- Beginners - Haskell.org

Install on "foreign" server problems
by Lawrence Bottorff 30 May '11

30 May '11

I'm trying to install the binary of 7.0.3 on a Bluehost.com server where I have an account and ssh shell for a web client of mine. I want to eventually run Yesod or happstack or some other sort of "haskell on the web." I've talked with their tech support and they're cool with it as long as it's an "in-place" install. Good. I try the ./configure and it errors saying things like: checking for path to top of build tree... utils/ghc-pwd/dist/build/tmp/ghc-pwd: /lib64/libc.so.6: version `GLIBC_2.9' not found (required by utils/ghc-pwd/dist/build/tmp/ghc-pwd) utils/ghc-pwd/dist/build/tmp/ghc-pwd: /lib64/libc.so.6: version `GLIBC_2.7' not found (required by utils/ghc-pwd/dist/build/tmp/ghc-pwd) configure: error: cannot determine current directory I'm not (nor ever was) much of a sysadmin type. Any advice? I also installed 7.0.3 on my home machine and got a lot of dependency problems I'm sure I'll face once I get past this GLIBC problem. Actually, I'm just trying to do Web with Haskell on a server/cloud other than my own home box. Again, I have a Web client and I'd like to get us off PHP. Any Haskell-friendly clouds/hosts to suggest? Somebody said they had Yesod running on Amazon EC2.... Olwe

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return IO () vs. ()
by Neil Jensen 29 May '11

29 May '11

I've been attempting to refactor some working code and running into confusion about returning IO values. The basic sequence is to query a database, calculate some values, and then store the results back in the database. The function which does the querying of the db and calculating results has the following type signature: calcCUVs :: AccountId -> IO [((ISODateInt, ISODateInt), CUV)] This function stores the results back into the database saveCUVs :: AccountId -> [((ISODateInt, ISODateInt), CUV)] -> IO () saveCUVs account cuvs = do r' <- mapM (\x -> storeCUV (snd $ fst x) account (snd x)) cuvs return () I had a working variation of the below using 'do' notation, but for some reason when I moved to using bind, I'm getting messed up with return values. processAccountCUVs :: AccountId -> ISODateInt -> ISODateInt -> IO () processAccountCUVs account prevMonthEnd monthEnd = -- do if (prevMonthEnd == 0 && monthEnd == 0) then calcCUVs account >>= (\cuvs -> saveCUVs account cuvs) >>= return () else calcCUVs account prevMonthEnd monthEnd >>= (\cuvs -> saveCUVs account cuvs) >>= return () The compiler gives the following error message: Couldn't match expected type `IO ()' against inferred type `()' In the first argument of `return', namely `()' In the second argument of `(>>=)', namely `return ()' In the expression: calcCUVs account >>= (\ cuvs -> saveCUVs account cuvs) >>= return () I thought the last return () would correctly return us IO () as we are in the IO monad... what am I missing? Thanks for any input you can provide. Neil

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question about round function
by Prashant Shah 29 May '11

29 May '11

Hi, This is my first time with Haskell. Here is what I found while trying out some examples : Prelude> round 3.499999999999999 3 it :: Integer Prelude> round 3.4999999999999999 4 it :: Integer --------------- Prelude> round 111111.49999999999 111111 it :: Integer Prelude> round 111111.499999999999 111112 it :: Integer Is it something to do with the CPU register size ?

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Data.Foldable/foldMap question
by Patrick LeBoutillier 28 May '11

28 May '11

Hi, I'm reading the LYAH book and on page 263 it says that if you define Data.Foldable.foldMap you get Data.Foldable.foldl and Data.Foldable.foldl for "free". The default implementation code of Data.Foldable.foldr is: foldr :: (a -> b -> b) -> b -> t a -> b foldr f z t = appEndo (foldMap (Endo . f) t) z I don't understand this code, but more specifically I don't get how there can a Monoid constraint on foldMap's return type and not on foldr/foldl. Can any one explain what Endo is and how it works? Thanks a lot, Patrick -- ===================== Patrick LeBoutillier Rosemère, Québec, Canada

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Simple UTF-8 output
by Christopher Howard 28 May '11

28 May '11

Hi. I'm trying to print out UTF-8 strings (pulled from a MySQL database through HDBC.ODBC). If I just print them out, the extended characters are printed out as question marks. If I use show on them (which is what I want to do in this case) they are printed as \NNNNN... (presumably the Unicode number). Google brought up hundreds of topics for "Haskell utf-8" but its like looking for a single tree in a forest. I took great care to ensure my Gentoo Linux system is UTF-8 ready, and I can type or echo utf-8 characters on the command line just fine. # locale LANG=en_US.UTF-8 LC_CTYPE="en_US.UTF-8" LC_NUMERIC="en_US.UTF-8" LC_TIME="en_US.UTF-8" LC_COLLATE="en_US.UTF-8" LC_MONETARY="en_US.UTF-8" LC_MESSAGES="en_US.UTF-8" LC_PAPER="en_US.UTF-8" LC_NAME="en_US.UTF-8" LC_ADDRESS="en_US.UTF-8" LC_TELEPHONE="en_US.UTF-8" LC_MEASUREMENT="en_US.UTF-8" LC_IDENTIFICATION="en_US.UTF-8" LC_ALL= -- frigidcode.com theologia.indicium.us

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How does one uninstall a Hackage package? :)
by KC 27 May '11

27 May '11

-- -- Regards, KC

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How to think like a functional programmer?
by Costello, Roger L. 26 May '11

26 May '11

Hi Folks, I am working on a project and am trying hard to write the code in a functional style. Sadly, upon reviewing what I've written, I realize that I am not succeeding. I just devised a recipe for solving the problem and then created functions corresponding to steps in the recipe. I am still thinking imperatively, not functionally. Is there a book or article that describes how to approach problems from a functional mindset? How did you "flip the switch" in your brain to the functional mindset? /Roger

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How to make a kinder, gentler "fail"?
by Sean Perry 26 May '11

26 May '11

This is probably a FAQ of sorts, but I found composing the proper search terms complicated. When I write simple parsers and the like I tend to prefer returning useful error strings instead of simply Nothing from Maybe. For example this is a common utility function of mine. Yes, I know with the addition of a Read n qualification I can make this more generic but it does not help with the current discussion. getNum :: String -> Either String Int getNum n = case reads n of [(x, "")] -> Right x [(x, cs)] -> Left $ "incomplete parse: " ++ cs cs -> Left $ "invalid number: " ++ cs But I would rather write the following so I am not bound to Either. This would even work with Maybe since Nothing just drops the string from fail. getNum :: Monad m => String -> m Int getNum n = case reads n of [(x, "")] -> return x [(x, cs)] -> fail $ "incomplete parse: " ++ cs cs -> fail $ "invalid number: " ++ cs Yeah, I know, no one likes the fact that fail raises an exception. What I would like to do in my code is define something like class (Monad a) => MonadGentle a where myreturn = return myfail s = fails s But I can not get an instance of this to compile because it insists myreturn and myfail are not visible. Since this comes up a lot in the tutorials and books, I am curious why there is not something like MonadGentle in Hackage or the libs. I use mzero occasionally, but as I said I usually prefer some information with my errors since it makes for more human usable results. Thanks.

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Re: [Haskell-beginners] How to make a kinder, gentler 'fail'?
by Sean Perry 26 May '11

26 May '11

Yes, this sounds like what I want but I can not get it to work how I want (-: Monad Reader 15 is where this is discussed. Apparently MonadFailure has been replaced with Failure. >From M.R 15 here is the example: safeHeadFailure :: Failure String m => [a] -> m a safeHeadFailure [] = failure "empty list" safeHeadFailure (x:xs) = return x safeHead' :: [a] -> Maybe a safeHead' = safeHeadFailure and sure enough, safeHead' works in Maybe. But if I change it to safeHead' :: [a] -> Either String a which is more like what will happen in real code I get: No instance for (Failure String (Either String)) arising from a use of `safeHeadFailure' Possible fix: add an instance declaration for (Failure String (Either String)) In the expression: safeHeadFailure In an equation for `safeHead'': safeHead' = safeHeadFailure Yes, I installed control-monad-failure-mtl and yes I have a 'import Control.Monad.Error' directive in the module. This is straight from the article: safeHeadPair' :: Failure String m => [a] -> [b] -> m (a, b) safeHeadPair' [] [] = failure "both lists empty" safeHeadPair' [] _ = failure "first list empty" safeHeadPair' _ [] = failure "second list empty" safeHeadPair' (x:xs) (y:ys) = return (x, y) safeHead2 :: [a] -> [b] -> Either String (a, b) safeHead2 = safeHeadPair' I get the same failure here. What am I missing? On Thu May 26 11:24 , Elvio Toccalino sent: >Well, there's the 'failure' package. Another package builds on top of >it, the control-monad-failure (easier to use, maybe). >I read in the monad reader (don't remember the issue number) about it, >and have used it ever since. The idea is to let the instanced monad >decide what a failure means. >Check it out, and comment back. > >Cheers. > >On Thu, 2011-05-26 at 11:15 -0700, Sean Perry wrote: >> This is probably a FAQ of sorts, but I found composing the proper search terms >> complicated. >> >> When I write simple parsers and the like I tend to prefer returning useful error >> strings instead of simply Nothing from Maybe. >> >> For example this is a common utility function of mine. Yes, I know with the >> addition of a Read n qualification I can make this more >> generic but it does not help with the current discussion. >> >> getNum :: String -> Either String Int >> getNum n = case reads n of [(x, "")] -> Right x >> [(x, cs)] -> Left $ "incomplete parse: " ++ cs >> cs -> Left $ "invalid number: " ++ cs >> >> But I would rather write the following so I am not bound to Either. This would >> even work with Maybe since Nothing just drops the string from fail. >> >> getNum :: Monad m => String -> m Int >> getNum n = case reads n of [(x, "")] -> return x >> [(x, cs)] -> fail $ "incomplete parse: " ++ cs >> cs -> fail $ "invalid number: " ++ cs >> >> Yeah, I know, no one likes the fact that fail raises an exception. What I would >> like to do in my code is define something like >> >> class (Monad a) => MonadGentle a where >> myreturn = return >> myfail s = fails s >> >> But I can not get an instance of this to compile because it insists myreturn and >> myfail are not visible. >> >> Since this comes up a lot in the tutorials and books, I am curious why there is >> not something like MonadGentle in Hackage or the libs. I use mzero occasionally, >> but as I said I usually prefer some information with my errors since it makes for >> more human usable results. >> >> Thanks. >> >> >> _______________________________________________ >> Beginners mailing list >> Beginners(a)haskell.org >> http://www.haskell.org/mailman/listinfo/beginners > >

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how does the State monad actually work?
by Sean Perry 26 May '11

26 May '11

I am trying to understand how the State ends up in the function. foo :: State Int String foo = do tmp <- get put (tmp + 1) return "blah" when invoked as print $ evalState foo 0 I get "blah" as expected. When I switch to execState I get the value 1. This is what confuses me. Where is the State coming from if it is not passed into foo? What are get and put operating on? Desugared foo becomes: (get >>= \tmp -> put (tmp + 1)) >> return "blah" Which shows my confusion perfectly. Neither get nor put take a parameter. Why is that? How does return know to include them in the new State value? Especially considering the use of ">>". Obviously I can use State instances following examples like this but I can not reason with them since there is still a feeling of magic to them. Thanks.

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