- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] List of nitpicks
by Johannes Waldmann 27 Aug '15

27 Aug '15

>> quickCheck $ ( (\ xs -> xs == reverse xs ) :: Eq a => [a] -> Bool) > This can be solved, by the way: > https://ro-che.info/articles/2013-02-19-smallcheck#required-type-annotations Very good indeed. But (and here's another nit to pick - about ghc) - your > smallCheck 5 $ \x y -> x == (y :: Integer) would look much clearer that way: > smallCheck 5 $ \ (x::Integer) (y :: Integer) -> x == y but ghc complains about missing ScopedTypeVariables and that error message is fully incomprehensible (e.g., to beginning students) as there is no type variable anywhere. There was a PatternSignatures option but it seems gone (deprecated). I believe that (PatternSignatures and) ScopedTypeVariables should be on by default. It is currently off only to help those that are too lazy to write "forall t . " in (inner) declarations? Leaving out quantifiers should never be encouraged. And putting type declarations next to the name declaration (that is, on the "x" under the lambda) should be the normal thing to do. Everything else feels like a work-around, and it is a distraction when teaching. Putting language options at the top of the source file, or in .ghci, is also a distraction. - J.W.

4 3

Elementary math question: primality test for huge numbers
by Christopher Howard 26 Aug '15

26 Aug '15

Hi. I was working through a math chapter explaining the RSA algorithm, and I was implementing it in Haskell for learning sake. However, the chapter does not explain how you efficiently pick the two prime numbers. I scanned through the WP page on primality tests but it was somewhat overwhelming. What would be the standard primality test for this sort of application, and is it already built in to Base somewhere? -- Biblical creationism: http://tinyurl.com/qfyeg4a Free Bible software: http://xiphos.org Software freedom: http://tinyurl.com/qjnpnsm Free computer operating system: http://tinyurl.com/7wczchu Alternative to MS Office: http://tinyurl.com/aw9p6d4

5 4

'Scientific' loses significant digits
by Alex 26 Aug '15

26 Aug '15

Hi: Does anyone know why is this true? ((fromJust . readMaybe) "1.00000000" :: Scientific) == ((fromJust . readMaybe) "1" :: Scientific) -- Alex

2 1

Munich Haskell Meeting, 2015-08-27 @ 19:30 Augustiner-Keller Biergarten
by Christian Neukirchen 25 Aug '15

25 Aug '15

Dear all, This week, our monthly Munich Haskell Meeting will take place again on Thursday, August 27 at **Augustiner-Keller Biergarten** (Arnulfstr. 52) in the self-service area (rougly here: https://goo.gl/maps/QmnMc) at 19h30. For details see: http://www.haskell-munich.de/dates Please note the changed location this time! The weather will be nice. If you plan to join, please add yourself to this dudle. It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-aug-2015/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

1 0

Compilation time
by Corentin Dupont 25 Aug '15

25 Aug '15

Hi guys, I have an application with around 7K loc in 4 packages. It takes 1min30 to compile with stack. Is it a normal compilation time? My computer is fairly recent with Intel i7... I run some music in background :). Is there some statistics about compilation time somewhere? Thanks. Corentin

3 8

[ANN] frpnow-0.12
by Atze van der Ploeg 25 Aug '15

25 Aug '15

Dear Cafe, We have released the (nearly) first version of FRPNow, the functional reactive programming library based on the ICFP 2015 paper "Principled Practical FRP: Forget the Past, Change the Future, FRPNow!" ( https://www.reddit.com/r/haskell/comments/3ai7hl/principled_practical_frp_f… ) The main package: http://hackage.haskell.org/package/frpnow Examples: https://github.com/atzeus/FRPNow/tree/master/Examples Gloss interoperability: http://hackage.haskell.org/package/frpnow-gloss GTK interoperability: http://hackage.haskell.org/package/frpnow-gtk (hackage doesn't like the newer GTK docs, so you can read the docs at http://www.cse.chalmers.se/~atze/frpnow-gtk/ ) Cheers, Atze

3 9

New Functional Programming Job Opportunities
by Functional Jobs 24 Aug '15

24 Aug '15

Here are some functional programming job opportunities that were posted recently: Haskell Engineer at Wagon http://functionaljobs.com/jobs/8857-haskell-engineer-at-wagon Cheers, Sean Murphy FunctionalJobs.com

1 0

Alternative to newtypes or orphan instances
by Clinton Mead 24 Aug '15

24 Aug '15

Lets say there's a type `T` I've imported from a package I don't control. I can easily write new functions to work on `T`. For example, I could write: f :: T -> T -> T f x y = ... I've extended T's functionality. No need for newtypes. This makes sense. Whilst a newtype in the following case: newtype Time = Time Int because in this case `Time` is conceptually different to `Int`. For example, you shouldn't multiply two `Time`s So in summary, we've just extended T's functionality appropriately, without introducing a newtype. But this could be dangerous. Lets say another module writer writes a similar function "f". Then if someone imports their module and ours, there will be a clash. Fortunately the compiler tells about this when it happens, and we can then use qualified imports to workaround the clash. This I'll get back to later. Now lets say I want to extend T's functionality some more, but this time by making it an instance of class C. Class C already exists in a package I don't control. Now it would be unreasonable for every class writer to write instances for every possible data type that is appropriate for their class. As you can see, it's an O(n^2) problem, so there will undoubtably be situations where one needs to write instances for data types and classes in separate packages. One recommended approach here is to newtype T, like the following: newtype MyT = MyT T however I find this incredibly ugly. Conceptually, there is no "newtype", and now there's different types of Ts bashing around which really are the same thing. Not only that, they're incompatible, for no good reason. Instead of these different Ts being different types because they're conceptually different things that shouldn't be mixed, the type of these objects depends solely on what operations I want to apply to them. Imagine a rule where: f :: T -> T -> T was only legal in the module T was defined. If you wanted to define T anywhere else, you'd have to do: f :: MyT -> MyT -> MyT That's how messy the newtype solution is, it wouldn't be accepted for ordinary functions, and I don't think it's reasonable to apply that messiness to class functions. A second approach is an orphan instance. The recommendation here is to put the orphan instances in their own module, so the user can choose to import them. This may works ok if your user is writing an executable. But what if your user is writing a library themselves. But once, you, or your user, directly uses one of the instances, they need to import it, and they pollute the global instance namespace for anyone that uses their package. This is okay if you're the only person doing it, but I think it's reasonable to say that if your practices would be bad if someone else was doing them also, then they're a bad practice. Indeed, if someone else writes their own instances and they clash with yours, people simply can't use your package and their package in the same program. This is bad. In the simple "function" case. We deal with the clash of two functions named "f" using qualified imports. We can't do that instances, nor can we even hide one of the instances. So to recap here we've tried: (1) Copying the data type. We've decided this has lots of problems. (2) Just making an orphan instance. This has serious problems too. I want to suggest a third option: (3) Copying the class. What would this involve: a) Making a new class (say C1), with the same class methods as the existing one (say C). b) Making an instance of the class like so: instance (C x) => C1 x where f = ModuleC.f ... which forwards all the calls to C. c) Add your new instances to C1 (these will overlap with the above but they will be more specific so with overlapping instances this is allowed). Now if your users want to use your instances, they import C1. They can still use the data type T as usual on the old instances, or on any existing functions defined on T directly. The big question is, what if someone else does this, and creates C2? Then they'll be a clash. But in this case, classes, unlike instances, can be explicitly imported and qualified. Your packages won't be impossible to use together, just the user will have to import one of them qualified. That's acceptable, we understand we have to do this sometimes with clashing function names too. So I guess my questions are: What do you think of my analysis? Are there parts that are incorrect? What do you think of my solution? Is there reasons why it is not as good as the "newtype" or "orphan instances" approach? Is there a way to make my solution easier to implement, i.e. less typing?

5 14

getNumCapabilities always returns 1 with GHC 7.10.2 on OS X
by Geraldus 24 Aug '15

24 Aug '15

I have OS X Yosemite 10.10.4. There are two bundles of GHC (from ghcformacosx.github.io): 7.10.1 and 7.10.2. The latter always shows me one core when running `getNumCapabilities` action: *GHC 7.10.2* > stack ghci +RTS -N8 Using resolver: lts-3.0 from global config file: /Users/arthurfayzrakhmanov/.stack/global/stack.yaml Configuring GHCi with the following packages: GHCi, version 7.10.2: http://www.haskell.org/ghc/ :? for help Prelude> :m +Control.Concurrent Prelude Control.Concurrent> getNumCapabilities 1 *GHC 7.10.1* > /Applications/ghc-7.10.1.app/Contents/bin/ghci +RTS -N8 GHCi, version 7.10.1: http://www.haskell.org/ghc/ :? for help Prelude> :m +Control.Concurrent Prelude Control.Concurrent> getNumCapabilities 8 It looks like a 7.10.2 bug, can some one confirm that?

3 5

ANN: MicrosoftTranslator-0.1.0.1
by Ernesto Rodriguez 23 Aug '15

23 Aug '15

Dear Cafe, I needed to translate text in my Haskell code and Microsoft Translator[2] seems to be the easiest and cheapest (free 2M chars/month) way to do it. I wrapped my code in this package[1]. I tried to make it easy to use. If you plan to use Microsoft Translator in Haskell but don't want to use my package, I still recommend you look at my source code since the documentation in Microsoft's page appears to be incorrect (I had to see the source code of the Python package to get it to work). In any case, it is very easy to use: > translate "clientId" "clientSecret" "hello" English German Right "hallo" It is in an early stage and I haven't added most of the languages supported by Microsoft Translator. Pull requests are welcome :) Best Regards, Ernesto Rodriguez [1] http://hackage.haskell.org/package/MicrosoftTranslator-0.1.0.1 [2] https://www.microsoft.com/en-us/translator/default.aspx -- Ernesto Rodriguez Masters Student Computer Science Utrecht University www.netowork.me github.com/netogallo

1 0