- Haskell-Cafe - Haskell.org

Type classes
by Johannes Gerer 28 May '13

28 May '13

Dear Haskellers, While trying to understand the interconnection and hierarchy behind the important typeclasses, I stumbled upon the following question that I am not able to answer: There seems to be a hierachy of the type classes, in the sense, that the more powerfull a class is, the more fleixblility you have for combining them to complex programs. (Functor -> Applicative -> Arrow[Choice,Plus,Apply,..] -> Monad). It was nice to read in the Typeclassopedia, that ArrowApply and Monad are equivalent, which is shown by deriving two instances from each other: instance Monad m => ArrowApply (Kleisli m) instance ArrowApply a => Monad (a anyType) The logic seems to be, that if I can derive from every instance of class A an instance of class B, then A is more powerfull than B and (in general) it is easier to be of class B than of class A (e.g. more types can be made Applicatives, than Monads) So far, I think I can follow. But what really hit me was the Cokleisli type. Using it and the logic from above, I can show that ANY type class is more (or equally) powerfull than the Monad: instance AnyClass m => Monad (Cokleilsi m anyType) I know this makes no sense, but where is the fallacy? Why even bother with the above derivation, if any type class can be made into a monad? I can see, that the Monad instance from above does not really transform the type "a", but instead simply fix its first argument. But then on the other hand, the ArrowApply Instance does transform the "m" type (in a way similar to Cokleisli). If attention needs to be paid to the details, then what are they and why did they not matter above? Thanks, Johannes

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Stream instance for Parsec + conduits
by Phil Scott 28 May '13

28 May '13

Hi all. I would like to have a Parsec Stream instance for Data.Text streams in yesod's ConduitM. So far, I have this: hpaste.org/87599 The idea is that because Yesod's conduits will be chunking values in Data.Text, I should have a wrapper StreamSource to cache chunked values. For some reason, the test parser fails, saying: [Left "Blah" (line 1, column 1): unexpected "g" expecting "h" or "g"] Any ideas? Cheers! Phil

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Interfacing Java/Haskell
by Hans Georg Schaathun 28 May '13

28 May '13

Hi all, I am rather new to the café, so I just hope my question fits in. Does anyone have experience with integrating Haskell and Java? I have done some searching, finding a lot of pointers but hardly anything in terms of evaluation, successes, or caveats. >From what I see Frege looks promising, arguably not haskell I suppose, but does it work? Other projects I have seen appear to have reached a stand-still for ages. The background for the question is that I will contribute some control algorithms based on machine learning or AI in a larger project. It would save me a lot of time if I could write in Haskell, but only assuming that interfacing with Java afterwards is trivial compared to writing everything in java in the first place. I am, perhaps, particularly worried that a Haskell-lookalike for JVM might be unable to optimise properly, like not being lazy. Any advice? TIA -- :-- Hans Georg

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Module import and use in GHC plugin?
by Conal Elliott 28 May '13

28 May '13

In writing GHC plugins, how can I (a) add a module import (preferably qualified) and (b) make vars/ids for names imported from the newly imported module (to insert in the transformed Core code)? Thanks, - Conal

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Generalizing unionWithKey, unionWith, ...
by Jose A. Lopes 28 May '13

28 May '13

Hello everyone, unionWithKey and unionWith have the following types unionWith :: Ord k => (a -> a -> a) -> Map k a -> Map k a -> Map k a unionWithKey :: Ord k => (k -> a -> a -> a) -> Map k a -> Map k a -> Map k a Since they are implemented by means of mergeWithKey, wouldn't it be possible to generalize these functions to the following types ? unionWith :: Ord k => (a -> b -> c) -> Map k a -> Map k b -> Map k c unionWithKey :: Ord k => (k -> a -> b -> c) -> Map k a -> Map k b -> Map k c Cheers, Jose -- José António Branquinho de Oliveira Lopes Instituto Superior Técnico Technical University of Lisbon

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Re: [Haskell-cafe] ANN: Haskell Platform 2013.2.0.0
by harry 28 May '13

28 May '13

Good to see it released! Was there a deliberate decision not to build a Windows x64 platform, or is it just that there wasn't anyone to do it?

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Munich Haskell Meeting
by Heinrich Hördegen 28 May '13

28 May '13

Dear all, once again, our monthly Haskell Meeting in Munich will take place. We meet on Thursday, 30 of May, 19h30. The venue is as usually, Cafe Puck. Please go here for details and click the button if you plan to join: http://www.haskell-munich.de/dates Have a successful day! Heinrich

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ANN: Haskell Platform 2013.2.0.0
by Mark Lentczner 28 May '13

28 May '13

On behalf of the Haskell Platform team, I'm happy to announce the release of Haskell Platform 2013.2.0.0 *featuring* GHC 7.6.3 52 packages 650+ public modules 4 tools *New packages this release:* attoparsec case-insensitive hashable unordered-containers *and a major upgrade to* OpenGL and GLUT Get it now: Download Haskell Platform <http://www.haskell.org/platform/index.html> — for Windows <http://www.haskell.org/platform/windows.html> — for Mac OS X <http://www.haskell.org/platform/mac.html> — for Linux <http://www.haskell.org/platform/linux.html> (distributions updating soon) *N.B.: You will likely need to explicitly re-fresh those links when visiting in your browser - those pages tend to get cached for very long periods of time.* — Mark "platform wrangler" Lentczner

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 117, Issue 40
by Diego Saa-temp 27 May '13

27 May '13

Sent from my Verizon Wireless 4G LTE smartphone -------- Original Message -------- Subject: Haskell-Cafe Digest, Vol 117, Issue 40 From: "haskell-cafe-request(a)haskell.org" <haskell-cafe-request(a)haskell.org> Date: Mon, 27-May-2013 06:00 To: "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> CC: Send Haskell-Cafe mailing list submissions to haskell-cafe(a)haskell.org To subscribe or unsubscribe via the World Wide Web, visit http://www.haskell.org/mailman/listinfo/haskell-cafe or, via email, send a message with subject or body 'help' to haskell-cafe-request(a)haskell.org You can reach the person managing the list at haskell-cafe-owner(a)haskell.org When replying, please edit your Subject line so it is more specific than "Re: Contents of Haskell-Cafe digest..."

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hackage update brigade (was Re: ANNOUNCE: new bridge! (prelude-prime))
by Evan Laforge 27 May '13

27 May '13

> Yes, it would break code. Probably a lot of code. So of course I volunteer to fix my code, but that's not much help, since it's a small minority of the code on hackage. So that made me think, maybe we should organize a kind of hackage community service brigade, which, when the time is right, would spring into action. They would download sections of hackage, update the code, and send patches to the maintainers. Then they'd keep track of which packages actually applied the patch, and that could go on an "update status" page. I'd sign up for that. Of course, package maintainers might prefer to do the change themselves, or more likely may be incommunicado. But the presence of a small army of organized volunteers waiting to update code might reduce friction to make necessary changes, and would take the weight off the shoulders of the few who wind up doing it anyway when a new ghc comes out. If people think this is a good idea, I volunteer to do the setup. I guess this means a short doc describing the process, and then a place where volunteers can sign up, and then keep the volunteer list up to date.

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