- Beginners - Haskell.org

Handling a MonadError
by Christopher Howard 31 May '11

31 May '11

Hi. I'm trying to learn Data.ConfigFile. I want to use the function readfile: [DESCRIPTION] readfile :: MonadError CPError m => ConfigParser -> FilePath -> IO (m ConfigParser) Loads data from the specified file. It is then combined with the given ConfigParser using the semantics documented under merge with the new data taking precedence over the old. However, unlike merge, all the options as set in the old object are preserved since the on-disk representation does not convey those options. May return an error if there is a syntax error. May raise an exception if the file could not be accessed. [/DESCRIPTION] I understand that this function returns a monad, and I pull the value out of it with (<-). And I believe this is similar to a "Maybe" situation where I can do one thing if it provides a syntax error and another thing if it provides the ConfigParser. But I'm not sure what this would look like in actual code. Could somebody give me a small, simple example? -- frigidcode.com theologia.indicium.us

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Re: [Haskell-beginners] Install on "foreign" server problems
by Michael Snoyman 31 May '11

31 May '11

On Mon, May 30, 2011 at 11:53 PM, Lawrence Bottorff <galaxybeingplan9(a)gmail.com> wrote: >> Hi Lawrence >> >> I think a lot of us are finding that the easiest solution is to >> compile the binaries locally and upload to the server. In my case, >> I've tried having the compiler on a VPS, but compiling is very >> memory-intensive, and it can be a real pain whenever you need to >> upgrade libraries. >> >> My newest deployment is simply an EC2 micro instance running Ubuntu >> 32-bit, just like I have on my local system. I compile the app, copy >> the binary and the static files, and deployment is done. I'll likely >> be adding some notes about this to the Yesod book, and perhaps even >> put together some scripts to automate the process more. >> >> Michael > > So I would do an "in situ/in-place" binary install on a home machine and > simply move that whole installation directory onto the foreign machine. > Hmmm.... What about dependent libraries? If it can't find libc (or the > GL-mesa libraries) won't it break? I'm guessing from the instructions that I > would do a > # ./configure --prefix=/home/me/haskell/ > then a > # make in-place > Is this right? Then move that /haskell directory onto the foreign box? > Lb > What I meant was not to have GHC on the server at all. Build your web apps locally, and then upload the binaries to the server. You can even install a virtual machine locally with the same OS as what your server is using. Michael

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Install on "foreign" server problems
by Lawrence Bottorff 31 May '11

31 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 29 May '11

29 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 29 May '11

29 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. 27 May '11

27 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 27 May '11

27 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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