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How to unify these three types with identical structures into one definition?
by ducis 16 Dec '18

16 Dec '18

Hello, Say I have a certain type for parse trees: data Expr = Var String | Enclosed String Expr String | Prefix Expr Expr | Ternary Expr String Expr String Expr deriving (Show,Eq,Ord,Generic)--,Typeable) Then I want a new type where every occurence of "Expr" definition changed to "[Expr]": data ExprL = VarL String | EnclosedL String [Expr] String | PrefixL [Expr] [Expr] | TernaryL [Expr] String [Expr] String [Expr] deriving (Show,Eq,Ord,Generic) Sometimes, I also want to use a DList instead of list. data ExprD = VarD String | EnclosedD String (DList Expr) String | PrefixD (DList Expr) (DList Expr) | TernaryD (DList Expr) String (DList Expr) String (DList Expr) deriving (Show,Eq,Ord,Generic) They have exactly the same structure, is there a way to unify the three definitions into one? Furthurmore, is it possible to generalise the latter two for all Functors ? Thanks, ducis

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fmap use
by Damien Mattei 14 Dec '18

14 Dec '18

hello again, another question with Haskell, again with my database but that is again related to Monads: i have this code in main: rows <- query_ conn "SELECT Nom,distance FROM AngularDistance WHERE distance > 0.000278" let lg = Prelude.length rows let tst = if lg > 1 then "some" else "one or zero" it compiles and i assume it works, and i have a new (for those that have follow my previous posts) function like this one: -- this function will return the list of N°BD from WDS for a given name -- note: there could be multiple result for a given name in WDS due to components getBD_WDS :: Connection -> String -> IO Float getBD_WDS conn name = trace "Entering getBD_WDS" noBDfp where qry_head = "select DNUM from WDS where DISC = ?" :: Query bd_rows :: IO [Only Text] bd_rows = query conn qry_head (Only (name::String)) lg = fmap Prelude.length bd_rows -- tst = if (fmap lg) > 1 then "some" else "one or zero" noBDtxt :: IO Text -- noBDtxt = trace "lg " (fmap (fromOnly . Prelude.head) bd_rows) noBDtxt = (fmap (fromOnly . Prelude.head) bd_rows) -- noBDstr :: String -- noBDstr = Text.unpack noBDtxt noBDstr :: IO String noBDstr = fmap Text.unpack noBDtxt -- noBDfp = read $ noBDstr :: Float noBDfp :: IO Float noBDfp = trace "Exiting getBD" (fmap read noBDstr) the problem is that first i noticed that in a function to make working the same code than in Main i had to had almost always fmap before the line, i admit ,even if i'm teacher in functional programming in Scheme at University, i begin in Haskell and i don't always understand what i'm using, i try to learn it by instinct even if sometimes i read pages about Functor and Monads i have no time to get in the Mathematics theoretical knowledge behind Haskell, i try to learn by getting some solutions in Haskell-cafe and it works, i know now a lot more about Haskell than 2 weeks ago,so.... if someon could explain me a few more about fmap or just give what i must place in my function instead of let lg = Prelude.length rows let tst = if lg > 1 then "some" else "one or zero" i replace this in the function by: lg = fmap Prelude.length bd_rows OK for compilation, but then tst = if lg > 1 then "some" else "one or zero" does not compile: *Main> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:80:18: error: • No instance for (Ord (IO Int)) arising from a use of ‘>’ • In the expression: lg > 1 In the expression: if lg > 1 then "some" else "one or zero" In an equation for ‘tst’: tst = if lg > 1 then "some" else "one or zero" | 80 | tst = if lg > 1 then "some" else "one or zero" | ^^^^^^ UpdateSidonie.hs:80:23: error: • No instance for (Num (IO Int)) arising from the literal ‘1’ • In the second argument of ‘(>)’, namely ‘1’ In the expression: lg > 1 In the expression: if lg > 1 then "some" else "one or zero" | 80 | tst = if lg > 1 then "some" else "one or zero" | ^ Failed, no modules loaded. and even the "fmap miraculous function" as i call it no more works: tst = if (fmap lg) > 1 then "some" else "one or zero" Prelude> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:80:24: error: • Couldn't match expected type ‘a0 -> b0’ with actual type ‘IO Int’ • In the first argument of ‘fmap’, namely ‘lg’ In the first argument of ‘(>)’, namely ‘(fmap lg)’ In the expression: (fmap lg) > 1 | 80 | tst = if (fmap lg) > 1 then "some" else "one or zero" | ^^ Failed, no modules loaded. a solution to this proble i think , will help me a lot, i sticked with problems in haskell i would have already solved in Scheme or Python, that's really discouraging me of using Haskell, i'm obstinating myself because i really think it's a great language that push the programming to high levels of abstractions but for now it's a bit a nightmare, i must admit it... -- Damien.Mattei(a)unice.fr, Damien.Mattei(a)oca.eu, UNS / OCA / CNRS

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MonadThrow, MonadReader and shake
by PICCA Frederic-Emmanuel 14 Dec '18

14 Dec '18

Hello , I would like your opinion, about this thread. http://mail.haskell.org/pipermail/beginners/2018-December/018403.html I do not know how to solve the issue. thanks Frederic

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Announce: Haskell Platform 8.6.3
by Gershom B 13 Dec '18

13 Dec '18

On behalf of the Haskell Platform team, I'm happy to announce the release of Haskell Platform 8.6.3 Now available at https://www.haskell.org/platform/ This includes GHC 8.6.3, cabal-install 2.4.1.0, and stack 1.9.3. This is the first platform released in the 8.6 series, as we have waited until a number of bugfix ghc releases stabilized things across all core platforms (linux, os x, windows). A full list of contents is available at https://www.haskell.org/platform/contents.html The list of GHC changes is available at: https://ghc.haskell.org/trac/ghc/blog/ghc-8.6.1-released https://ghc.haskell.org/trac/ghc/blog/ghc-8.6.2-released https://ghc.haskell.org/trac/ghc/blog/ghc-8.6.3-released A list of cabal changes is available at: https://hackage.haskell.org/package/cabal-install-2.4.1.0/changelog A list of stack changes is available at: https://docs.haskellstack.org/en/stable/ChangeLog/#v193 There are a number of important changes in this release, with more changes planned in the future. First: Win32 builds are working again and provided. Thanks to all the folks (especially Tamar) who helped sort out the build issues on that platform. Second, only core builds are provided, not "full" builds. This release is the first one where cabal-install warns on legacy commands and asks users to either use the v1-prefix for them or the v2/new prefix to move to the new-build system. As such, providing additional global packages outside of the core set now makes even less sense than in the past, where we had been already discouraging it for some time. Finally, there are no linux generic builds provided, and instead we recommend use of the ghcup tool (https://github.com/haskell/ghcup/) in combination with the stack install script. We feel this gives a smoother and better experience than the existing install process, being less invasive (not requiring root) and more flexible (by running ghc's own configure script it can better detect differences in configuration between distros). What does this all mean for the future of the platform? What I would like to move towards is the following. First: replacing the mac installer by ghcup as well in the near future. While a native installer has its advantages, the same reasons that ghcup is recommended on linux hold for mac as well, although with somewhat less force. Because of how Windows works, the difficulties of moving from a native installer are much more real, and we would anticipate keeping a native installer for the time being. Second: with the platform installers (or recommended installers) now really a delivery mechanism for core binaries and nothing else, to move to split the platform into two components. A) a set of recommended install instructions for major platforms (and a native windows installer), and B) a set of recommended and known-compatible packages which cover most "extended standard-lib" bases and which we again grow much more freely, as in the past. This will require some redesign and reconceptualization of the website, and would be a great opportunity for people that want to chip-in to move things forward to get involved. Please reach out if you'd like to lend a hand! Happy Haskell Hacking all, Gershom

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Call for Haskell.org Committee Nominations
by George Wilson 13 Dec '18

13 Dec '18

Dear Haskellers, It is time to put out a call for new nominations (typically but not necessarily self-nominations) to the haskell.org committee. We have one member of our committee due for retirement -- our chair Gershom Bazerman. To nominate yourself, please send an email to committee at haskell.org by the 7th of January, 2019. Retiring members are eligible to re-nominate themselves. Please feel free to include any information about yourself that you think will help us to make a decision. The Haskell.org committee serves as a board of directors for Haskell.org, a 501(c)3 nonprofit which oversees technical and financial resources related to Haskell community infrastructure. Being a member of the committee does not necessarily require a significant amount of time, but committee members should aim to be responsive during discussions when the committee is called upon to make a decision. Strong leadership, communication, and judgement are very important characteristics for committee members. The role is about setting policy, providing direction/guidance for Haskell.org infrastructure, planning for the long term, and being fiscally responsible with the Haskell.org funds (and donations). As overseers for policy regarding the open source side of Haskell, committee members must also be able to set aside personal or business related bias and make decisions with the good of the open source Haskell community in mind. We seek a broad representation from different segments of the Haskell world -- including but not limited to those focused on education, those focused on industrial applications, those with background in organizing users-groups, and those focused directly on our technical infrastructure. More details about the committee's roles and responsibilities are on https://wiki.haskell.org/Haskell.org_committee If you have any questions about the process, please feel free to e-mail us at committee at haskell.org, or contact one of us individually. Regards, George Wilson

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ZuriHac 2019 takes place 14 to 16 June in Zurich, Switzerland
by Jasper Van der Jeugt 12 Dec '18

12 Dec '18

Hello Friends of Haskell, It is our great pleasure to announce that the next ZuriHac will take place from Friday the 14th to Sunday the 16th of June 2019. It will be hosted at the Hochschule Rapperswil on the shores of beautiful Lake Zurich. This will mark the 8th anniversary of ZuriHac since our beginnings in 2010. The Zurich Haskell Hackathon is a free, international, grassroots, and collaborative coding festival. Our goal is to connect Haskellers, expand the community, learn things from each other, and to work on Haskell libraries, tools, and infrastructure. This year, we will enjoy keynotes from: - Simon Peyton Jones - Susan Potter - Richard Eisenberg - Ryan Trinkle More keynote speakers will be announced. The event is open to any experience level, from beginners to gurus. We want to make a special effort to ensure that the event is welcoming and accessible to people completely new to Haskell. That is why we are super excited that, Julie Moronuki, co-author of Haskell Programming from first principles [1] and Joy of Haskell [2], has kindly agreed to teach a beginners course in one of the classrooms we have available. Additionally, there will be mentors that you can approach during the whole event with any Haskell-related questions. This is a great opportunity to meet your fellow Haskellers in real life, find new contributors for your project, improve existing libraries and tools or even start new ones! We will have space for over 400 attendees. Registration is free and will open on 26th of December, as a late holiday present to Haskellers around the globe. You can find this information and more on our website: https://zfoh.ch/zurihac2019 We would also like to thank our sponsors Digital Asset [3], DFINITY [4] and HSR [5] for their strong commitment to the Haskell community and for supporting this great event! Looking forward to seeing you there, The Zurich Friends of Haskell association [6] [1]: https://www.goodreads.com/book/show/25587599-haskell-programming [2]: https://joyofhaskell.com/ [3]: https://digitalasset.com/careers.html [4]: https://dfinity.org/jobs [5]: https://www.hsr.ch/ [6]: https://zfoh.ch/

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Fwd: [Haskell-beginners] monad and variable result
by Damien Mattei 12 Dec '18

12 Dec '18

i'v got no solution from Haskell-beginners... perheaps is there some expert here? -------- Message transféré -------- Sujet : [Haskell-beginners] monad and variable result Date : Mon, 10 Dec 2018 11:32:23 +0100 De : Damien Mattei <mattei(a)oca.eu> Répondre à : The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell <beginners(a)haskell.org> Pour : The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell <beginners(a)haskell.org> have some code that works but want to put it in a simple function without sucess: getBD :: Connection -> String -> Float getBD conn name = noBDfp where qry_head = "select `N° BD` from sidonie.Coordonnées where Nom = ?" :: Query bd_rows = do local_bd_rows <- query conn qry_head (Only (name::String)) return local_bd_rows i want the variable local_bd_rows accessible in the 'where' clause how can i do that? note the goal is to do the same thing of my main function that works : main :: IO () --main :: Int main = do conn <- connect defaultConnectInfo { connectHost = "moita", connectUser = "mattei", connectPassword = "******", connectDatabase = "sidonie" } -- we get all the Double Stars that have angular distance superior to a threshold of 1 second = 0.000278 degree rows <- query_ conn "SELECT Nom,distance FROM AngularDistance WHERE distance > 0.000278" forM_ rows $ \(name,distance) -> putStrLn $ Text.unpack name ++ " " ++ show (distance :: Double) -- we will get the Durchmusterung Number BD from Sidonie and WDS and compare them for a given name -- Warning: there could be multiple result in WDS for a given entry name (due to components) -- first we get the N°BD from sidonie let name = "A 20" -- let qry = "select `N° BD` from Coordonnées where Nom = " ++ name let qry_head = "select `N° BD` from sidonie.Coordonnées where Nom = ?" :: Query -- bd_rows <- query_ conn "select `N° BD` from sidonie.Coordonnées where Nom = 'A 20'" bd_rows <- query conn qry_head (Only (name::String)) putStrLn $ show bd_rows putStrLn $ show name let resLst = Prelude.map fromOnly bd_rows let noBDtxt = fromOnly (Prelude.head bd_rows) :: Text -- let noBD2 = _ (Prelude.head bd_rows) putStrLn $ show resLst putStrLn $ show noBDtxt forM_ bd_rows $ \(Only a) -> putStrLn $ Text.unpack a let noBDstr = Text.unpack noBDtxt :: String let noBDfp = read $ noBDstr :: Float putStr "noBDfp =" (putStrLn (show noBDfp)) close conn print "Exit." for now i have errors in the function: Prelude> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:47:28: error: • Ambiguous type variable ‘r0’ arising from a use of ‘query’ prevents the constraint ‘(QueryResults r0)’ from being solved. Relevant bindings include bd_rows :: IO [r0] (bound at UpdateSidonie.hs:46:9) Probable fix: use a type annotation to specify what ‘r0’ should be. These potential instances exist: instance Result a => QueryResults (Only a) -- Defined in ‘Database.MySQL.Simple.QueryResults’ instance (Result a, Result b) => QueryResults (a, b) -- Defined in ‘Database.MySQL.Simple.QueryResults’ instance (Result a, Result b, Result c) => QueryResults (a, b, c) -- Defined in ‘Database.MySQL.Simple.QueryResults’ ...plus 21 others (use -fprint-potential-instances to see them all) • In a stmt of a 'do' block: local_bd_rows <- query conn qry_head (Only (name :: String)) In the expression: do local_bd_rows <- query conn qry_head (Only (name :: String)) return local_bd_rows In an equation for ‘bd_rows’: bd_rows = do local_bd_rows <- query conn qry_head (Only (name :: String)) return local_bd_rows | 47 | local_bd_rows <- query conn qry_head (Only (name::String)) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Failed, no modules loaded. -- Damien.Mattei(a)unice.fr, Damien.Mattei(a)oca.eu, UNS / OCA / CNRS _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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Re: [Haskell-cafe] Faster set intersections?
by Olaf Klinke 11 Dec '18

11 Dec '18

> Note that a concrete set "concretizes" anything it touches. Don't take > unions of these sets, though, it'll just be a mess. > > > Won't a union just be the same as intersection but using || instead of && ? > > > -Jan-Willem Maessen Unions of predicates and concrete sets are easy, thanks to Set.member: union (Pred p) (Concrete s) = Pred (\k -> p k || member k s) What you can not do, of course, is enumerate and fold these sets. There is a set type [1] which supports a litte bit more: Set a = Maybe ((a -> Bool) -> a) It has unions, intersections and a Monad instance and can represent infinite sets. If the base type has an Ord instance, the set can be enumerated. If the base type has an Eq instance, so has (Set a). Some functions usually implemented using Foldable are also possible, e.g. minimum and maximum. Caveat: Performance can be poor, depending on how the function inside the set was defined. Cheers, Olaf [1] http://hackage.haskell.org/package/infinite-search

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Is unfoldr too strict?
by Isaac Elliott 11 Dec '18

11 Dec '18

I was reading this article https://wiki.haskell.org/Correctness_of_short_cut_fusion on the Haskell wiki. It presents an example (2.1.2) where destroy/unfoldr fusion behaves oddly. If I use a lazier definition of unfoldr, then this problem goes away: unfoldr :: (b -> Maybe (a, b)) -> b -> [a] > unfoldr f b = > case f b of > Nothing -> [] > Just z -> fst z : unfoldr f (snd z) Does this mean unfoldr is 'too strict'? Or is there a good reason for not writing it this way (performance, perhaps?)

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References for topological arguments of programs?
by Siddharth Bhat 11 Dec '18

11 Dec '18

Hello, I was recently intrigued by this style of argument on haskell cafe: One can write a function Eq a => ((a -> Bool) -> a) -> [a] that enumerates the elements of the set. Because we have universal quantification, this list can not be infinite. Which makes sense, topologically: These so-called searchable sets are topologically compact, and the Eq constraint means the space is discrete. Compact subsets of a discrete space are finite. ------- I've seen arguments like these "in the wild" during Scott topology construction and in some other weird places (hyperfunctions), but I've never seen a systematic treatment of this. I'd love to have a reference (papers / textbook preferred) to self learn this stuff! Thanks Siddharth -- Sending this from my phone, please excuse any typos!

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