April 2010 - Beginners

ANN: scan-0.1.0.3, a Haskell style scanner
by Christian Maeder 19 Apr '10

19 Apr '10

Dear Haskell friends, I like to announce a Haskell style scanner at http://hackage.haskell.org/package/scan documented under http://projects.haskell.org/style-scanner/ It's best used in conjunction with hlint http://community.haskell.org/~ndm/hlint/ and gives many suggestions regarding the file format. Feedback is welcome. Thanks Christian (Discussions should only be posted to either haskell-cafe@ or beginners@)

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Re: [Haskell-beginners] Beginner trying to install Haskell support in Eclipse and failing
by Alan Shaw 17 Apr '10

17 Apr '10

I installed scion from hackage ok, but then I remembered this, from http://eclipsefp.sourceforge.net/ : You cannot use the version from Hackage (0.1.0.2) since commands have been added for eclipsefp. You'll need to build Scion from source (git clone git://github.com/JPMoresmau/scion.git, runhaskell Setup.hs configure/build/install) So I did the other thing: cabal install -f-cabal_1_8 And now I have got updates in Eclipse from http://eclipsefp.sf.net/updates, and I'm about to "Create a Haskell Project." so if that process doesn't mystify me I'm good to go for now. And I plan to update ghc "soon." Thanks, all! -A On Sat, Apr 17, 2010 at 7:43 PM, Daniel Fischer <daniel.is.fischer(a)web.de> wrote: > > Am Sonntag 18 April 2010 01:07:17 schrieb Alan Shaw: > > Hi, > > I'm using cabal for the first time, and I'm confused. > > > > I'm trying to build scion as instructed at > > http://eclipsefp.sourceforge.net/build.html . > > I have done: > > > > git clone git://github.com/JPMoresmau/scion.git > > cd scion > > cabal install > > > > > > I'm now at the point where this tells me the following: > > > > Resolving dependencies... > > cabal.exe: dependencies conflict: ghc-6.10.4 requires Cabal ==1.6.0.3 > > however > > Cabal-1.6.0.3 was excluded because scion-0.1.0.2 requires Cabal ==1.8.* > > && ==1.8.* > > That's odd, the .cabal file says > > if flag(cabal_1_8) > build-depends: Cabal == 1.8.* > cpp-options: -DCABAL_VERSION=108 > else > build-depends: Cabal == 1.6.* > cpp-options: -DCABAL_VERSION=106 > > and since you don't have Cabal-1.8.*, it should set the flag to false and > try again. > > Anyway, try setting the flag to false on the command line: > > cabal install -f-cabal_1_8 > > or install the hackage package: > > cabal install scion > > > > > Some more information: > > > > $ cabal -V > > cabal-install version 0.6.2 > > using version 1.6.0.3 of the Cabal library > > $ cabal update > > Downloading the latest package list from hackage.haskell.org > > Note: there is a new version of cabal-install available. > > To upgrade, run: cabal install cabal-install > > $ cabal install cabal-install > > (lots and lots of output, ending with:) > > Linking dist\build\cabal\cabal.exe ... > > Installing executable(s) in C:\Program Files (x86)\Haskell\bin > > Is that directory in your path (and before the directory cabal- > install-0.6.2 is in)? > > > $ cabal -V > > cabal-install version 0.6.2 > > using version 1.6.0.3 of the Cabal library > > perhaps > > cabal install cabal-install-0.8.0 (or whatever the latest version working > on windows is) > > > > > > > Help? > > > > Thanks, > > -A >

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Beginner trying to install Haskell support in Eclipse and failing
by Alan Shaw 17 Apr '10

17 Apr '10

Hi, I'm using cabal for the first time, and I'm confused. I'm trying to build scion as instructed at http://eclipsefp.sourceforge.net/build.html . I have done: git clone git://github.com/JPMoresmau/scion.git cd scion cabal install I'm now at the point where this tells me the following: Resolving dependencies... cabal.exe: dependencies conflict: ghc-6.10.4 requires Cabal ==1.6.0.3 however Cabal-1.6.0.3 was excluded because scion-0.1.0.2 requires Cabal ==1.8.* && ==1.8.* Some more information: $ cabal -V cabal-install version 0.6.2 using version 1.6.0.3 of the Cabal library $ cabal update Downloading the latest package list from hackage.haskell.org Note: there is a new version of cabal-install available. To upgrade, run: cabal install cabal-install $ cabal install cabal-install (lots and lots of output, ending with:) Linking dist\build\cabal\cabal.exe ... Installing executable(s) in C:\Program Files (x86)\Haskell\bin $ cabal -V cabal-install version 0.6.2 using version 1.6.0.3 of the Cabal library Help? Thanks, -A

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calling polymorphic function in Selenium question
by MH 16 Apr '10

16 Apr '10

I am running the following code that is using Selenium. If you look at the function "start", you will see that the return type of the function is polymorphic. Now in main function, I call start function to get selenium IO monad and sequentially call selenium commands (open, doCommand etc...). The problem that I have here is, while I can call all Selenium commands with signature (String -> Selenium String), I can't call commands with signature (String -> Selenium Bool). As I understand it, even though "start" function shall return IO (Selenium a -> IO (Either String a)), it actually return IO (Selenium String -> IO (Either String String)). How shall go about fixing this problem? I need to be able to call both types of Selenium commands 1. doCommand :: SCommand -> [String] -> Selenium String OR selectFrame :: String -> Selenium String AND 2. isTextPresent :: String -> Selenium Bool Thanks. ========================================================= module SeleniumTest where import Control.Monad.Error import Data.Maybe import Network.BSD import Network.URI import Test.Selenium.Server import Test.Selenium.Syntax infixr 0 $$ -- | Starts up a session and returns a wrapper function that will run -- commands. Gives common defaults for browser and host. start :: String -> IO (Selenium a -> IO (Either String a)) start url = do -- host <- getHostName start' Firefox "localhost" url -- | Starts up a session and returns a wrapper function that will run -- commands. start' :: Browser -> HostName -> String -> IO (Selenium a -> IO (Either String a)) start' browser host url = do let uri = fromJust (parseURI url) sel = mkSeleniumRCSession host browser uri result <- runSelenium sel startSelenium return $ runSelenium (either (\msg -> error msg) id result) ($$) :: Show t => (Selenium () -> r) -> Selenium t -> r ($$) s c = s $ do r <- c; liftIO (putStrLn $ "Result: " ++ show r); return () -- | Stops a session (in the wrapper returned by start) stop :: Selenium () stop = stopSelenium --main::IO() main = do selenium <- start "http://www.google.com" selenium $ open "/" selenium $ doCommand SWindowMaximize [] selenium $ typeText (Name "q") "stuff" selenium $ clickAndWait (Name "btnG") return selenium ===============================================================

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Forcing evalation in the IO Monad
by Philip Scott 16 Apr '10

16 Apr '10

Hi folks, This question is so trivial it must have been answered before, but I have been googling all morning to no avail so please bear with me. I've been wading through mires of $! and seq, deepSeq, $!! etc.. and I have leaned a lot of useful stuff about lazy evaluation - but I haven't been able to solve my problem.. What I would like to do is to force an expression to be fully evaluated to normal form, from inside the IO monad, right there and then. For example, in GHCI, calling putStrLn does just what I want: Debug.Trace Prelude> let a = trace "Hello" 42 Debug.Trace Prelude> putStrLn $ show a Hello 42 Except that it only takes showable things, and it puts an annoying message on the screen. What I want is something like putStrLn that doesn't print anything, and takes any type as its input, evaluates it to normal form and then does nothing else. It might be called something like 'evaluate' have a type like this evaluate :: a -> IO() It _must_ exist - and it _must_ be easier to find this sort of thing than I am making it, or else how did all you haskell gurus attain your status? Am I missing something :) All the best, Philip

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Re: calling polymorphic function in Selenium question
by MH 16 Apr '10

16 Apr '10

I tried that and I got the following error: A pattern type signature cannot bind scoped type variables `a' unless the pattern has a rigid type context In the pattern: x :: Selenium a -> IO (Either String a) In a stmt of a 'do' expression: (x :: Selenium a -> IO (Either String a)) <- start " https://www.google.com" In the expression: do { (x :: Selenium a -> IO (Either String a)) <- start " https://www.google.com"; Then I tried to add "forall a" but the Selenium library doesn't use forall so there was a conflict. Is there a way to work around the rigid type error you know of? Thanks On Apr 15, 3:14 pm, Brent Yorgey <byorgey(a)seas.upenn.edu> wrote: > On Sun, Apr 04, 2010 at 09:25:16PM -0400, MH wrote: > > I am running the following code that is using Selenium. If you look at > > the function "start", you will see that the return type of the > > function is polymorphic. Now in main function, I call start function > > to get selenium IO monad and sequentially call selenium commands > > (open, doCommand etc...). The problem that I have here is, while I can > > call all Selenium commands with signature (String -> Selenium > > String), I can't call commands with signature (String -> Selenium > > Bool). As I understand it, even though "start" function shall return > > IO (Selenium a -> IO (Either String a)), it actually return IO > > (Selenium String -> IO (Either String String)). > > Hmm, I must confess that I don't understand exactly what is going on > here. One thing to try is to give an explicit type annotation at > start's call site, something like > > > main = do > > (selenium :: Selenium a -> IO (Either String a)) <- start " http://www.google.com" > > selenium $ open "/" > > selenium $ doCommand SWindowMaximize [] > > selenium $ typeText (Name "q") "stuff" > > selenium $ clickAndWait (Name "btnG") > > return selenium > > Does that help? > > -Brent > _______________________________________________ > Beginners mailing list > Beginners@haskell.orghttp://www.haskell.org/mailman/listinfo/beginners

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Re: [Haskell-beginners] Compiling C into Haskell
by Hein Hundal 16 Apr '10

16 Apr '10

Thanks Allan! I was hoping the C-Haskell mix would work. I am glad to hear that you have good things to say about it. My main reason for thinking about the C-to-Haskell compiler was to address the question "Say you had a C program. Can you always convert it to Haskell in such a way that the compiled Haskell is not too slow and does not need too much memory?" Supposing that too slow means slower than 1/4 the speed of C and too much memory means twice the memory of C. Do you know the answer to this question? Cheers, Hein

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Trying to find an alternative StateMonad
by edgar klerks 14 Apr '10

14 Apr '10

Hi All, I wanted to write the statemonad in a different way, because I want to track mutations to the state. And also to be able to undo an done or redo an undone mutation. Therefore I wrote a small dsl with an evaluator, which finds out the current state. The problem is, I don't know how to create a get and put function for my monad. I have a function which evaluate the state and gives back the current State. Can someone help me a bit along? I have the feeling I do something wrong, but I am not sure what. With kind regards, Edgar {-# LANGUAGE GADTs,DeriveFunctor #-} import Control.Monad import Data.Maybe data StateCmd a where Put :: a -> StateCmd a Undo :: StateCmd a Redo :: StateCmd a deriving (Show, Functor) newtype State s a = State { unState :: ([StateCmd s],a) } deriving Show joinState :: State s a -> State s b -> State s b joinState (State (xs, a)) (State (ys, b)) = State (xs ++ ys, b) instance Monad (State s) where return a = State $ ([], a) (>>=) = bindState -- m a -> ( a -> m b) -> m b bindState st@(State (_,a)) f = st `joinState` st' where st' = f a unPut :: StateCmd a -> Maybe a unPut (Put a) = Just a unPut _ = Nothing test = State $ ([Put 4, Undo, Redo, Undo, Put 5, Undo, Redo, Put 6, Undo, Redo, Undo], ()) getCurrent = fromJust.unPut.head.snd.(foldr current' ([],[])).reverse.fst.unState where current' x (ul,cl) = case x of Put n -> (ul, Put n : cl) Undo -> (head cl : ul, tail cl) Redo -> (tail ul, head ul : cl)

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Re: [Haskell-beginners] Compiling C into Haskell
by Hein Hundal 13 Apr '10

13 Apr '10

Heinrich wrote: >What are the complex operations you want to do on matrices? There >is the hmatrix that binds to LAPACK and is a pleasure to use, >but it's mainly about matrix multiplication and other numerical >matrix operations. The main operations I need are Matrix Inversion, Transpose, Singular Value Decomposition, and QR Factoring. LAPACK takes care of all of these. I will have to look into hmatrix. Thanks! Cheers, Hein

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Compiling C into Haskell
by Hein Hundal 13 Apr '10

13 Apr '10

Hi, I have been playing around with Haskell for about a month now and reading the nice book "Real World Haskell." My main reason for learning Haskell is that I want to code up some machine learning projects (heavy use of matricies). Normally, I use Mathematica and mix it with C++, but Mathematica is proprietary, slow, and can't produce executable while C++ is verbose. Learning Haskell has been fun; however, I have been a little worried that I will sacrifice too much performance when coding in Haskell. So, I recently asked one of my friends the following question, "Say you had a C program. Can you always convert it to Haskell in such a way that the compiled Haskell is not too slow and does not need too much memory?" Supposing that too slow means slower than 1/4 the speed of C and too much memory means twice the memory of C. Hopefully, someone that knows Haskell well can comment on this question. I am not sure, but I think the answer is yes, such a conversion can always be done and creating a C to Haskell compiler with the above performance constraints is not extremely hard. I started thinking about how a compiler might convert a simple C program into Haskell. Below I will paste a C program and the compile-by-hand Haskell code. It seems to me that the ideas I used to create the Haskell code can be implemented in a compiler that converts a simple subset of C into Haskell. I was thinking about restricting the C to one data type 32bit-integers, arithmetic (+-*/%), assignment (=), comparison (<,>,==,<=,>=), the if-condition-codeblock construct, and the while-condition-codeblock construct. (I would also like to do integer arrays, but I have not read about mutable arrays and monads yet.) Any comments? Cheers, Hein H. --------------------C to Haskell------------------------ module Main where -- Convert a C-Program line by line into Haskell -- -- 1-- #include <stdio.h> -- 2-- void main() -- 3-- { -- 4-- long i,j; -- 5-- i=7; -- 6-- j=0; -- 7-- while(j<10000) -- 8-- { -- 9-- if ((i % 17)== 11) --10-- i = i*2; --11-- if ((i % 35)== 12) --12-- i = i+13; --13-- if (i> 1000) --14-- i = i - 1000; --15-- i++; --16-- j++; --17-- } --18-- printf("%ld", i); --19--} --assignment statements line5 (i,j) = (7,j) line6 (i,j) = (i,0) line10 (i,j) = (i*2,j) line12 (i,j) = (i+13,j) line14 (i,j) = (i-1000,j) line15 (i,j) = (i+1,j) line16 (i,j) = (i,j+1) --while statement line7test (i,j) = j >=100000000 line7 (i,j) = until line7test body9To16 (i,j) --if statements line9 (i,j) = if ((mod i 17) == 11) then line10 (i,j) else (i,j) line11 (i,j) = if ((mod i 35) == 12) then line12 (i,j) else (i,j) line13 (i,j) = if (i>1000) then line14 (i,j) else (i,j) -- code blocks mainprogram :: (Int, Int) -> (Int, Int) mainprogram = line7 . line6 . line5 body9To16 = line16 . line15 . line13 . line11 . line9 main :: IO () main = putStrLn(show(mainprogram (0,0))) -----------------end of program-------------------------

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