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Re: [Haskell-cafe] Most Important GHC extensions to learn/use?
by wren ng thornton 01 Jun '12

01 Jun '12

On 6/1/12 6:11 AM, Gábor Lehel wrote: > On Fri, Jun 1, 2012 at 6:29 AM, wren ng thornton<wren(a)freegeek.org> wrote: >> >> TypeFamilies (aka TFs) >> These are really nifty and they're all the rage these days. In >> a formal sense they're equivalent to fundeps, but in practice >> they're weaker than fundeps. > > Is that still true? The reason used to be that we didn't have > superclass equalities, but we do have them now since 7.2. The only > drawbacks I know of relative to FDs are that it's sometimes more > typing, not supported by GeneralizedNewtypeDeriving, and doesn't allow > OverlappingInstances (ick). The superclass equalities was a big thing, but the disparity remains even still. The main problem is that type inference with fundeps remains more powerful than type inference for TFs. There are still patterns of use which infer/check easily with fundeps but which don't have an equivalent TF implementation. I haven't messed with TFs enough to know the details here, but there are other folks on the list who do. In addition there's the issue of overlapping instances, which is currently being discussed elsewhere. -- Live well, ~wren

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Re: [Haskell-cafe] Most Important GHC extensions to learn/use?
by wren ng thornton 01 Jun '12

01 Jun '12

On 6/1/12 12:45 AM, Jonathan Geddes wrote: > Thanks, Wren, I really appreciate the detailed response! Though I am > surprised that Template Haskell isn't on your list. From the little I know > of TH it seems like all of the interesting generic/generative stuff is done > with TH. Do the other extensions subsume the need for TH, or is it just not > terribly interesting? TH is plenty interesting, but it's a very different sort of direction to head. The extensions I mentioned are the ones I think everyone assumes a seasoned Haskeller will know. With the exception of TFs and GADTs, those extensions have all been around since the days of GHC 6.6 and Hugs. Thus, most people consider them normal parts of "Haskell" even if they're not in in the Report. Consequently, understanding them is necessary to understand most of the non-H98 code on Hackage. TH, on the other hand, is very much GHC-only and that's unlikely to change. Also, there's a lot of unnecessary(?) grunge there. The basic theory of staged computation which led to TH was laid out in the MetaML/MetaOCaml papers. The version in MetaML is a lot nicer in terms of the theory, so I'd suggest people start there before diving into TH. One of the nice things in MetaML is that there's no limit on the levels of meta-ness. This was removed from TH because it would require having the compiler present in every executable; a sensible limitation, though it complicates the theory. Another major difference is that MetaML only did staging at the term level, whereas TH also allows splices in type signatures, splices which generate type class instances, etc. These extensions make TH *much* more powerful than MetaML, but also make it *much* harder to understand and reason about. IMO the formal theory of these non-term splices hasn't been worked out very well, which is part of the reason why TH is so grungy to work with. I'm a big fan of staged computation, and I'd definitely suggest any sophisticated functional programmer should read the MetaML papers (and the TH papers if so inclined, and the Flask papers to get a different perspective on the theme). But very few packages require understanding TH in order to understand them, and even fewer require understanding TH in order to use them. -- Live well, ~wren

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Requesting Feedback: I Love Haskell, but can't find a place to use it
by Jonathan Geddes 01 Jun '12

01 Jun '12

I love Haskell. It is my absolute favorite language. But I have a very hard time finding places where I can actually use it! I had hoped that compiling Haskell to C with -fvia-C (or would it be just -C?) would allow Haskell to run in new, uncharted territory such as Android (with NDK), IOS, Google's NaCl, etc. But today I learned that GHC's C backend has been deprecated! Is it more difficult than I am imagining to get Haskell to work in these environments? Is it simply a matter of low interest in this kind of work? Or something more fundamental? Am I missing something? I'm hoping that the Haskell->JavaScript efforts will mature enough to make Haskell viable for client-side web apps. (I think the first sign of this will be a self-hosting Haskell->JavaScript compiler.) I use Haskell for Server-Side code with various web frameworks, but over the years more and more of the app logic is moved into client-side JavaScript, leaving the server-side code as little more than a simple validation and security layer over the database and other services. Haskell doesn't have any trouble with this, of course, but it's not exactly a role where it can shine. (Of course this is not true of ALL server-side code, just the kind of apps I have been writing.) So anyway I'd like to request feedback: where can I use Haskell besides simple CLI utilities, dull server code, or project Euler problems? Even if it's just to contribute to getting Haskell in the environments mentioned above, any feedback is welcome! Thanks for reading, --J Arthur

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Re: [Haskell-cafe] Requesting Feedback: I Love Haskell, but can't find a place to use it
by Doug McIlroy 01 Jun '12

01 Jun '12

> > I love Haskell. It is my absolute favorite language. > > But I have a very hard time finding places where I can actually use it! > > have you considered "your head" as such a place that should be easy to find. Excellent advice. Haskell shines unusually brightly on applications that have an algebraic structure. And laziness relieves a plethora of sequencing concerns. I particularly treasure one experience: I asked a guru about the complexity of converting regular expressions to finite-state automata without epsilon transitions (state transitions that don't produce output). The best he knew was O(n^3), which is the cost of removing epsilon transitions from arbitrary automata. Then I wrote about a dozen lines of Haskell to do the job--and running time turned out to be O(n^2). Once I'd written the code, it became clear how to do it in other languages, but I never would have found the theorem without the help of Haskell. (This without even bringing in the heavy artillery of higher-order functions and monads.) Doug McIlroy

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Re: [Haskell-cafe] Requesting Feedback: I Love Haskell, but can't find a place to use it
by Doug McIlroy 01 Jun '12

01 Jun '12

> > I love Haskell. It is my absolute favorite language. > > But I have a very hard time finding places where I can actually use it! > > have you considered "your head" as such a place that should be easy to find. An excellent reason. Haskell shines unusually brightly on applications that have an algebraic structure. Laziness relieves a plethora of sequencing concerns. I particularly treasure one experience: I asked a guru about the complexity of converting regular expressions to finite-state automata without epsilon transitions (state transitions that don't produce output). The best he knew was O(n^3), which is the cost of removing epsilon transitions from arbitrary automata. Then I wrote about a dozen lines of Haskell to do the job--and running time turned out to be O(n^2). Once I'd written the code, it became clear how to do it in other languages, but I never would have found the theorem without the help of Haskell. (This without even bringing in the heavy artillery of higher-order functions and monads.) Doug McIlroy

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Tests by properties: origin?
by Yves Parès 01 Jun '12

01 Jun '12

Out of curiosity, does someone know if QuickCheck was the first test framework working through test by properties associated with random generation or if it drew the idea from something else? Because the idea has be retaken by a lot of frameworks in several languages (see http://en.wikipedia.org/wiki/Quickcheck) but I can't find what was QuickCheck inspiration. (This question pop up when arguing with a Java user about what Haskell brings to the field of development, he pretends it would be astonishing that QuickCheck is the first one using this testing paradigm).

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Confused by ghci output
by Clark Gaebel 31 May '12

31 May '12

*X> 3^40 `mod` 3 == modexp2 3 40 3 False *X> modexp2 3 40 3 0 *X> 3^40 `mod` 3 0 I'm confused. Last I checked, 0 == 0. Using GHC 7.4.1, and the file x.hs (which has been loaded in ghci) can be found here: http://hpaste.org/69342 I noticed this after prop_sanemodexp was failing. Any help would be appreciated, - Clark

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Re: [Haskell-cafe] How to write Source for TChan working with LC.take?
by Hiromi ISHII 30 May '12

30 May '12

Oops, sorry. The last case's behaviour was not as I expected... A correct log is below: ~~~~ ghci> sourceTChanRaw ch $$ LC.isolate 10 =$= LC.mapM_ print () () () () () () () () () () ghci> sourceTChanState ch $$ LC.isolate 10 =$= LC.mapM_ print () () () () () () () () () () ghci> sourceTChanYield ch $$ LC.isolate 10 =$= LC.mapM_ print () () () () () () () () () () *blocks* ~~~~ So again, sourceTChanYield blocks here even if it is already supplied with enough values! -- Hiromi ISHII konn.jinro(a)gmail.com

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Finding the average in constant space
by Chris Wong 30 May '12

30 May '12

Hello all I just came up with a way of executing multiple folds in a single pass. In short, we can write code like this: average = foldLeft $ (/) <$> sumF <*> lengthF and it will only traverse the input list once. The code is at: https://gist.github.com/2802644 My question is: has anyone done this already? If not, I might release this on Hackage -- it seems quite useful. Chris

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Haskell Weekly News: Issue 229
by Daniel Santa Cruz 30 May '12

30 May '12

Welcome to issue 229 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of May 20 to 26, 2012. Quotes of the Week * Peaker: Harrop taught me that I am a team of experts * applicative: Even if no one else understands me, still, the Glasgow Haskell Compiler understands me * JoeyA: Answers in #haskell are automatically sorted by readability descending. * monochrom: monochromism is a graph morphism that maps graphs to 1-colorable graphs :) * sipa: can we write a TutorialMonad t ? which is used to build and combine monad tutorials? Top Reddit Stories * my Haskell project was 100% funded on Kickstarter Domain: self.haskell, Score: 125, Comments: 22 On Reddit: [1] http://goo.gl/E2wkY Original: [2] http://goo.gl/E2wkY * Building intuition for monads, without mentioning monads (Ertugrul Söylemez) Domain: haskell.org, Score: 56, Comments: 32 On Reddit: [3] http://goo.gl/2leRd Original: [4] http://goo.gl/kMbJR * A gamified tutorial on the sequent calculus Domain: logitext.ezyang.scripts.mit.edu, Score: 51, Comments: 23 On Reddit: [5] http://goo.gl/tD8fZ Original: [6] http://goo.gl/E6zBl * pipes 2.0 - Pipe Finalization Domain: haskellforall.com, Score: 45, Comments: 32 On Reddit: [7] http://goo.gl/boq03 Original: [8] http://goo.gl/AABwV * Haste: Haskell to Javascript Compiler Domain: github.com, Score: 40, Comments: 45 On Reddit: [9] http://goo.gl/sJCcJ Original: [10] http://goo.gl/ZhYbP * Monad Reification in Haskell and the Sunroof Javascript compiler Domain: ittc.ku.edu, Score: 37, Comments: 26 On Reddit: [11] http://goo.gl/LVSVO Original: [12] http://goo.gl/s0PVp * HaskSymb, An Experiment in Haskell Symbolic Algebra. Feedback appreciated! Domain: github.com, Score: 36, Comments: 34 On Reddit: [13] http://goo.gl/d6Xo8 Original: [14] http://goo.gl/TUiJt * Using Ragel, Clang, LLVM and GHC to produce really fast stockmarket data parsers Domain: breaks.for.alienz.org, Score: 35, Comments: 8 On Reddit: [15] http://goo.gl/qEmKb Original: [16] http://goo.gl/ABzsA * Looks like Bryan finished importing Cabal issues to Github — thanks! Domain: github.com, Score: 32, Comments: 6 On Reddit: [17] http://goo.gl/9O8Sa Original: [18] http://goo.gl/bWrLg * What if haskellwiki were run by a Haskell wiki? Domain: haskellwiki.gitit.net, Score: 29, Comments: 14 On Reddit: [19] http://goo.gl/713Yb Original: [20] http://goo.gl/chSPM * New release of Elm available. Looking for feedback on the language so far. Domain: self.haskell, Score: 24, Comments: 21 On Reddit: [21] http://goo.gl/ejS9p Original: [22] http://goo.gl/ejS9p * Generics and Protocol Buffers Domain: breaks.for.alienz.org, Score: 17, Comments: On Reddit: [23] http://goo.gl/z0obj Original: [24] http://goo.gl/R5b3l Top StackOverflow Questions * What's the theoretical basis for existential types? votes: 32, answers: 3 Read on SO: [25] http://goo.gl/ZdAJb * How can a gc find out about object references done from the stack? votes: 20, answers: 4 Read on SO: [26] http://goo.gl/AtXUr * Why is GHC complaining about non-exhaustive patterns? votes: 14, answers: 3 Read on SO: [27] http://goo.gl/9oIsB * Evolving data structure votes: 13, answers: 2 Read on SO: [28] http://goo.gl/gbeLN * What makes a good name for a helper function? votes: 11, answers: 5 Read on SO: [29] http://goo.gl/b6B7o * Haskell “Apply”? [closed] votes: 11, answers: 3 Read on SO: [30] http://goo.gl/EBwsy * Round to nearest integer votes: 10, answers: 1 Read on SO: [31] http://goo.gl/1RaVk * Haskell: lazy versus eager evaluation for insertion sort votes: 6, answers: 1 Read on SO: [32] http://goo.gl/zGrie * Loss of polymorphism after pattern matching votes: 6, answers: 1 Read on SO: [33] http://goo.gl/YmA1y * Church-Rosser Theorem Example in a Functional Programming Language votes: 6, answers: 1 Read on SO: [34] http://goo.gl/FmSMh Until next time, Daniel Santa Cruz References 1. http://www.reddit.com/r/haskell/comments/u2vt5/my_haskell_project_was_100_f… 2. http://www.reddit.com/r/haskell/comments/u2vt5/my_haskell_project_was_100_f… 3. http://www.haskell.org/pipermail/haskell-cafe/2012-May/101338.html 4. http://www.reddit.com/r/haskell/comments/u04vp/building_intuition_for_monad… 5. http://logitext.ezyang.scripts.mit.edu/logitext.fcgi/tutorial 6. http://www.reddit.com/r/haskell/comments/tzp7f/a_gamified_tutorial_on_the_s… 7. http://www.haskellforall.com/2012/05/pipes-20-pipe-finalization.html 8. http://www.reddit.com/r/haskell/comments/txkb0/pipes_20_pipe_finalization/ 9. https://github.com/valderman/haste-compiler#haste 10. http://www.reddit.com/r/haskell/comments/tw997/haste_haskell_to_javascript_… 11. http://www.ittc.ku.edu/csdlblog/?p=88 12. http://www.reddit.com/r/haskell/comments/tvwn7/monad_reification_in_haskell… 13. https://github.com/colah/HaskSymb 14. http://www.reddit.com/r/haskell/comments/u1t8n/hasksymb_an_experiment_in_ha… 15. http://breaks.for.alienz.org/blog/2012/02/09/parsing-market-data-feeds-with… 16. http://www.reddit.com/r/haskell/comments/txfhq/using_ragel_clang_llvm_and_g… 17. https://github.com/haskell/cabal/issues 18. http://www.reddit.com/r/haskell/comments/u2faj/looks_like_bryan_finished_im… 19. http://haskellwiki.gitit.net/ 20. http://www.reddit.com/r/haskell/comments/txn1p/what_if_haskellwiki_were_run… 21. http://www.reddit.com/r/haskell/comments/u69wo/new_release_of_elm_available… 22. http://www.reddit.com/r/haskell/comments/u69wo/new_release_of_elm_available… 23. http://breaks.for.alienz.org/blog/2012/05/20/generics-and-protocol-buffers/ 24. http://www.reddit.com/r/haskell/comments/tw6dq/generics_and_protocol_buffer… 25. http://stackoverflow.com/questions/10753073/whats-the-theoretical-basis-for… 26. http://stackoverflow.com/questions/10710603/how-can-a-gc-find-out-about-obj… 27. http://stackoverflow.com/questions/10699532/why-is-ghc-complaining-about-no… 28. http://stackoverflow.com/questions/10761583/evolving-data-structure 29. http://stackoverflow.com/questions/10740515/what-makes-a-good-name-for-a-he… 30. http://stackoverflow.com/questions/10767407/haskell-apply 31. http://stackoverflow.com/questions/10738569/round-to-nearest-integer 32. http://stackoverflow.com/questions/10681507/haskell-lazy-versus-eager-evalu… 33. http://stackoverflow.com/questions/10728897/loss-of-polymorphism-after-patt… 34. http://stackoverflow.com/questions/10728997/church-rosser-theorem-example-i…

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