mir zustehende Erlaubnis zum Führen des akademischen Titels Doktor der Naturwissenschaft (gerne cum maxima laude) erteilt zu bekommen und nutzbringend zu verwenden dürfen. Ich versichere Ihnen hiermit mit Eideskraft, dass ich Ihnen keinerlei Fremdleistung betrügerisch als eigene vorgaukele, sondern meine wesentlichen Vorarbeiter_innen mit den mir bekanntesten Namen attributiert und meine den menschlichen Erkenntnisgewinn schallmauerbrechend fördernde kleine Synthese allein mithilfe eigener geistiger Arbeit und im Dialog mit unter Zuhilfenahme meines Internetterminals zugreifbarer automatischer Informationen selbst produziert habe. Zur Formwahrung noch einen weiteren tabellarischer Lebenslauf, welcher eigentlich Zeit vergeudet, solange Herr Pütz einem ständig Millionenwerte von der Platte putzt, und man heutzutage einfach nur noch http://facebook.com/roman.czyborra/about ins Adressfeld einzutragen braucht: 1970 http://czyborra.com/diplom/kopfgeburt.pdf 1976 http://czyborra.com/diplom/scheidungsurteil.pdf 1980-1984 http://czyborra.com/diplom/kinderausweis.jpg 1986-1989 http://czyborra.com/diplom/abitur.pdf 1989-∞ http://czyborra.com/diplom/fahrerlaubnis.pdf 1990-1992 http://czyborra.com/asf/p.pdf 1989-1993 http://czyborra.com/diplom/vordiplom.pdf 1993-∞ http://czyborra.com/bisexy/ 1994-1995 http://www.cs.tu-berlin.de/www/ 1994-1996 http://www.cs.tu-berlin.de/~czyborra/job.html 1996-1997 http://tal.cs.tu-berlin.de/lehre/thegi1/ 1998 http://czyborra.com/diplom/abgabe.html 1999-2002 unixadmin(a)taz.de mit http://woffs.de/ 2003 http://czyborra.com/diplom/zeugnis.pdf 2003 http://czyborra.com/diplom/wolisz1.pdf >>= http://czyborra.com/chemnitz/ 2004-2005 Versauern in Depressionen verursacht durch 2001-2012 millionenschwere Psychiatrieeinweisungen durch Ärzte und Richter in Cantor-Gödelschem Ausmaß 2006-2007 http://de.boywiki.org/wiki/Archiv:WP/Benutzersperrung/Roman_Czyborra 2007-2012 Systemadminstration in der WfbM http://via-werkstaetten.de/ 2011-2012 http://czyborra.com/zeta/ 2012-∞ http://czyborra.com/diplom/reimmatrikulation.pdf 2013-∞ http://czyborra.com/diplom/marriagecertificate.pdf 2013 http://www.unicode.org/mail-arch/unicode-ml/y2013-m07/0015.html 2013 http://czyborra.wordpress.com/2013/07/05/hacking-the-universe-with-weltform… 2013 http://czyborra.com/diplom/plank.hs Ihren Antworten freudig entgegensehend verbleibe ich als: Ihr Roman Czyborra, doctorandus informaticae, tel:+491789794164

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Empty type
by KwangYul Seo 28 Oct '13

28 Oct '13

Hello, It seems there are three different ways to declare an empty type in Haskell. http://www.haskell.org/haskellwiki/Empty_type 1) data E0 = E0 2) newtype Void = Void Void 3) data Void I'd like to know how the second trick works. Is it possible to create a new type from itself? How should I interpret this? Thanks, Kwang Yul Seo

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reifying based-on type of a newtype or data
by AntC 25 Oct '13

25 Oct '13

So I have (or rather the user of my package has): > {-# LANGUAGE DeriveDataTypeable #-} > > newtype Foo = Foo Int deriving (Read, Show, Typeable, Data, ...) > someFoo = Foo Int > Note: * the `newtype` could be `data` -- if that would help. * this is _not_ a parameterised type, but a 'baked in' `Int`. * the data constr is named same as the type -- if that would help. I can ask for `typeOf someFoo` and get `Foo` OK. I can ask for `typeOf Foo` and get `Int -> Foo` OK. If I ask for `typeOf (typeOf someFoo)` I get `TypeRep`. `typeOf (show $ typeOf someFoo`) gets me `[Char]` (aka `String`) So far very logical, but not very helpful. What I want is to get the based-on type baked inside `someFoo` -- that is: `Int` (It would also be handy to get the name of the data constr, just in case it's different to the type.) Do I need to get into `deriving (..., Generic)` ? That looks like serious machinery! Thanks AntC

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better exception handling
by Miro Karpis 25 Oct '13

25 Oct '13

Please, can you help me with following?... I have a working code but am not very happy with the error handling in "updateStocks" function. It does what it should, but I would like to implement some kind of guards. Problem is that it always ends it some error + am not sure if I can use guards in inside defined variable. Pls. what would you suggest? httpExceptionHandler :: HttpException -> IO L.ByteString httpExceptionHandler e = (putStrLn "Error: simpleHttp returned exception ") >> (return L.empty) getStocks :: String -> IO L.ByteString getStocks url = (simpleHttp url) `X.catch` httpExceptionHandler updateStocks :: IO String updateStocks = do yqlResult <- getStocks testQuery case yqlResult of x | x == L.empty -> return "return exception" | otherwise -> return "here I will call another function" thanks, m.

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Re: [Haskell-beginners] Signature of monadic functions
by Lorenzo Tabacchini 22 Oct '13

22 Oct '13

I think I am looking at the problem in a wrong way... What I want to do is using higher-order functions with IO (or an IO-based transformer). For example, let's say I want to apply a function to all the keys in a Map. With pure functions, I would do: Map.mapKeys doSomething myMap But let's suppose the function has a signature: doSomething :: a -> IO b Is there an easy way to apply it?

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Re: [Haskell-beginners] -with-rtsopts ignored at compile time
by Brandon Allbery 21 Oct '13

21 Oct '13

On Mon, Oct 21, 2013 at 1:51 PM, Kees Bleijenberg < k.bleijenberg(a)lijbrandt.nl> wrote: > I understand the problem. I’ve set the GHCRTS environment variable and > when I run the program I get : <unknown>: Most RTS options are disabled. > Link with –rtsopts to enable them. **** > > Then I added rtsopts to the command line: ghc -shared -rtsopts -O2 -o > TBGlas.dll TBGlasDll.hs**** > > and I get the same errormessage again: Most RTS options are…… What is > the problem? > Exact same issue. Those cannot be per DLL, they apply to the final program (what would it mean for only some DLLs to allow RTS options? There's only one runtime!); and you'd probably have to use the same mechanism I pointed at before if you're invoking hs_main() from C instead of using a Haskell main function. -- brandon s allbery kf8nh sine nomine associates allbery.b(a)gmail.com ballbery(a)sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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Re: [Haskell-beginners] -with-rtsopts ignored at compile time
by Brandon Allbery 21 Oct '13

21 Oct '13

On Mon, Oct 21, 2013 at 8:01 AM, Kees Bleijenberg < k.bleijenberg(a)lijbrandt.nl> wrote: > When I run my program, I get a stack overflow message (current stacksize > is 8388608 bytes) and the advice to use +RTS –Ksize -RTS**** > > The Haskell code creates a Windows dll, so no command line. That’s why I > want to change the stack size at compile time. > I think the default RTS options are only settable in a compile that produces an executable; otherwise there would have to be a chunk of code that retrieved all the rtsopts from every Haskell-compiled DLL and somehow reconciled any conflicts between them. Also note that if the main program is in C, you don't get -with-rtsopts processing at all; see http://www.haskell.org/ghc/docs/latest/html/users_guide/ffi-ghc.html#using-… how you set RTS options in this case. (See at "To use +RTS flags with hs_init()".) In this particular case you may be better off just using the %GHCRTS% environment variable. -- brandon s allbery kf8nh sine nomine associates allbery.b(a)gmail.com ballbery(a)sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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-with-rtsopts ignored at compile time
by Kees Bleijenberg 21 Oct '13

21 Oct '13

When I run my program, I get a stack overflow message (current stacksize is 8388608 bytes) and the advice to use +RTS -Ksize -RTS The Haskell code creates a Windows dll, so no command line. That's why I want to change the stack size at compile time. I tried ghc -shared -O2 -o -with-rtsopts="-H128m -K64m" TBGlas.dll TBGlasDll.hs Ghc tells that it links with "-H128m -K64m". When I run the program I still get a message overflow with again the same (?) 8388608 stack size. I also changed the stacksize of the calling program, but no luck (same message). I wonder whether ghc ignores the -with-rtsopts parameter? Probably the problem is caused by not enough strictness, but still I want te make sure that it is not the stacksize (it is a big program). Kees

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storage leak in NetSNMP?
by Lieven Marchand 21 Oct '13

21 Oct '13

Hi, I have a small script that computes a list of network devices, directs an SNMP query at each of them, computes a report from the result of that query and prints that. I would expect that to run in constant space but instead memory usage keeps going up until the script is killed by the OOM killer. Is it possible that the NetSNMP library is keeping hold of all the results of its session? Here's part of my code: analyzeSNMP :: Line -> IO() analyzeSNMP x = do snmp <- getSNMP x putStrLn $ getNetworks (x, snmp) getSNMP :: Line -> IO (Either String [SnmpResult]) getSNMP line = snmpWalk snmp_version_2c (encodeUtf8 (primaryRouterLoopback line)) (DBC.pack "XXX") mib2 getNetworks :: (Line, Either String [SnmpResult]) -> String main = do initialize args <- arguments wans <- case wanInventoryFileName args of Just s -> parseWANInventory s Nothing -> return DefaultWAN.wans let lines = makeLines (tail wans) mapM_ analyzeSNMP lines

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Re: [Haskell-beginners] Actions v. Functions
by MJ Williams 20 Oct '13

20 Oct '13

For example, `main = do input <- getLine ...', where `getLine' is an action as distinguished from a function such as `main'. Other examples of actions in Haskell include `print', `putStrLn' and any process that reads from or writes to a file. Of course, the list of actions doesn't end there, though the examples here should answer your question. Sincerely, Matt At 21:44 19/10/2013, you wrote: >What do you mean by "action"? > >Antoine > > >On Fri, Oct 18, 2013 at 9:26 PM, MJ Williams ><<mailto:matthewjwilliams101@gmail.com>matthewjwilliams101(a)gmail.com> wrote: >Hello >What are the formal properties of an action? >How does an action differ from a function? >Sincerely, >Matthew > >_______________________________________________ >Beginners mailing list ><mailto:Beginners@haskell.org>Beginners(a)haskell.org >http://www.haskell.org/mailman/listinfo/beginners > > >_______________________________________________ >Beginners mailing list >Beginners(a)haskell.org >http://www.haskell.org/mailman/listinfo/beginners

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Actions v. Functions
by MJ Williams 19 Oct '13

19 Oct '13

Hello What are the formal properties of an action? How does an action differ from a function? Sincerely, Matthew

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Re: [Haskell-beginners] How do I give a type for this code?
by Todd Wilson 18 Oct '13

18 Oct '13

Thanks, Daniel, for your suggestion, but I think it misses a key aspect of my situation (unless I've misunderstood). Every model is supposed to be a quadruple (m, q, f, p), with m a list, q an element of m, f a list of ordered pairs of elements of m, and p a sublist of m. By contrast, it looks like your definition makes a model either a single element, or a single pair, or a single list, etc. --Todd On Thu, Oct 17, 2013 at 4:12 AM, Daniel Trstenjak < daniel.trstenjak(a)gmail.com> wrote: > > Hi Todd, > > the problem is, that 'model_of' tries to return different types: Model a, > Model (a,a) and Model [a]. > > I think you have to use some kind of ADT also for the Model, like you > already did for ModName. > > Something like: > > data Model a = Model a > | ProductModel (a,a) > | PowerModel [a] > ... > > > Greetings, > Daniel > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners >

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How do I give a type for this code?
by Todd Wilson 17 Oct '13

17 Oct '13

Haskell community, I'm in one of those situations where I have the code I want to write, but need to find the right types to get it to typecheck. What follows is a generic version of my situation. Suppose I want to represent the finite models of a language (in the sense of mathematical logic) with a single constant c, unary function symbol f, and predicate P. I can represent the carrier as a list m, the constant as an element of m, the function as a list of ordered pairs of elements of m, and the predicate as the list of the elements in the model that satisfy it: -- a model is (m, c, f, p) type Model a = ([a], a, [(a,a)], [a]) -- a is the "base type" I can then construct particular models and operations on models. The details aren't important for my question, just the types: unitModel :: Model () unitModel = ([()], (), [((),())], []) cycleModel :: Int -> Model Int cycleModel n = ([0..n-1], 0, [(i, (i+1)`mod`n) | i<-[0..n-1]], [0]) -- function application, assuming function is total ap :: Eq a => [(a,b)] -> a -> b ap ((x',y'):ps) x = if x == x' then y' else ap ps x -- cartesian product of models productModel :: (Eq a, Eq b) => Model a -> Model b -> Model (a,b) productModel (m1, c1, f1, p1) (m2, c2, f2, p2) = (m12, c12, f12, p12) where m12 = [(x1,x2) | x1 <- m1, x2 <- m2] c12 = (c1, c2) f12 = [(x12, (ap f1 (fst x12), ap f2 (snd x12))) | x12 <- m12] p12 = [x12 | x12 <- m12, elem (fst x12) p1 && elem (snd x12) p2] -- powerset of model powerModel :: (Eq a, Ord a) => Model a -> Model [a] powerModel (m, c, f, p) = (ms, cs, fs, ps) where ms = subsequences m cs = [c] fs = [(xs, nub (sort (map (ap f) xs))) | xs <- ms] ps = [xs | xs <- ms, elem c xs] Now, I want to give a name to all of these models: data ModName = UnitModel | CycleModel Int | Product ModName ModName | Power ModName deriving (Show, Eq) And finally, I want to associate each name with its associated model: model_of UnitModel = unitModel model_of (CycleModel n) = cycleModel n model_of (Product m1 m2) = productModel (model_of m1) (model_of m2) model_of (Power m1) = powerModel (model_of m1) I've tried a number of ways of getting this to work, in the sense of defining types so that I can use exactly this definition of model_of (some involving GADTs and type families) but haven't found a best way. How should I do it? Todd Wilson

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Signature of monadic functions
by Lorenzo Tabacchini 17 Oct '13

17 Oct '13

Hi, is there an easy way to transform a function from: Monad m => a -> m b to: Monad m => m (a -> b) ?

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capital letter name in record syntax (parsing json)
by Miro Karpis 17 Oct '13

17 Oct '13

Hi, please,...is it possible to use a capital letter in record syntax data definition? For example data Car = Car { CarName :: !Text ,CarColor :: !Text } deriving (Show,Generic) When I try this I get: parse error on input `CarName' The thing is that I'm trying to parse a json file (with Aeson package) that has records starting with capital letter. After that I would like to parse the JSON file with following code: d <- (eitherDecode <$> getJSON) :: IO (Either String [Car]) where getJSON gets the json file from url If not, what alternative do I have? thanks, m.

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Exercise of "Programming with Arrows"
by Thiago Negri 16 Oct '13

16 Oct '13

The paper "Programming with Arrows" of John Hughes gives some exercises to do [1]. I'm trying to solve it and would like to receive a feedback if I'm doing it right or not before reading the rest of the paper. I didn't find the answers in the internet (if someone could point me to it, please do so). Exercise 2 (section 2.5) is asking to create a Stream Processor that can map more than one output per input (e.g. 3 outcomes for a single consume of the stream). The paper says that implementing "first" will be tricky, and it really is. I've came up to the solution listed below, *is it right?* * * * * module SP where import Prelude hiding (id, (.)) import Control.Category import Control.Arrow data SP a b = Put b (SP a b) | Get (a -> SP a b) runSP :: SP a b -> [a] -> [b] runSP (Put b s) as = b:runSP s as runSP (Get k) (a:as) = runSP (k a) as runSP (Get k) [] = [] compose :: SP b c -> SP a b -> SP a c compose (Put a s) g = Put a (compose s g) compose (Get k) (Put a s) = compose (k a) s compose f (Get k) = Get (\a -> compose f (k a)) instance Category SP where id = arr id (.) = compose instance Arrow SP where arr f = Get (\a -> Put (f a) (arr f)) first (Put a s) = Get (\(a', c) -> Put (a, c) (delayed (a', c) s)) first (Get k) = Get (\(a, c) -> firstWithValue (k a) c) delayed :: (a, c) -> SP a b -> SP (a, c) (b, c) delayed (a, c) (Put b s) = Put (b, c) (delayed (a, c) s) delayed (a, c) (Get k) = firstWithValue (k a) c firstWithValue :: SP a b -> c -> SP (a, c) (b, c) firstWithValue (Put a s) c = Put (a, c) (firstWithValue s c) firstWithValue (Get k) _ = Get (\(a, c) -> firstWithValue (k a) c) input :: [(String, String)] input = [("a1", "a2"), ("b1", "b2"), ("c1", "c2"), ("d1", "d2")] myArrow :: SP (String, String) (String, String) myArrow = (delay "db1" >>> delay "da1") *** (delay "db2" >>> delay "da2") delay :: a -> SP a a delay b = Put b (arr id) main :: IO () main = let output = runSP myArrow input in mapM_ f output where f (a, b) = putStrLn $ "(" ++ show a ++ ", " ++ show b ++ ")" * * The output of "main" is: *SP> main ("da1", "da2") ("da1", "db2") ("da1", "a2") ("db1", "a2") ("a1", "a2") ("b1", "b2") ("c1", "c2") ("d1", "d2") [1] http://www.cse.chalmers.se/~rjmh/afp-arrows.pdf

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Problems with cabal install hakyll
by Britt Anderson 16 Oct '13

16 Oct '13

I have a fresh installation of ghc 7.6.3-1 and cabal 1.16. From this I was able to cabal update and then cabal install diagrams and pandoc, but when trying to install hakyll (either version 4 or 3) I get a lot of dependency conflicts. Can anyone give me some suggestions on how to proceed in this case? Thank you. Below follow the messages from cabal install --dry-run *** Trying version 4 *** cabal install --dry-run hakyll Resolving dependencies... cabal: Could not resolve dependencies: trying: hakyll-4.4.1.0 trying: hakyll-4.4.1.0:+checkexternal trying: http-conduit-1.9.5.1/installed-1fe... rejecting: tls-extra-0.6.6/installed-302... (conflict: tls-extra => cryptohash==0.11.1/installed-ae4..., hakyll => cryptohash>=0.7 && <0.11) rejecting: tls-extra-0.6.6, 0.6.5, 0.6.4, 0.6.3, 0.6.1, 0.6.0, 0.5.1, 0.5.0, 0.4.7.1, 0.4.7, 0.4.6.1, 0.4.6, 0.4.5, 0.4.4, 0.4.3, 0.4.2.1, 0.4.2, 0.4.1, 0.4.0, 0.3.1, 0.3.0, 0.2.3, 0.2.2, 0.2.1, 0.2.0, 0.1.9, 0.1.8, 0.1.7, 0.1.6, 0.1.5, 0.1.4, 0.1.3, 0.1.2, 0.1.1, 0.1.0 (conflict: http-conduit => tls-extra==0.6.6/installed-302...) ***Trying version 3 *** cabal install --dry-run hakyll-3.5.3.0 Resolving dependencies... In order, the following would be installed: HUnit-1.2.5.2 (new package) List-0.5.1 (new package) blaze-html-0.5.1.3 (new version) bytestring-mmap-0.2.2 (new package) entropy-0.2.2.4 (new package) crypto-api-0.12.2.2 (new package) cryptohash-0.8.4 (new version) enumerator-0.4.20 (new package) attoparsec-enumerator-0.3.1 (new package) blaze-builder-enumerator-0.2.0.5 (new package) hashable-1.1.2.5 (new version) case-insensitive-1.1 (reinstall) changes: hashable-1.2.1.0 -> 1.1.2.5 hexpat-0.20.3 (new package) highlighting-kate-0.5.5 (reinstall) changes: blaze-html-0.6.1.1 -> 0.5.1.3, filepath-1.3.0.1 added lrucache-1.1.1.3 (new package) regex-tdfa-1.1.8 (new package) syb-0.3.7 (new version) hs-bibutils-5.0 (new package) json-0.7 (new package) pandoc-types-1.9.1 (new version) citeproc-hs-0.3.9 (new package) system-filepath-0.4.8 (new package) system-fileio-0.3.11 (new package) shakespeare-1.2.0.1 (new package) hamlet-1.1.7.3 (new package) tagsoup-0.12.8 (new version) texmath-0.6.4 (reinstall) changes: pandoc-types-1.12.2.3 -> 1.9.1, syb-0.4.1 -> 0.3.7 pandoc-1.9.4.5 +blaze_html_0_5 (new version) unix-compat-0.4.1.1 (new package) unordered-containers-0.2.3.3 (reinstall) changes: hashable-1.2.1.0 -> 1.1.2.5 zlib-enum-0.2.3 (new package) snap-core-0.9.4.1 (new package) snap-server-0.9.3.4 (new package) hakyll-3.5.3.0 (new package) Warning: The following packages are likely to be broken by the reinstalls: http-types-0.8.1 pandoc-1.12.0.2 http-conduit-1.9.5.1 yaml-0.8.5.1 semigroups-0.11 void-0.6.1 zlib-conduit-1.0.0 lens-3.9.2 force-layout-0.2 diagrams-contrib-0.7 diagrams-0.7.1.1 conduit-1.0.8 blaze-builder-conduit-1.0.0 MemoTrie-0.6.1 vector-space-0.8.6 vector-space-points-0.1.2.1 diagrams-core-0.7.0.1 diagrams-svg-0.8.0.1 diagrams-lib-0.7.1.1 active-0.1.0.7 semigroupoids-3.1 semigroupoid-extras-3.0.1 profunctor-extras-3.3.3.1 monoid-extras-0.3.2.2 dual-tree-0.1.0.4 groupoids-3.0.1.1 comonad-transformers-3.1 comonads-fd-3.0.3 bifunctors-3.2.0.1 comonad-3.1 profunctors-3.3.0.1 aeson-0.6.2.1 pandoc-types-1.12.2.3 Use --force-reinstalls if you want to install anyway.

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Re: [Haskell-beginners] [Haskell-cafe] Haskell Group Dresden?
by Daniel Trstenjak 15 Oct '13

15 Oct '13

Hi Stephan, > I'm very much interested. Do you need any help organizing a first meeting? > > For what it's worth, we once had an event about Yesod at the local CCC > group[1]. well, normally I'm not the kind of guy who enjoys performing in front of a lot of people, so I'm trying to find a way to get in touch with some folks without getting a heart attack =). So looking at your yesod event, the perception to get filmed, isn't really helping here :D. I was thinking about something "more" informal or if someone else wants to talk, I have no issues to sit somewhere in the back ;). Ok, but perhaps we can work out something fitting. Greetings, Daniel

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Haskell Group Dresden?
by Daniel Trstenjak 15 Oct '13

15 Oct '13

Hi, is there any interest in having something like a Haskell Group in Dresden? Perhaps I could say something about the lens library, which would be more of a practical talk, how to define lenses and working with lenses, traversals, isos and prisms. Greetings, Daniel

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Re: [Haskell-beginners] monads do not fit together?
by Brandon Allbery 15 Oct '13

15 Oct '13

On Tue, Oct 15, 2013 at 5:00 AM, Kees Bleijenberg < k.bleijenberg(a)lijbrandt.nl> wrote: > I wonder, do you always have to create a monadtransformer if your program > uses 2 kinds of IO? > In one sense the answer is "yes"; but the real answer is "that's not how it works". There is only one IO. Monad transformers allow you to add functionality on top of it, but at some point you need to do actual I/O in the IO monad. One reason it's done this way is that CGIT could in theory be written such that you could swap out the raw IO monad for something that lets you do testing. -- brandon s allbery kf8nh sine nomine associates allbery.b(a)gmail.com ballbery(a)sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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Re: [Haskell-beginners] monads do not fit together?
by Brandon Allbery 15 Oct '13

15 Oct '13

On Mon, Oct 14, 2013 at 10:58 AM, Kees Bleijenberg < k.bleijenberg(a)lijbrandt.nl> wrote: > I get the same message (Couldn't match expected type `CGIT IO a0' with > actual type `IO [String]'). I think I do understand where this message is > coming from. But what can I do to fix it? > For now, you just correct the type of runRefreshFirewall; it should be runRefreshFirewall :: String -> CGIT IO [String] If you are intending to run actual IO actions in the real thing, you'll need to use liftIO to "reach" the IO embedded in the CGIT IO a. -- brandon s allbery kf8nh sine nomine associates allbery.b(a)gmail.com ballbery(a)sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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Re: [Haskell-beginners] Exercise of "Programming with Arrows"
by Kim-Ee Yeoh 14 Oct '13

14 Oct '13

On Tue, Oct 8, 2013 at 4:08 AM, Thiago Negri <evohunz(a)gmail.com> wrote: > I agree with "no-op assembly line", but when I'm using `first` on a processor, I want to process the first stream *only*. The second stream should remain as it was not touched, so future processors will receive the same sequence from the second stream. We are in violent agreement! > I mean, I think I need to guarantee that this definition holds: > > `g *** f` is the same as `first g >>> swap >>> first f >>> swap` Excellent! Let's turn to laws and definitions to be _precise_ in what we're saying. > If my implementation of `first` uses a real no-op assembly line for `c` (i.e., `arr id`), then I would lose the stream. To that end, consider this easy problem: using (1) the arrow laws that you know off the top of your head, and (2) the above definition of (***) can you show that g *** (arr id) = first g? Because arr id, as you noted, is a no-op. Do you see what's going on? -- Kim-Ee

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Re: [Haskell-beginners] monads do not fit together?
by Kim-Ee Yeoh 14 Oct '13

14 Oct '13

On Mon, Oct 14, 2013 at 6:51 PM, Kees Bleijenberg < k.bleijenberg(a)lijbrandt.nl> wrote: > The error at the problem line is: parse error on input `<-' **** > > If I move the line above the ’if’ I get: Couldn't match expected type > `CGIT IO t0' with actual type `IO [String]'. **** > Looks like you're missing a "do". You're writing code like this: if blah then x <- foo bar x else quux when you need to write if blah then do x <- foo bar x else quux Recommended exercises: * can you see in your mind's eye the desugaring of do-syntax? * how would you write it in a single line, i.e. without using indentation a.k.a. "layout" -- hint: look up curly braces -- Kim-Ee

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monads do not fit together?
by Kees Bleijenberg 14 Oct '13

14 Oct '13

The program belows reads the get fields 'un' and 'ip'. If these get fields both exist, the program modifies a firewall script. The problem is in cgiMain. The error at the problem line is: parse error on input `<-' If I move the line above the 'if' I get: Couldn't match expected type `CGIT IO t0' with actual type `IO [String]'. How can I extract values from the IO [String] monad in cgiMain? I don't want to alter the type of runRefreshFirewall. Kees module Main( main ) where import Network.CGI import Text.XHtml import Text.XHtml.Transitional import Data.Maybe runRefreshFirewall :: String -> String -> IO [String] runRefreshFirewall un ip = do return ["test" ] inputCgiOkPage :: String -> String -> [String] -> Html inputCgiOkPage un ip msgs = body << h1 << ("IP: " ++ (show ip)) undefinedFieldsPage :: Html undefinedFieldsPage = body << h1 << "Undefined Fields" cgiMain :: CGI CGIResult cgiMain = do maybeUn <- getInput "un" maybeIp <- getInput "ip" if ((isJust maybeUn) && (isJust maybeIp)) then msgs <-runRefreshFirewall (fromJust maybeUn) (fromJust maybeIp) -- problem output $ renderHtml $ inputCgiOkPage (fromJust maybeUn) (fromJust maybeIp) msgs else output $ renderHtml undefinedFieldsPage main :: IO () main = runCGI $ handleErrors cgiMain

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