March 2010 - Haskell-Cafe

installing Haskell
by love_pku 20 Mar '10

20 Mar '10

I am sorry to trouble you but I have problems to install Haskell. Things run smoothly when I do the command ‘./configure’, But some errors appeared when I run the command ‘make’. The details are as follows(in red font): Distribution/Client/HttpUtils.hs:127:49: Couldn't match expected type `network-2.2.1.2:Network.URI.URI' against inferred type `URI' In the fourth argument of `AuthBasic', namely `uri' In the first argument of `Just', namely `(AuthBasic "" usr pwd uri)' In the expression: Just (AuthBasic "" usr pwd uri) Distribution/Client/HttpUtils.hs:135:37: Couldn't match expected type `network-2.2.1.2:Network.URI.URI' against inferred type `URI' In the `rqURI' field of a record In the expression: Request {rqURI = uri, rqMethod = GET, rqHeaders = [Header HdrUserAgent userAgent], rqBody = ByteString.empty} In the definition of `mkRequest': mkRequest uri = Request {rqURI = uri, rqMethod = GET, rqHeaders = [Header HdrUserAgent userAgent], rqBody = ByteString.empty} where userAgent = "cabal-install/" ++ display Paths_cabal_install.version Error: Building the cabal-install-0.6.2 package failed make: *** [build.stamp] Error 2 If I install cabal-install manually, the errors will be: # /usr/bin/runhaskell Setup configure Configuring cabal-install-0.6.2... Warning: This package indirectly depends on multiple versions of the same package. This is highly likely to cause a compile failure. package HTTP-4000.0.6 requires network-2.2.1.2 package cabal-install-0.6.2requires network-2.2.1.4

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Why does `flip` cause function type so different ?
by zaxis 20 Mar '10

20 Mar '10

>let f x xs = [x:xs,xs] > :t f f :: a -> [a] -> [[a]] >:t (>>=) .f (>>=) .f :: a -> ([[a]] -> [a] -> b) -> [a] -> b > :t (flip (>>=) .f) (flip (>>=) .f) :: a -> [[a]] -> [[a]] Why is the type of `(>>=) .f` and `flip (>>=) .f` so different ? Sincerely! ----- fac n = let { f = foldr (*) 1 [1..n] } in f -- View this message in context: http://old.nabble.com/Why-does-%60flip%60-cause-function-type-so-different-… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Haskell hackathon in Philadelphia (Hac phi 2) in May?
by Brent Yorgey 20 Mar '10

20 Mar '10

We are gauging interest in another Haskell hackathon to be hosted at U Penn in May, the weekend of either May 15 or May 22. If you might be interested in attending such a thing, send me an email (please DON'T reply to all!) with your name and whether you have a strong preference for one or the other weekend. Feel free to respond even if you are not sure; will be a separate official registration procedure once details have been nailed down. -Brent

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Call for Praatjes: Dutch HUG Day
by Sean Leather 20 Mar '10

20 Mar '10

>From ZuriHac, we bring you... *Call for Praatjes: Dutch HUG Day* To celebrate our first anniversary, the Dutch Haskell Users Group (HUG) invites Haskell and functional programming enthusiasts in the Netherlands and surrounding area to a short symposium. The Dutch HUG is an informal group with monthly meetings including talks on novel applications of Haskell and socializing in the neighborhood pub. The group was founded on Sunday, April 19, 2009 at the Haskell Hackathon hosted in Utrecht. After a successful year, we will continue the trend with the Dutch HUG Day. The Dutch HUG Day will be held in Utrecht on the afternoon of Saturday, April 24, 2010. The actual location is not confirmed; however, we will send out a notice as soon as it is known. *PRAATJE PROPOSALS* We seek people to give talks or "praatjes" in Dutch. The topic can cover anything in the functional programming or, more specifically, Haskell world. It may be your latest pet project, current research, or simply a demonstration of something cool that you found. The audience will be coming with a broad range of backgrounds, so praatjes should be oriented towards people having a general knowledge of Haskell programming. You can choose from one of the formats and topics below, or feel free to suggest something different. If you are interested in presenting, please send your proposal to Sean Leather by Saturday, April 3. More details are below. *FORMATS* - Talks (20 min) - Tutorials (25 min) - Lightning talks (5 min) - Demos (5 min) *TOPICS* - Development or design of a library or application - Comparison of libraries, applications, or languages - Lead a focused discussion (e.g. web programming options in Haskell) - Functional pearl (a "fun" talk) - Current research topic - Functional programming in industry - Experience report - Your favorite piece of software (and why) - Anything else (related to Haskell or functional programming) that you find interesting *IMPORTANT DATES* Praatje Proposal Deadline: Saturday, April 3 Dutch HUG Day: Saturday, April 24 *CONTACT* Send your proposal to Sean Leather: leather(a)cs.uu.nl Dutch HUG: http://dutchhug.nl/ Dutch HUG Day: http://dutchhug.nl/DutchHugDay/

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Takusen and iteratee problem
by Vasyl Pasternak 20 Mar '10

20 Mar '10

Hello Cafe. Today I stuck with the following problem: I want to read a file with iteratee package, and but it to database through Takusen package, but it doesn't work. The `Takusen` was built with `mtl` package, and `iteratee` - with `transformers`, so they are conflicting when used simultaneously. To be concrete, I want to run the code, that looks like: withSession (connect "dbname.db") (do execDDL (sql "create table x (value string)") fillFromFile "file.dat" ) fillFromFile fname = fileDriverFd iter fname iter = iterator that uses execDML (sql "Insert into ...." ).... I have no ideas how to solve the problem, so appreciate any recommendations, that will make it possible. Thanks, Vasyl

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re: [Haskell-cafe] HTTP package freezes on Windows 7
by Phil 20 Mar '10

20 Mar '10

Scrap my original query - the problem isn't as black & white as I thought. The below works fine - I've changed the response type from json to xml.... strange, but for some reason downloading json doesn't work.... it's fine on Linux. I'm guessing this is more likely to be a Windows issue rather than a Haskell issue - any ideas? import qualified Network.HTTP as HTTP main :: IO () main = do x <- getLocation print x getLocation = (HTTP.simpleHTTP $ HTTP.getRequest url) >>= HTTP.getResponseBody where url = "http://maps.google.com/maps/api/geocode/xml?address=London&sensor=false"

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hdbc-mysql on windows
by Roderick Ford 19 Mar '10

19 Mar '10

Hello, In order to build (cabal install HDBC-mysql) the HDBC-mysql package, I am having to make the following changes: (1) I had to change the mysql (5.1.45 community distrib msi) to use a wrapper exe on the mysql_config.pl and put this in c:\program files\mysql\bin since although the c:\program files\mysql\scripts\, where mysql_config.pl resides, was in the path, even the commandline call to it was not finding or using it...very strange. This of course doesn't directly have to do with the HDBC-mysql package. --The quick wrapper was: import System.Environment(getArgs) import System.Process(readProcessWithExitCode) import System.Exit(exitWith,ExitCode(ExitSuccess)) main = do args <- getArgs (ecode,out,err) <- readProcessWithExitCode "c:\\Perl64\\bin\\perl.exe" ("C:\\Program Files\\MySQL\\scripts\\mysql_config.pl" : args) "" if ecode == ExitSuccess then mapM_ putStrLn (lines out) else mapM_ putStrLn (lines err) exitWith ecode -- so now the output is: "C:\Program Files\MySQL\lib\mysqlclient.lib" "wsock32.lib" "advapi32.lib" "user3 2.lib" (2) Once that was done, the "cabal install HDBC-mysql" call was able to find the mysql_config. However, it was splitting the values returned by mysql_config on space, and thus failed to find mysql.h, as the resulting command was: C:\Program Files (x86)\Haskell Platform\2009.2.0.2\gcc.exe -c -BC:\ Program Files (x86)\Haskell Platform\2009.2.0.2\gcc-lib -IC:\Program Files (x86) \Haskell Platform\2009.2.0.2\include\mingw -D__GLASGOW_HASKELL__=610 -IIC:\Program -Iles\MySQL\include\mysql" ... -- then I tried taking the -I out of the mysql_config.pl script and got the following from the "cabal install HDBC-mysql" command was: C:\Program Files (x86)\Haskell Platform\2009.2.0.2\gcc.exe -c -BC:\ Program Files (x86)\Haskell Platform\2009.2.0.2\gcc-lib -IC:\Program Files (x86) \Haskell Platform\2009.2.0.2\include\mingw -D__GLASGOW_HASKELL__=610 -I:\Program -Iles\MySQL\include\mysql" ... blah blah blah cabal: Error: some packages failed to install: HDBC-mysql-0.6.1 failed during the building phase. The exception was: exit: ExitFailure 1 ... so finally I am downloading your package to see if I can figure out a fix for windows boxes. Just thought I would give you a heads up on this windows-related bug. Thanks, Roderick

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Re: explicit big lambdas
by oleg＠okmij.org 19 Mar '10

19 Mar '10

Paul Brauner wrote: > is there a way in some haskell extension to explicit (system F's) big > lambdas and (term Type) applications in order to help type inference? Actually, yes: newtype constructor introductions may act as a big lambda, with constructor elimination acting as type applications: http://okmij.org/ftp/Haskell/types.html#some-impredicativity Newtypes are also handy for turning type functions (defined by type families) into real lambdas. For example, given the following code > {-# LANGUAGE NoMonomorphismRestriction #-} > {-# LANGUAGE TypeFamilies, EmptyDataDecls #-} > {-# LANGUAGE ScopedTypeVariables #-} > > module S where > > data Z > data S a > > class Nat a where nat :: a -> Int > instance Nat Z where nat _ = 0 > instance Nat n => Nat (S n) where nat _ = succ (nat (undefined::n)) > > type family Add a b > type instance Add Z b = b > type instance Add (S a) b = S (Add a b) we would like to compose the type function Add (S (S Z)) twice. We may first try *> newtype F2 f x = F2{unF2 :: f (f x)} *> tf1 :: F2 (Add (S (S Z))) (S Z) *> tf1 = undefined alas, without success: Type synonym `Add' should have 2 arguments, but has been given 1 In the type signature for `tf1': tf1 :: F2 (Add (S (S Z))) (S Z) Indeed, type functions defined via type families can't be turned into closures. Newtypes help, introducing the needed Lambda behind the scenes: > newtype A2 a = A2{unA2 :: Add (S (S Z)) a} > > appA2 :: x -> A2 x > appA2 = undefined > > -- Now we can compose the type function, many times > tc = unA2 . appA2 . unA2 . appA2 > -- Inferred type: > -- tc :: a -> Add (S (S Z)) (Add (S (S Z)) a) > > tcrun = nat $ tc (undefined::S Z) > -- 5 One can see from the inferred type of tc that the type-checker has composed the partially applied Add function. I'll show a bit more elaborate example next week.

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Why no instance of Happstack.Data.Default.Default for Data.ByteString.Lazy
by Mads Lindstrøm 19 Mar '10

19 Mar '10

Hi Why is that Happstack has: instance Happstack.Data.Default.Default Data.ByteString But not one for: instance Happstack.Data.Default.Default Data.ByteString.Lazy I am trying to make a HTTP proxy using Happstack and the client part with the Network.HTTP (see Hackage). Network.HTTP.simpleHttp returns a lazy Bytestring when given a lazy Bytestring (which makes sense). Happstack on the other hand contains a lazy ByteString in its request type: data Request = Request { ... rqHeaders :: Headers, ... rqBody :: RqBody ... } newtype RqBody = Body Data.ByteString.Lazy.Internal.ByteString which Happstack.Server.SimpleHttp.simpleHTTP feeds into my application. However Happstack.Server.SimpleHttp.simpleHTTP expects something returned which is an instance of Happstack.Data.Default.Default which lazy ByteString is not. Confused? So am I. Who thought String handling could be so complex. Greetings, Mads Lindstrøm

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Trace
by Ben Derrett 19 Mar '10

19 Mar '10

Hi, I'm a new Haskell programmer and am trying to output the values of some of the variables (for debugging) as the program executes. At the moment to output the value of the function I use: f x y z = trace (show moves) moves where moves = [complicated expression of x y z] But really what I want is to be able to write: f x y z = trace2 [complicated expression of x y z] Is there a standard function to do this? I tried: trace2 a = trace (show a) a But I got the error: Couldn't match expected type `t1 -> t' against inferred type `String' In the expression: (show a) a In the definition of `trace2': trace2 a = (show a) a How do I fix it? Thanks, Ben

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