- Beginners - Haskell.org

Can't remove old haskell
by Lawrence Bottorff 15 May '18

15 May '18

I've followed the steps here <https://www.haskell.org/platform/linux.html#linux-generic>; however, it doesn't replace or supercede an older version of ghci from my Ubuntu 18.04: 12:47:02:~$ which ghci /opt/ghc/8.0.2/bin/ghci 12:47:14:~$ ghci --version The Glorious Glasgow Haskell Compilation System, version 8.0.2 I've tried various removes, but to no avail. The newer version is on my system: 12:50:32:/usr/local/haskell/ghc-8.4.2-x86_64/bin$ ./ghci --version The Glorious Glasgow Haskell Compilation System, version 8.4.2 But I don't know what to do about this old "default" version. LB

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Re: [Haskell-beginners] Adding 1 to Just 9
by Alex Belanger 14 May '18

14 May '18

Applicative is taking one step further, where the transformation itself is also in such container/context. <*> :: Functor => f (a -> b) -> f a -> f b <*> ~ Maybe (a -> b) -> Maybe a -> Maybe b <*> ~ Num n => Maybe (n -> n) -> Maybe n -> Maybe n And the implementation: (<*>) (Just f) (Just n) = Just (f n) (<*>) Nothing (Just n) = Just n (<*>) (Just f) Nothing = Nothing (<*>) Nothing Nothing = Nothing Thus, with everything we've learned, we should be able to deal with any situation thrown at use, using Functor or Applicative. Given (+1) and 42, I can basic function application with ($) or writing it (+1) 42.Given (+1) and Just 42, I can use fmap to apply my transformation on that functor, fmap (+1) (Just 42).Given Just (+1) and Just 42, I can use <*> to apply my transformation inside a functor (applicative functor) to another functor, Just (+1) <*> Just 42.Given Just (+1) and 42, I can wrap 42 with Just and again use <*> as earlier, Just (+1) <*> Just 42. Hope this helps. nitrix On Mon, May 14, 2018, at 9:17 AM, Alex Belanger wrote: > A first approximative intuition is to think of Functors as containers > (or more like a context) and of fmap as a way to apply a > transformation function of your choice on the contained value, > respecting the signification of that context.> > For example, Maybe represents the possibility of having or not having > a value, therefore, fmap will apply your transformation on that value > if it exists, otherwise you're still left with nothing.> > This example might seem straight forward but it had to be defined > somewhere, thus, made possible by the Functor instance implementation > for the type Maybe.> > Let's have a look at it: > > fmap :: Functor f => (a -> b) -> f a -> f b > > Specialized: > > fmap ~ (a -> b) -> Maybe a -> Maybe b > > And concretize further: > > fmap ~ Num n => (n -> n) -> Maybe n -> Maybe n > > As you can see, given a transformation function and maybe some > numeral, you'll get maybe another numeral.> > The implementation lools like this: > > fmap f (Just n) = Just (f n) > fmap f Nothing = Nothing > > Thus, starting with Nothing, we cannot apply our tranformation so you > stay with Nothing. Similarly, with Just n, we're able to pattern match > to obtain that n, apply our transformation f on that n, and then > rewrap everything back into Just.> > You can see how the value cannot escape its container / context. > > Of course there are more complex such containers / context. > > Either represents a choice between two values. [] contains multiple > values, IO contains (side-)effects, and so on.> > Hope this helps. > > nitrix > > On Mon, May 14, 2018, 08:18 David McBride <toad3k(a)gmail.com> wrote: >> let foo = \x -> Just (x + 1) >> fmap foo (Just 9) >> >> Just (Just 10) >> >> >> On Mon, May 14, 2018 at 8:15 AM, Olumide <50295(a)web.de> wrote: >>> Dear List, >>> >>> Chapter 14 of LYH appears to suggest that a Just value can be added >>> to an Int. Quote from >>> http://learnyouahaskell.com/for-a-few-monads-more#useful-monadic-functions>>> >>> "For instance, say we have Just 9 and the function \x -> Just >>> (x+1). If we map this function over Just 9, we're left with Just >>> (Just 10).">>> >>> I've tried the following in ghci but got the error: >>> >>> <interactive>:12:1: error: >>> • Non type-variable argument in the constraint: Num (Maybe a) >>> (Use FlexibleContexts to permit this) >>> • When checking the inferred type >>> it :: forall a. (Num (Maybe a), Num a) => Maybe a >>> >>> >>> Am I reading the quote wrong? Is Just (Just 10) a hypothetical? >>> >>> Regards, >>> >>> - Olumide >>> _______________________________________________ >>> Beginners mailing list >>> Beginners(a)haskell.org >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners >> >> _______________________________________________ >> Beginners mailing list >> Beginners(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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Adding 1 to Just 9
by Olumide 14 May '18

14 May '18

Dear List, Chapter 14 of LYH appears to suggest that a Just value can be added to an Int. Quote from http://learnyouahaskell.com/for-a-few-monads-more#useful-monadic-functions "For instance, say we have Just 9 and the function \x -> Just (x+1). If we map this function over Just 9, we're left with Just (Just 10)." I've tried the following in ghci but got the error: <interactive>:12:1: error: • Non type-variable argument in the constraint: Num (Maybe a) (Use FlexibleContexts to permit this) • When checking the inferred type it :: forall a. (Num (Maybe a), Num a) => Maybe a Am I reading the quote wrong? Is Just (Just 10) a hypothetical? Regards, - Olumide

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Re: [Haskell-beginners] EasyPlot on Windows
by Gregory Guthrie 11 May '18

11 May '18

Thanks; good idea - but nope. I'd file a report to EasyPlot, but not sure where to do that. Older discussions on this issue note that there is a "persist" option for gnuplot, but I didn't yet see how to pass that to try it. -----Original Message----- My first hunch was that, since the program ends right after plotting, it cleans up the plot window and exits. But trying it on an X11 system, I got a persistent window despite the program exiting. That may be a difference between Unix and Windows child process handling; I don't have a Windows machine handy right now to test, but you could try this: main = do plot Windows $ Data2D [Title "Demo Title"] [] tData _ <- getLine return () If that doesn't fix it, my only guess is there's either a difference between the Unix and Windows versions of gnuplot, or a bug in easyplot's Windows handling.

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Graph path
by mike h 11 May '18

11 May '18

Does anyone know of a good description of the algorithm to visit (if possible) each node in a graph? I can find a few links that describe the imperative way of doing it but I struggle converting to functional style :) Many Thanks Mike

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EasyPlot on Windows
by Gregory Guthrie 10 May '18

10 May '18

A very simple test program with Graphics.EasyPlot, flashes up a window, but too fast to see it. There are several old posts on this issue with gnuplot, but I haven't seen any current solutions. Example: import Graphics.EasyPlot tData = [ (1,2), (2,3), (3,1), (4,5), (5,3), (6,1) ] main = do plot Windows $ Data2D [Title "Demo Title"] [] tData

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Join'ing a State Monad, example from LYH
by Olumide 09 May '18

09 May '18

Dear List, I've been stumped for a few months on the following example, from chapter 13 of LYH http://learnyouahaskell.com/for-a-few-monads-more#useful-monadic-functions runState (join (State $ \s -> (push 10,1:2:s))) [0,0,0] I find the following implementation of join in the text is hard to understand or apply join :: (Monad m) => m (m a) -> m a join mm = do m <- mm m In contrast, I find the following definition(?) on Haskell Wikibooks https://en.wikibooks.org/wiki/Haskell/Category_theory#Monads join :: Monad m => m (m a) -> m a join x = x >>= id easier to understand, and although I can apply it to the following Writer Monad example, in the same section of LYH, runWriter $ join (Writer (Writer (1,"aaa"),"bbb")) I cannot apply it to the State Monad example. Regards, - Olumide

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Efficient Binary Multiplication
by Quentin Liu 28 Apr '18

28 Apr '18

Hi all, Suppose I want to multiply two binary numbers whose representation uses lists (e.g. 14 is represented as [1, 1, 1, 0]). Is there any efficient way to do binary multiplication? The way I could come up with involves a lot of intermediate lists that will be discarded eventually and is extremely inefficient. I know one fast algorithm that uses Array but would it be possible to do the multiplication with only lists? Regards, Qingbo Liu

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phantom type
by PICCA Frederic-Emmanuel 18 Apr '18

18 Apr '18

Hello, Here my problem. I extract some information from a database and put them into a type with 50 parameters Type MyData = MyData Int String ... But I need to be able to differenciate two different case. depending on the String content. something like if the String start with "ref-" it is a Caracterization and if not it is a Collect So I end up with this solution data Unchecked data Caracterization data Collect data MyData t = MyData Int String ... now I defined a function getMyDataFromDatabase :: ... -> Mydata Unchecked and isCollect in order to verify that I have a Collect and not something Else isACollect :: MyData Unchecked -> Either Text (MyData Collect). which use coerc in order to avoid copying all the members. It is great. now Here is my problem. I create a method myMethod :: MyData Collect -> IO () but I want my method to work for MyData Collect and Mydata Caracterization but not Unchecked. So It seems to me that I can not just do myMethod :: MyData t -> IO () what is the right way to solve this problem ? thanks for your help. Frederic

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Call for Participation: Summer School on Advanced Functional Programming
by Alejandro Serrano Mena 16 Apr '18

16 Apr '18

# Call for Participation SUMMER SCHOOL ON ADVANCED FUNCTIONAL PROGRAMMING Utrecht, the Netherlands, 27-31 August 2018 http://www.afp.school ## ABOUT The Advanced Functional Programming summer school has been running for more than ten years. We aim to educate aspiring Haskell programmers beyond the basic material covered by many textbooks. The lectures will cover several more advanced topics regarding the theory and practice of Haskell programming, including topics such as: * lambda calculus; * monads and monad transformers; * lazy evaluation; * generalized algebraic data types; * type families and type-level programming; * concurrency and parallelism. The summer school consists of a mix of lectures, labs, and a busy social program. ## LECTURERS Utrecht staff: * Johan Jeuring * Alejandro Serrano Mena * Doaitse Swierstra * Wouter Swierstra Guest lectures: * Manuel Chakravarty * Koen Claessen * Gabriele Keller ## PREREQUISITES We expect students to have a basic familiarity with Haskell already. You should be able to write recursive functions over algebraic data types, such as lists and trees. There is a great deal of material readily available that covers this material. If you’ve already started learning Haskell and are looking to take your functional programming skills to the next level, this is the course for you. ## DATES Registration deadline: 1 August, 2017 School: 27-31 August ## COSTS €1700 - Housing and registration €1500 - Registration only We offer a €1000 discount for students and staff members affiliated with a university. ## SCHOLARSHIPS If you’re struggling to finance your trip to Utrecht, please let us know. We have a limited number of scholarships or discounts available for students that would not be able to attend otherwise, especially for women and under represented minorities. ## FURTHER INFORMATION Further information, including instructions on how to register, is available on our website: http://www.afp.school

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