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Understanding type variables in Haskell class declarations
by PATRICK BROWNE 21 May '18

21 May '18

I am trying to understand how to interpret type variables in Haskell class declarations from a paper <http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=1EA31D76684217BC72…> . With respect to the code below I have the following questions: 1. What is the difference between "a b" in the header of the Container class and "a b" in the signature of the class methods? Does the whitespace in header mean 2 distinct types and in the methods mean function application? 2. In the Boathouse class what is the difference between the first ocurance of "b p" and the second bracketed "(b p)"? 3. When I try to make instance of these classes I seem to need FlexibleInstances. Why is this? {-# LANGUAGE MultiParamTypeClasses #-} -- Containers a b stands for all container types a holding things of type b. -- from :info command a has kind *->*, b has kind * class Containers a b where insert :: b -> a b -> a b remove :: b -> a b -> a b whatsIn :: a b -> [b] class Surfaces a b where put :: b -> a b -> a b takeOff :: b -> a b -> a b whatsOn :: a b -> [b] -- from :info command "p" has kind *, "h" and "b" have kind *->* class People p class Containers h p => Houses h p where class (People p, Surfaces h p) => Boats h p where class (Boats b p,Houses h (b p)) => BoatHouses h b p where class (People p, Houses h (b p),Boats b p) => HouseBoats h b p -- This email originated from DIT. If you received this email in error, please delete it from your system. Please note that if you are not the named addressee, disclosing, copying, distributing or taking any action based on the contents of this email or attachments is prohibited. www.dit.ie <http://www.dit.ie/> Is ó ITBÁC a tháinig an ríomhphost seo. Má fuair tú an ríomhphost seo trí earráid, scrios de do chóras é le do thoil. Tabhair ar aird, mura tú an seolaí ainmnithe, go bhfuil dianchosc ar aon nochtadh, aon chóipeáil, aon dáileadh nó ar aon ghníomh a dhéanfar bunaithe ar an ábhar atá sa ríomhphost nó sna hiatáin seo. www.dit.ie <http://www.dit.ie/> Tá ITBÁC ag aistriú go Gráinseach Ghormáin – DIT is on the move to Grangegorman <http://www.dit.ie/grangegorman>

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merge error
by trent shipley 18 May '18

18 May '18

The below produces an error. And I am very proud that I could use the GHCi debugging tools to get this far. merge [] [] works. merge [1] [] works. I don't know why the failing example fails. It should return: [4,5] Help to unstuck is appreciated. :step merge [4,5] [] *** Exception: ex6_8.hs:(12,1)-(16,66): Non-exhaustive patterns in function merge Given: merge :: Ord a => [a] -> [a] -> [a] merge [] [] = [] merge [x] [] = [x] merge [] [y] = [y] merge first@(x:xs) second@(y:ys) | x <= y = x : merge xs second | otherwise = y : merge first ys

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ANNOUNCE: Haskell Communities and Activities Report (34th ed., May 2018)
by Mihai Maruseac 15 May '18

15 May '18

On behalf of all the contributors, we are pleased to announce that the Haskell Communities and Activities Report (34th edition, May 2018) is now available, in PDF and HTML formats: http://haskell.org/communities/05-2018/report.pdf http://haskell.org/communities/05-2018/html/report.html All previous editions of HCAR can be accessed on the wiki at https://wiki.haskell.org/Haskell_Communities_and_Activities_Report Many thanks go to all the people that contributed to this report, both directly, by sending in descriptions, and indirectly, by doing all the interesting things that are reported. We hope you will find it as interesting a read as we did. If you have not encountered the Haskell Communities and Activities Reports before, you may like to know that the first of these reports was published in November 2001. Their goal is to improve the communication between the increasingly diverse groups, projects, and individuals working on, with, or inspired by Haskell. The idea behind these reports is simple: Every six months, a call goes out to all of you enjoying Haskell to contribute brief summaries of your own area of work. Many of you respond (eagerly, unprompted, and sometimes in time for the actual deadline) to the call. The editors collect all the contributions into a single report and feed that back to the community. When we try for the next update, six months from now, you might want to report on your own work, project, research area or group as well. So, please put the following into your diaries now: ======================================== End of September 2018: target deadline for contributions to the October 2018 edition of the HCAR Report ======================================== Unfortunately, many Haskellers working on interesting projects are so busy with their work that they seem to have lost the time to follow the Haskell related mailing lists and newsgroups, and have trouble even finding time to report on their work. If you are a member, user or friend of a project so burdened, please find someone willing to make time to report and ask them to "register" with the editors for a simple e-mail reminder in November (you could point us to them as well, and we can then politely ask if they want to contribute, but it might work better if you do the initial asking). Of course, they will still have to find the ten to fifteen minutes to draw up their report, but maybe we can increase our coverage of all that is going on in the community. Feel free to circulate this announcement further in order to reach people who might otherwise not see it. Enjoy! -- Mihai Maruseac (MM) "If you can't solve a problem, then there's an easier problem you can solve: find it." -- George Polya

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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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