May 2011 - Beginners - Haskell.org

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 26 May '11

26 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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Large binary overhead?
by Christopher Howard 26 May '11

26 May '11

I'm still going through the GHC tutorial, so be patient with me: But why is it that my GHC-produced executables are always so large? Even my one line Hello World program ends up being over 500KB. I thought maybe it was because all of the functionality was statically compiled in, but I used ldd and saw that my hello world binary links dynamically to 10 different C libraries. (I compile on Gentoo Linux x86, if that makes a difference.) Anyway, is that just the way it is, or does this become a non-issue once I learn proper optimization/linking techniques? -- frigidcode.com theologia.indicium.us

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Quickcheck Preconditions
by Guy 25 May '11

25 May '11

I have some functions with preconditions. For example, foo::[a]->IO Bool requires a list of at least three elements, and bar::[a]->Int->IO () requires that its second argument is some position within the list. How can I test these with quickcheck? If I use ==>, quickcheck gives up with very few test cases.

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Handling Maybes
by Christopher Howard 25 May '11

25 May '11

Since I started into Haskell, I frequently run into a scenario like this: some function returns a Maybe value. If it is a Nothing, I want to do one thing, but if it isn't, I want to do something with the value encapsulated by the Maybe. But before I can use that value (say, to print it) I've got to pull it out of the Maybe in a way that satisfies the compiler. So one option is to use a separate function with pattern matching, for example: handleMaybeVal Nothing = do someAction someOtherActionEtc handleMaybeVal (Just a) = putStrLn ("The string in reverse is " ++ (reverse a)) Or I can use the maybe function with a conditional main = do val <- someIOFunc -- val is a Maybe String if val == Nothing then do someAction someOtherActionEtc else putStrLn ("The string in reverse is " ++ ((reverse . unpack) val)) where unpack a = maybe (error "unreachable error") (\b -> b) a The second option I like better, for some reason, but it is a bit dangerous, as I have to double check my code to make sure the default value passed to "maybe" cannot possibly be evaluated. (Alternatively, I could pass in a regular default value, like a blank string, though that is actually more dangerous in my mind because a bad value might be used silently.) Anyway: how do you pros go about handling situations like these? -- frigidcode.com theologia.indicium.us

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