- Beginners - Haskell.org

Providing Haskell Libraries wrapped for other Languages
by Leonhard Applis 28 Nov '19

28 Nov '19

Hi, I am currently writing a small library. I will provide a small web-api for it, however I have some colleagues who neither have experience in haskell, nor in web-dev. Also they maybe want to produce programs using my library without internet / docker / etc. So in addition I'd like to provide the library "wrapped" for other languages, especially python, but maybe java as well. These wrappers should do nothing except invoking their pardons on the library, and should somewhat contain the library. What is the common way to do this? I've seen "hyphen" as a super heavyweight solution, and I've seen people starting the haskell library listening on system-pipes. I don't feel that these are the "best" solutions, atleast for my intention. best regards Leonhard

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list doesn't end
by Alexander Chen 25 Nov '19

25 Nov '19

Hi, I am doing some further learning with euler now in Haskell instead of Julia. But I am getting something strange. [x+y+z | x <- a2, y <- b3, z <- c4, (x+y+z) < 50] a2, b3, c4 are all finite lists derived from the lazy list of the primes package. the output is: [28,47,33,49 this is not a typo it is still calculating hence the missing bracket (for the last couple of hours, when to a christmas fair in between). The only thing is that, this should be the answer: [28,47,33,49] so it should be done but its not. what am I doing wrong? best,

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Re: [Haskell-beginners] Configuration of Environment
by Leonhard Applis 22 Nov '19

22 Nov '19

> Hi All, > > I’m a newcomer to Haskell and mailing list, so I am not sure if my question is proper. > > I use notepad++ to learn Haskell now. Are there any suggestions or conventions which can help me learn Haskell? > > Best regards, > Juncheng Hi Juncheng, I think for learning "code", as in writing nice code, get yourself the Hlinter [1] running. It really spots great improvements and some very haskelly ways. For Learning purposes I would suggest that you first "try it your way" step by step, and check every step with hlint. So: fun1 -> Hlint -> fun2 -> Hlint Additionally you might want to enable a spell-checker (like, a normal one, for english). Last thing you might find very useful is a local Hoogle [2] install. It's amazingly convenient if you want to know, e.g. the function which gives a length of a list, but you don't remember if it was len, length, size, count ... So you just search for [a] -> Int and voila. Second Hoogle can help you find similar functions to what you are currently building, often you are implementing something that is somewhere as a higher order function already. best regards, Leonhard [1] Hlint: http://hackage.haskell.org/package/hlint [2] Hoogle: http://hackage.haskell.org/package/hoogle

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Configuration of Environment
by juncheng_hsk＠outlook.com 22 Nov '19

22 Nov '19

Hi All, I’m a newcomer to Haskell and mailing list, so I am not sure if my question is proper. I use notepad++ to learn Haskell now. Are there any suggestions or conventions which can help me learn Haskell? Best regards, Juncheng

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How to use the Data.Map inbuild Monoid
by Leonhard Applis 18 Nov '19

18 Nov '19

Hello Francesco, > `Map k v` is already an instance of `Monoid` (when `v` is an instance of `Ord`), are you sure you need to write another one? Thanks for your answer! My "Values" is a Monoid, however it seems that the Graph cannot properly be seen correctly as a Monoid using mconcat does not (properly) work, meaning that <> is not correctly applied to the someMonoidsso instead of type G = Map Text someMonoid mconcat [gs] I have to write: foldr (Map.unionWith (<>)) Map.empty [gs] This passes the tests. Same Problem with g1 <> g2 --(Does not work properly) and Map.unionWith (<>) g1 g2 --(Does work) I have declared someMonoid myself, do I need to declare something special about it? I feel like my G is messing up with <> being about him, or about someMonoid A broken down piece of code is: importData.MapasMap dataSum=SumIntderiving (Eq,Show) instanceSemigroupSumwhere (<>) (Sum a) (Sum b)=Sum(a + b) instanceMonoidSumwhere mempty =Sum0 typeG=Map.MapStringSum And to verify my problem: GHCI > v = Map.singleton "A" (Sum 1) GHCI > u = Map.singleton "A" (Sum 1) GHCI> c= v <> u GHCI> Map.lookup "A" c Just (Sum 1) but I want Just (Sum 2) thanks Leonhard

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Export two modules from one.hs file?
by Leonhard Applis 18 Nov '19

18 Nov '19

Hey All, I currently want to make a module, which "truly" exposes 3 functions. However, there are more functions which I want to test. I would like to export two modules from the same file, "ModuleToUse" and "ModuleOpen". Then I can use ModuleToUse everywhere else, and ModuleOpen in the UnitTests. I know I can declare ModuleOpen, and export only certain functions with ModuleToUse. But I am not sure, if that is a smart / normal thing to do. In my case this would double my modules, and ... hence files? What is the "real world" approach for this? Thanks Leonhard

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importing Data.Digits
by Alexander Chen 18 Nov '19

18 Nov '19

Hi All, This is my first 'post' not quite sure if I am doing it right. I have a question about the importing of modules that are available on the Hackage.Haskell site: http://hackage.haskell.org/package/digits if I write this in my wordpad and then load it to GCI i get the following error: chapter4.hs:1:1: error: Could not find module ‘Data.Digits’ Perhaps you meant Data.Bits (from base-4.12.0.0) Use -v to see a list of the files searched for. | 1 | import Data.Digits | ^^^^^^^^^^^^^^^^^^ However if I load Data.Char than it loads fine. What am I doing wrong? best,

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Re: [Haskell-beginners] Expose every Function of a wrapped type
by 鲍凯文 16 Nov '19

16 Nov '19

Hi, Have you considered using `Control.Lens.Wrapped` or `Control.Newtype`? You'd have to move the wrapping/unwrapping bits to the sites in your code at which you use underlying `Map` functions (quite repetitive), but it could be easier than redefining all the functions you want (and definitely easier than having to redefine a new one when you think of needing it?). There's probably a canonical way to handle this sort of thing, but I can't find it ATM. Good luck! On Fri, Nov 15, 2019 at 5:00 AM <beginners-request(a)haskell.org> wrote: > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. Expose every Function of a wrapped type (Leonhard Applis) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Fri, 15 Nov 2019 11:36:03 +0000 > From: Leonhard Applis <Leonhard.Applis(a)protonmail.com> > To: "beginners(a)haskell.org" <beginners(a)haskell.org> > Subject: [Haskell-beginners] Expose every Function of a wrapped type > Message-ID: > > <8VyrlRTZwYrqqkI95kJ3UHQ41YSXFwhcqA4orOfVyau1ULCyCnVVkPI__Ya6rLaYFCUrVB3yM0IqT_l5NcCiMHhyfjsRg0S2xmxG3Cmvkq0=@ > protonmail.com> > > Content-Type: text/plain; charset="utf-8" > > Hi, > > I currently have a newtype definition of a typed Data.Map. > > newtype G = G Data.Map String Values > > I have done this because for my specific use-case, I can define G as a > monoid and get some nice benefits. > > However, I find myself often just re-declaring functions of Data.Map and > wrap/unwrap them in my Constructor G, such as > > singleton :: String -> Value -> G > singleton s v = G $ (Data.Map.singleton s v) > > I think I do not need to limit the scope of G's functionality, I basically > want everything from Data.Map + Bonus , like my Monoid. > Also I want G to be a specific Data.Map, there is never any other case > where it's not a Map String Values. > > However, if I have > > type G2 = Data.Map String Values > > I cannot instance Monoid. > > So, I am very sure that I'm not the first person ever doing this. Is there > a language extension providing what I want? Or is what i want "bad"? > Am I just missing a higher-order base function which does what I want? > > best regards & thanks > Leonhard >

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Expose every Function of a wrapped type
by Leonhard Applis 15 Nov '19

15 Nov '19

Hi, I currently have a newtype definition of a typed Data.Map. newtype G = G Data.Map String Values I have done this because for my specific use-case, I can define G as a monoid and get some nice benefits. However, I find myself often just re-declaring functions of Data.Map and wrap/unwrap them in my Constructor G, such as singleton :: String -> Value -> G singleton s v = G $ (Data.Map.singleton s v) I think I do not need to limit the scope of G's functionality, I basically want everything from Data.Map + Bonus , like my Monoid. Also I want G to be a specific Data.Map, there is never any other case where it's not a Map String Values. However, if I have type G2 = Data.Map String Values I cannot instance Monoid. So, I am very sure that I'm not the first person ever doing this. Is there a language extension providing what I want? Or is what i want "bad"? Am I just missing a higher-order base function which does what I want? best regards & thanks Leonhard

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What is this?
by Dimitri DeFigueiredo 13 Nov '19

13 Nov '19

Hello, Can anyone shed a light on the proper way to write the function below. It looks pretty standard, but it's not clear to me what it is? data Abortable res = Aborted | Executed res deriving Functor -- i.e. Maybe abortableThroughEither :: Abortable (Either a b) -> Either a (Abortable b) abortableThroughEither Aborted = Right Aborted abortableThroughEither (Executed (Left a)) = Left a abortableThroughEither (Executed (Right b)) = Right (Executed b) Thanks! Dimitri -- 2E45 D376 A744 C671 5100 A261 210B 8461 0FB0 CA1F

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