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list monad and controlling the backtracking
by isto 24 Nov '06

24 Nov '06

Hi all, Weekly news had a link to article "Local and global side effects with monad transformers" and the following is from there (minor modification done): import Control.Monad.List import Control.Monad.State import Control.Monad.Writer test5 :: StateT Integer (ListT (Writer [Char])) Integer test5 = do a <- lift $ mlist aList b <- lift $ mlist bList lift $ lift $ tell ("trying "++show a++" "++show b++"\n") modify (+1) guard $ a+b<5 return $ a+b go5 = runWriter $ runListT $ runStateT test5 0 If the aList = [1..5] as well as bList, then there will be 25 tryings. If aList and bList are [1..10000000], there will be a lot of tryings... However, guard is saying that we are interested in only values whose sum is less than 5. Is it possible to control easily in the above example that when we have tried out pairs (1,1), (1,2), (1,3), (1,4), that now we are ready to stop trying from the bList? And then similarly when we finally arrive to a pair (4,1), that now we are ready to finish also with aList? This would give a nice way to build branch & bounding algorithms, even though it does not take much more lines to do it in some other way. br, Isto

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strange type mismatch when trying to use takusen
by Seth Gordon 24 Nov '06

24 Nov '06

I have a simple test program for takusen and PostgreSQL: > import Database.Enumerator > import Database.PostgreSQL.Enumerator > import Control.Monad.Trans > > gazdbSession dbname = connect [CAdbname dbname] > > resultCollector :: (Monad m) => String -> IterAct m [String] > resultCollector str accum = result' (str:accum) > > main = do withSession (connect [CAdbname "template1"]) $ main' > > main' = do r <- doQuery (sql "select 'fred'") resultCollector [] > liftIO $ putStrLn $ show r But it won't compile: [1 of 1] Compiling Main ( takusen-test.hs, takusen-test.o ) takusen-test.hs:11:57: Couldn't match expected type `forall mark. DBM mark Session a' against inferred type `DBM mark sess ()' In the second argument of `($)', namely `main'' In the expression: (withSession (connect [CAdbname "template1"])) $ main' In the expression: do (withSession (connect [CAdbname "template1"])) $ main' Adding an explicit "main' :: DBM mark Session ()" type signature doesn't change the error message. The takusen source that I'm using was updated from darcs on November 14 and compiled with ghc 6.6.

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Re: [Haskell] Re: [Haskell-cafe] SimonPJ and Tim Harris explain STM - video
by C.M.Brown 24 Nov '06

24 Nov '06

Hi, I got this working on Mac OS X. I had to download media player 9: http://www.microsoft.com/windows/windowsmedia/software/Macintosh/osx/defaul… This contains the WMV3 codec. Cheers, Chris. On Fri, 24 Nov 2006, James William Pye wrote: > On Fri, Nov 24, 2006 at 10:26:38AM +0100, Tomasz Zielonka wrote: > > Does anybody know how to watch this on Linux? I would prefer to simply > > download the movie file and use MPlayer on that, but I failed. > > > > . or on Mac OS X (haven't tried yet) > > The latest mplayer works for me on FreeBSD/amd64 (1.0rc1, iirc). > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell >

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Implicit params and typeclasses
by Grzegorz Chrupała 24 Nov '06

24 Nov '06

Hi all, When I try to compile the following in GHC (v 6.6) (with -fno-monomorphism-restriction -fimplicit-params) class Configuration a where thestring:: a -> String foo c = let { ?c = c } in bar bar = thestring ?c I get: Ambiguous type variable `a' in the constraint: `Configuration a' arising from use of `thestring' at /home/grzegorz/Foo.hs:9:6-17 Possible cause: the monomorphism restriction applied to the following: bar :: (?c::a) => String (bound at /home/grzegorz/Foo.hs:9:0) foo :: a -> String (bound at /home/grzegorz/Foo.hs:8:0) Probable fix: give these definition(s) an explicit type signature or use -fno-monomorphism-restriction Adding the type signature bar :: (Configuration a,?c :: a) => String the compiler says: Ambiguous constraint `Configuration a' At least one of the forall'd type variables mentioned by the constraint must be reachable from the type after the '=>' In the type signature for `bar': bar :: (Configuration a, ?c :: a) => String Similar code without use of implicit params compiles fine: class Configuration a where thestring:: a -> String foo c = thestring c Why do implicit params cause this error and is there a way to make it work? Thanks, -- Grzegorz

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Foldl
by ClareZako 24 Nov '06

24 Nov '06

Hello, i'd like to write a function that given a list like [1,2,3,4...] returns a list of couple where the first element is the corresponding element of the string, and the second is the sum of the previous elements. An example: input: [1,2,3,4] output: [(1,0)(2,1)(3,3)(4,6)] The problem is that I'd like to use foldl and a solution like: buildCouples = snd . foldl op (0,[]) where op (v,xs) x = (v+x,xs++[(x,v)]) is not good since the operator ++ let the function take 2*n time and not n. Can anyone help me please? Clare -- View this message in context: http://www.nabble.com/Foldl-tf2698202.html#a7524410 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Command line prompt templates
by Dougal Stanton 24 Nov '06

24 Nov '06

I was trying to write my own equivalent to Don Stewart's mkcabal but ended up getting sidetracked. I made some generalised prompts for use at the command line and wanted to get some feedback on them. The full code can be found at [1] but the basic summary is like this: > prompt :: String -> IO String Simple prompt - supply a question and receive the user's answer. > promptYesNo :: String -> Maybe String -> IO Bool A yes/no question, with an optional value to ask about. I suppose this could be simplified to just one string. > promptList :: String -> [String] -> Maybe Integer -> IO Integer Ask the user to choose from a list, where the result is the number chosen. The optional value is a default if the user doesn't pick a number. > promptListEdit :: String -> [String] -> Maybe Integer -> IO String As above, but the last option in the list is an invitation to provide your own answer. Again the optional value is the default choice if nothing else is chosen. The result here is necessarily the string rather than the integer cos I didn't want to complicate matters further with stuff like Either types. If you can see any flaws or have any suggestions please let me know! Cheers, D. [1]: <http://brokenhut.no-ip.org/~dougal/darcs/joincabal/CommandPrompts.hs>

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Equivalent of if/then/else for IO Bool?
by Dougal Stanton 23 Nov '06

23 Nov '06

Is there some sort of equivalent of the if/then/else construct for use in the IO monad? For instance the following can get quite tedious: > do bool <- doesFileExist filename > if bool > then sth > else sth' Is there a more compact way of writing that? Something akin to: > condM (doesFileExist filename) (sth) (sth') Or maybe there's a more sensible way of doing the above that I've missed. I seem to use endless alternate conditions sometimes and there's bound to be a better way.

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Re: [Haskell-cafe] working with lists of couples
by Clare 23 Nov '06

23 Nov '06

Yes the code you are suggesting is certainly linear and it takes without doubt time n. But I was looking for a solution using foldl that of course takes time n. The problem of the following solution is that it gives a reversed result, and of course i can't just reverse the result. couples = snd . foldl f (0,[]) where f (s,[]) x = (s+x, [(x,0)]) f (s,xs) x = (s+x, (:) (x,s) xs) Clare 2006/11/17, Valentin Gjorgjioski <valentin.gjorgjioski(a)ijs.si>: > > On 17.11.2006 21:04 Clare wrote: > > I'm not sure it takes time n couse of the operator ++ and the lazy > > stuffs in haskell. > Ok, you can use > buildCouples (x:xs) s = (x,s) : (buildCouples xs (x+s)) > instead of ++ > > this algorithm is linear, I don't know why(?) you think it is not. > > Valentin > > -- > Valentin Gjorgjioski > Bachelor of Computer Science > Department of Knowledge Technologies, Jozef Stefan Institute > Jamova 39, SI-1000 Ljubljana, Slovenia > Phone: +386 1 477 3343 > Fax: +386 1 477 3315 > Web: http://kt.ijs.si/ValentinGjorgjioski/ > Email: Valentin.Gjorgjioski(a)ijs.si > >

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type keeping rounding, typeable (and a difficulty)
by isto 23 Nov '06

23 Nov '06

Hi, I've been trying to compile the following function (rounding to a desired degree): roundDec :: (Num a, Typeable a) => Int -> a -> a roundDec d a = let t = show (typeOf a) in case t of "Double" -> roundDDec d a "Complex Double" -> roundCDec d a otherwise -> a -- or something The two other functions are roundCDec :: (RealFloat a) => Int -> Complex a -> Complex a roundCDec d (c :+ b) = (roundDDec d c :+ roundDDec d b) and roundDDec :: (RealFloat a) => Int -> a -> a roundDDec d a = a -- or somegthing Compiler gives the following error message: Couldn't match expected type `Complex a' against inferred type `a1' (a rigid variable) `a1' is bound by the type signature for `roundDec' at FFT.hs:57:17 In the second argument of `roundCDec', namely `a' In the expression: roundCDec d a In a case alternative: "Complex Double" -> roundCDec d a If in the roundDDec a's are replaced with Double, there will be similar error message from the "Double"-line. The functionality can be written differently, but I wanted to try write rounding having in a signature at least "(Num a) => Int -> a -> a". Again, any help would be appreciated a lot! Thanks in advance! br, Isto

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Type Error. Why!?
by Tim Newsham 23 Nov '06

23 Nov '06

Can anyone tell me why I get a type error in the following code? I have to define liftA2 twice to avoid the type error. Both times its defined identically but with different type signature. If I use the original liftA2 in place of liftA2' I get: whyerror.lhs:36:25: Ambiguous type variable `a' in the constraint: `Arrow a' arising from use of `>>>' at whyerror.lhs:36:25-27 Possible cause: the monomorphism restriction applied to the following: liftA2' :: forall b a1 b1 c. (a1 -> b1 -> c) -> a b a1 -> a b b1 -> a b c (bound at whyerror.lhs:36:1) unsplit' :: forall a1 b c. (a1 -> b -> c) -> a (a1, b) c (bound at whyerror.lhs:34:1) split' :: forall b. a b (b, b) (bound at whyerror.lhs:33:1) Probable fix: give these definition(s) an explicit type signature or use -fno-monomorphism-restriction ----- whyerror.lhs ---- > {-# OPTIONS_GHC -farrows #-} > module WhyError where > import Control.Arrow > newtype SimpleFunc a b = SimpleFunc { runF :: (a -> b) } > instance Arrow SimpleFunc where > arr f = SimpleFunc f > first (SimpleFunc f) = SimpleFunc (mapFst f) > where mapFst g (a,b) = (g a, b) > (SimpleFunc f) >>> (SimpleFunc g) = SimpleFunc (g . f) > split :: (Arrow a) => a b (b, b) > split = arr (\x -> (x,x)) > unsplit :: (Arrow a) => (b -> c -> d) -> a (b, c) d > unsplit = arr . uncurry > -- arr (\op (x,y) -> x `op` y) > liftA2 :: (Arrow a) => (b -> c -> d) -> a e b -> a e c -> a e d > liftA2 op f g = split >>> first f >>> second g >>> unsplit op > -- f &&& g >>> unsplit op > f, g :: SimpleFunc Int Int > f = arr (`div` 2) > g = arr (\x -> x*3 + 1) > h = liftA2 (+) f g > hOutput = runF h 8 > -- XXX I am getting type problems with split, unsplit and liftA2! why? > split' = arr (\x -> (x,x)) > unsplit' = arr . uncurry > -- liftA2' :: (Arrow a) => (b -> c -> d) -> a e b -> a e c -> a e d > liftA2' op f g = split' >>> first f >>> second g >>> unsplit' op > plusminus, double, h2 :: Kleisli [] Int Int > plusminus = Kleisli (\x -> [x, -x]) > double = arr (* 2) > h2 = liftA2 (+) plusminus double > h2Output = runKleisli h2 8 > main = do > print hOutput > print h2Output Tim Newsham http://www.thenewsh.com/~newsham/

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