October 2014 - Haskell-Cafe

`zip` doesn't work with infinite `Seq`s
by Matteo Ferrando 09 Oct '14

09 Oct '14

Hello, We are writing a compiler[1] for a course and found that the `zip` function included in the `Data.Sequence` module, `zip :: Seq a -> Seq b -> Seq (a, b)` would hang on the following code: > -- using the `zip` from `Data.Sequence` > zip ys (fomList $ repeat x) We checked the implementation[2] in the source of `Data.Sequence` and found the following: > zip :: Seq a -> Seq b -> Seq (a, b) > zip = zipWith (,) > -- Here `zipWith` assumes *non-infinite `Seq`s* > zipWith :: (a -> b -> c) -> Seq a -> Seq b -> Seq c > zipWith f xs ys > | length xs <= length ys = zipWith' f xs ys > | otherwise = zipWith' (flip f) ys xs > -- Function not exported by `Data.Seq`, assumes `length xs <= length ys` > zipWith' :: (a -> b -> c) -> Seq a -> Seq b -> Seq c > zipWith' f xs ys = snd (mapAccumL k ys xs) > where > k kys x = case viewl kys of > (z :< zs) -> (zs, f x z) EmptyL -> error "zipWith': unexpected EmptyL" In the lazy reading of the documentation we did, we didn't find any warning of using infinite `Seq`s for zips. (Maybe there are warings that we didn't see). But looking at the code of `zip` in the `Prelude`: > zip :: [a] -> [b] -> [(a,b)] > zip (a:as) (b:bs) = (a,b) : zip as bs > zip _ _ = [] We see that we could just *pattern-match* both heads, instead of making assumptions. Maybe this should be better explained in the documentation[3] of `zip` for `Seq`s: > zip :: Seq a -> Seq b -> Seq (a, b) > O(min(n1,n2)). zip takes two sequences and returns a sequence of > corresponding pairs. If one input is short, excess elements are discarded > from the right end of the longer sequence. Or just change the implementation for it to work with infinite `Seq`s. For those of you who are curious, we ended up using the following code to fix the *infinite `Seq`s problem*: > -- using the `zip` from `Prelude` > zip (toList ys) (repeat x) [1] https://github.com/chamini2/sapphire [2] http://hackage.haskell.org/package/containers-0.5.5.1/docs/src/Data-Sequenc… [3] http://hackage.haskell.org/package/containers-0.5.5.1/docs/Data-Sequence.ht…

4 5

Re: [Haskell-cafe] type families with non-type indices
by oleg＠okmij.org 09 Oct '14

09 Oct '14

Doug McIlroy wrote: > An example of such a family is integers mod p. One wants to define the > operations once for all p, so that a type for any particular p can be > introduced by a one-line declaration. That was precisely the motivating example for the 2004 Haskell Workshop paper Implicit configurations -- or, type classes reflect the values of types ftp://ftp.deas.harvard.edu/techreports/tr-15-04.pdf (The Technical Report is the extended version of the paper) which discusses several solutions to the problem.

1 0

Haskell Weekly News: Issue 309
by Daniel Santa Cruz 09 Oct '14

09 Oct '14

Welcome to issue 309 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from September 28 to October 4, 2014 Quotes of the Week * Uh... I'ma have to read the IRC logs... No good quotes @remember'd Top Reddit Stories * Announcing needle: ASCII-fied arrow notation Domain: scrambledeggsontoast.github.io, Score: 103, Comments: 119 Original: [1] http://goo.gl/wCPh8K On Reddit: [2] http://goo.gl/UwTJSt * How we might abolish Cabal Hell, part 1 Domain: well-typed.com, Score: 95, Comments: 69 Original: [3] http://goo.gl/N19hPq On Reddit: [4] http://goo.gl/CQQRF0 * After some failed attempts to learn Parsec I came across this incredibly patient tutorial and now I'm writing parsers! Domain: github.com, Score: 79, Comments: 17 Original: [5] http://goo.gl/msqtkb On Reddit: [6] http://goo.gl/p3EgqQ * Neil Mitchell's Haskell Blog: Why Traversable/Foldable should not be in the Prelude Domain: neilmitchell.blogspot.co.il, Score: 71, Comments: 289 Original: [7] http://goo.gl/sykNse On Reddit: [8] http://goo.gl/4wIxBX * Tutorial: Implementing highly efficient data structures with Monoids and Fingertrees Domain: codementor.io, Score: 47, Comments: 37 Original: [9] http://goo.gl/WB7Jwg On Reddit: [10] http://goo.gl/jGrLX1 * There is No Haskell Topology Domain: immanence.org, Score: 45, Comments: 32 Original: [11] http://goo.gl/JAoEDw On Reddit: [12] http://goo.gl/wuuF4i * Introduction to Low Level Haskell Optimization - Dan Doel Domain: youtu.be, Score: 44, Comments: 2 Original: [13] http://goo.gl/vYS1Hy On Reddit: [14] http://goo.gl/dG9Pn5 * Turn an optparse-applicative program into a CGI program! Domain: github.com, Score: 39, Comments: 10 Original: [15] http://goo.gl/sjCqsb On Reddit: [16] http://goo.gl/BXxeno * Yesod 1.4 released Domain: yesodweb.com, Score: 36, Comments: 1 Original: [17] http://goo.gl/T5Vqe8 On Reddit: [18] http://goo.gl/977ey0 * Updating Auto-Update Domain: yesodweb.com, Score: 36, Comments: 20 Original: [19] http://goo.gl/OwOrGZ On Reddit: [20] http://goo.gl/J2zr5n * Type-safe Routing for Spock Web Framework released Domain: github.com, Score: 32, Comments: 39 Original: [21] http://goo.gl/QZS9gU On Reddit: [22] http://goo.gl/BokvJ8 * Lattice-Based Data Structures for Deterministic Parallel and Distributed Programming (related to CRDTs; implementation in Haskell) Domain: composition.al, Score: 30, Comments: 9 Original: [23] http://goo.gl/mVQqqX On Reddit: [24] http://goo.gl/tZ0XrW * A non-recursive sorting algorithm Domain: haskellexists.blogspot.com, Score: 27, Comments: 36 Original: [25] http://goo.gl/jD0z0K On Reddit: [26] http://goo.gl/3HpTBy * Dijkstra monads Domain: why-lambda.blogspot.co.uk, Score: 25, Comments: 4 Original: [27] http://goo.gl/kx1Q6w On Reddit: [28] http://goo.gl/C86igH * Improving Haskell-related documentation Domain: self.haskell, Score: 23, Comments: 62 Original: [29] http://goo.gl/rQeuUT On Reddit: [30] http://goo.gl/rQeuUT * Haskell, Monads and Purity • via Hacker News Domain: jelv.is, Score: 23, Comments: 11 Original: [31] http://goo.gl/vm7juH On Reddit: [32] http://goo.gl/1P5OgF * FP Complete is hiring: Systems engineer for cloud and cluster work Domain: fpcomplete.com, Score: 21, Comments: 11 Original: [33] http://goo.gl/MLT5Ud On Reddit: [34] http://goo.gl/Ql8uIX * The future of the haskell2010/haskell98 packages for GHC 7.10 Domain: haskell.org, Score: 21, Comments: 22 Original: [35] http://goo.gl/94bnK3 On Reddit: [36] http://goo.gl/0c80JU * ANN: Nomyx V0.7, the only game where you can change the rules Domain: self.haskell, Score: 20, Comments: 7 Original: [37] http://goo.gl/1gGtqb On Reddit: [38] http://goo.gl/1gGtqb * Let's Build a Browser Engine in Haskell: part 5 Domain: hrothen.github.io, Score: 19, Comments: 6 Original: [39] http://goo.gl/zzSF2D On Reddit: [40] http://goo.gl/9xzqdR Top StackOverflow Questions * How fundamentaly different push-pull and arrowized FRP are? votes: 10, answers: 0 Read on SO: [41] http://goo.gl/XbbbVW * What are hashes (#) used for in Haskell's library's source? votes: 10, answers: 1 Read on SO: [42] http://goo.gl/mvNNlb * What is the inverse of a promise? votes: 9, answers: 3 Read on SO: [43] http://goo.gl/usQIiW * Is there a default polymorphic unit type haskell votes: 7, answers: 1 Read on SO: [44] http://goo.gl/5Ozy9G * nvcc + c2hs on OS X 10.9.5 votes: 7, answers: 1 Read on SO: [45] http://goo.gl/0XMNFu * How to sort a list using partial order in Haskell? votes: 7, answers: 2 Read on SO: [46] http://goo.gl/GLMiwA * Relationship between forward and backward map in Isomorphism (Lens package) votes: 6, answers: 1 Read on SO: [47] http://goo.gl/icIdIR * How to implement a generic neural network efficiently in Haskell? votes: 6, answers: 0 Read on SO: [48] http://goo.gl/HIHY8e * How do I unify two or more Signals in elerea? votes: 6, answers: 1 Read on SO: [49] http://goo.gl/jacb0r * Ambiguous type variable fixed with type equality constraint votes: 6, answers: 1 Read on SO: [50] http://goo.gl/JADmBj Until next time, [51]+Daniel Santa Cruz References 1. http://scrambledeggsontoast.github.io/2014/09/28/needle-announce/ 2. http://www.reddit.com/r/haskell/comments/2hpzpu/announcing_needle_asciified… 3. http://www.well-typed.com/blog/2014/09/how-we-might-abolish-cabal-hell-part… 4. http://www.reddit.com/r/haskell/comments/2hvm59/how_we_might_abolish_cabal_… 5. https://github.com/JakeWheat/intro_to_parsing 6. http://www.reddit.com/r/haskell/comments/2ia5u2/after_some_failed_attempts_… 7. http://neilmitchell.blogspot.co.il/2014/10/why-traversablefoldable-should-n… 8. http://www.reddit.com/r/haskell/comments/2hzqii/neil_mitchells_haskell_blog… 9. https://www.codementor.io/haskell-tutorial/monoids-fingertrees-implement-ab… 10. http://www.reddit.com/r/haskell/comments/2i3ttm/tutorial_implementing_highl… 11. http://immanence.org/post/there-is-no-haskell-topology/ 12. http://www.reddit.com/r/haskell/comments/2i85k7/there_is_no_haskell_topolog… 13. http://youtu.be/McFNkLPTOSY 14. http://www.reddit.com/r/haskell/comments/2hy6cp/introduction_to_low_level_h… 15. https://github.com/maxpow4h/webcloud#webcloud 16. http://www.reddit.com/r/haskell/comments/2huhrg/turn_an_optparseapplicative… 17. http://www.yesodweb.com/blog/2014/09/announcing-yesod-1-4 18. http://www.reddit.com/r/haskell/comments/2hvoxb/yesod_14_released/ 19. http://www.yesodweb.com/blog/2014/10/updating-auto-update 20. http://www.reddit.com/r/haskell/comments/2i5d7m/updating_autoupdate/ 21. https://github.com/agrafix/Spock 22. http://www.reddit.com/r/haskell/comments/2i2zfw/typesafe_routing_for_spock_… 23. http://composition.al/blog/2014/09/29/my-thesis-defense-talk/ 24. http://www.reddit.com/r/haskell/comments/2i17zj/latticebased_data_structure… 25. http://haskellexists.blogspot.com/2014/09/a-non-recursive-sorting-algorithm… 26. http://www.reddit.com/r/haskell/comments/2hsea9/a_nonrecursive_sorting_algo… 27. http://why-lambda.blogspot.co.uk/2014/09/dijkstra-monads.html 28. http://www.reddit.com/r/haskell/comments/2hzu5b/dijkstra_monads/ 29. http://www.reddit.com/r/haskell/comments/2i1z9u/improving_haskellrelated_do… 30. http://www.reddit.com/r/haskell/comments/2i1z9u/improving_haskellrelated_do… 31. http://jelv.is/blog/Haskell-Monads-and-Purity/ 32. http://www.reddit.com/r/haskell/comments/2i8tnl/haskell_monads_and_purity_v… 33. https://www.fpcomplete.com/page/systems-engineer 34. http://www.reddit.com/r/haskell/comments/2hz6ue/fp_complete_is_hiring_syste… 35. http://www.haskell.org/pipermail/glasgow-haskell-users/2014-September/02528… 36. http://www.reddit.com/r/haskell/comments/2i0bhr/the_future_of_the_haskell20… 37. http://www.reddit.com/r/haskell/comments/2hw529/ann_nomyx_v07_the_only_game… 38. http://www.reddit.com/r/haskell/comments/2hw529/ann_nomyx_v07_the_only_game… 39. http://hrothen.github.io/2014/10/01/lets-build-a-browser-engine-in-haskell-… 40. http://www.reddit.com/r/haskell/comments/2i0wnr/lets_build_a_browser_engine… 41. http://stackoverflow.com/questions/26164135/how-fundamentaly-different-push… 42. http://stackoverflow.com/questions/26169990/what-are-hashes-used-for-in-has… 43. http://stackoverflow.com/questions/26084439/what-is-the-inverse-of-a-promise 44. http://stackoverflow.com/questions/26097797/is-there-a-default-polymorphic-… 45. http://stackoverflow.com/questions/26125870/nvcc-c2hs-on-os-x-10-9-5 46. http://stackoverflow.com/questions/26158233/how-to-sort-a-list-using-partia… 47. http://stackoverflow.com/questions/26082221/relationship-between-forward-an… 48. http://stackoverflow.com/questions/26092063/how-to-implement-a-generic-neur… 49. http://stackoverflow.com/questions/26104735/how-do-i-unify-two-or-more-sign… 50. http://stackoverflow.com/questions/26146983/ambiguous-type-variable-fixed-w… 51. https://plus.google.com/105107667630152149014/about

1 0

Fingerprinting Haskell Objects
by Ozgun Ataman 09 Oct '14

09 Oct '14

Hello everybody, I have a little question I wanted to run by the folks here. I've run into it several times over the past few years and would love to lock down a good answer. What's the best way to "fingerprint" a Haskell object into, say, ByteString, so that this fingerprint can be used as the "lookup key" in a database (for example) and be trusted that it will remain constant over time even as the underlying libraries evolve? Here's a simple example: - Say I'm building a manual index on top of a key-value store (redis, dynamodb, etc.) - I want my keys to be arbitrary tuples (or similar records) that may contain various fields in them - I would like to avoid ad-hoc, hand-written MyTuple -> ByteString and ByteString -> MyTuple conversions. However, Generic derivations, template-haskell, etc. are acceptable - Notice how your fingerprint, which is used as a lookup key in the database, has to remain stationary. If it changes even by a single bit over time for the same MyTuple, the key-value store will NOT be able to find the index associated with MyTuple at this later time Here are some ideas (and related concepts) I've considered and used over the years: - Hand-write a "Prism' MyTuple ByteString". This works, but is tedious and error-prone. - Use Serialize/Binary and trust that the encode/decode pair will produce results consistently in 5 years (dangerous territory!) - Use SafeCopy, which is great for ensuring timeless decoding of the *value* in the index, but can we be sure that fingerprint (MyTuple -> ByteString) conversion is persistent? What if SafeCopy authors one day decide to encode tuples differently? They would write the migrations to transparently handle legacy code for *values*, but not for *keys*. Also notice here how migrations help with the ByteString -> MyTuple leg, but do not ensure MyTuple -> ByteString produces the same ByteString over time. - Hashable would've been nice, but there is NO guarantee of persistent results, even across multiple runs of the same code What would be your preferred solution? Thank you, Oz

3 4

Looking for a Google Hangouts tutor/advisor on Haskell
by Steven Arnold 09 Oct '14

09 Oct '14

I am looking for a Haskell developer who can review my code, suggest improvements, and generally help me become a better Haskell programmer more quickly. As such, you should be very proficient with Haskell yourself. Maybe you do Haskell work professionally; maybe you have contributed to a couple open-source projects, maybe you're a well-known Haskell blogger, writer or teacher. I am a beginner, although I have been programming professionally for many years. I do have a basic foundation in functional programming languages, including Haskell, but most of my background is with dynamic languages like Ruby and Python. I'm thinking we would meet once a week on a Google Hangout (or other technology) for an hour or so and I'd show you my code and have you review it. (Take it away and mark it up if you want.) I'd ask questions and we can discuss them. You can tell me about anything you think I might need to know to be more effective. I might also email questions to you or ask for more time than described above. You would help me as your schedule allows. (I understand you no doubt have a day job.) I am willing to pay for your time. Please let me know your hourly rate. I can compensate you with Paypal, a check....whatever works. Please feel free to contact me off-list if you're interested in helping. Thanks for taking the time to read this. Best Regards, steven

1 0

Jop opening: web developers (with a taste for Haskell!) at Anchor
by Andrew Cowie 09 Oct '14

09 Oct '14

Web Developers and Web Designers ================================ The engineering department at Anchor is building the analytics capabilities, internal systems, cloud infrastructure, deployment tooling, and operations practices that will be needed to take the company to the next level. These things all need front-ends. It goes without saying that the web is the platform for bringing new products to market, but we're a hosting company; most of our staff are operations-focused and our customers are too. What they really want are better tools to manipulate infrastructure. And even though Real Sysadmins™ prefer command-line tools (we build those too) there's still the need to cleanly present and visualize information, and the web is the place to do that. Haskell is the working language for internal development; the benefits of type safety, resilience when refactoring, and stability over time added to the power of functional programming has already paid dividends. An open question is how far we can push strongly-typed functional programming out to the edge. JavaScript is unavoidable browser-side, but does it need to be written by hand? The plethora of alt-js implementations clearly say “no!”, but _they_ all seem to force you into contortions. If you're going to be working in something other than JavaScript, then why not a real language? Having the codebase be common on both sides of the network would really be something; are haste or ghcjs up to the challenge? It's an exciting time. We have every intention of changing the hosting industry to make developing applications — and operating them at scale — easier for people facing massive growth. Interested in helping? Requirements ------------ A solid knowledge of HTML, JavaScript, and CSS is a prerequisite, obviously. Experience with — and hatred of — a number of different server-side frameworks (hack 'n slash PHP, enterprisey JSP, hipster RoR, unmaintainable PyThings, etc) is necessary to give you an appropriately jaded outlook on just how hard the web problem is. Understanding (or at least strong opinions about) how to build sites where content and presentation, design and implementation are separated is paramount. Can your designers make it beautiful without needing to run the entire stack? Can your systems developers improve performance without needing to run the website to find out if it works? Exposure to the harsh realities of IT operations and experience of the thrill of Linux systems administration will stand you in good stead. Knowledge of the history, progress, and problems in configuration management will be helpful. Familiarity with open source development practices, and the ability to tolerate other people refactoring your code is something we'll want you to demonstrate. Experience programming in Haskell would be a definite asset. We need to see a track record of working well in the open (regardless of language or project). Good documentation is as important as good code, and command of written English is necessary both to collaborate with your peers and to describe and promote your results. A Bachelor's in science, engineering, or computational biology will be well thought of. Your specific field of study doesn't matter, but how you've used computing to solve problems in your chosen field does. A degree in engineering will be a distinct plus. An undergraduate degree in computer science will not be held against you (much). Applying -------- Anchor Systems, based in Sydney Australia, has successfully been providing managed hosting and infrastructure to clients around the world for almost 15 years. The critical difference in the service we offer our customers often boils down to providing operations expertise and infrastructure configuration balancing the short term needs to get a new installation running with the long term imperatives of security, scalability, and flexibility. If you're interested in applying, send a PDF copy of your CV to <jobs(a)anchor.com.au>, along with the URL to your GitHub account. We also invite you to include a pointer to something that shows you in a good light: a mailing list thread where you argued a contentious position, a piece of particularly thorny code in which you found an elegant solution to a problem, or even an essay or article you've written. We don't do international relocations (sorry everyone who wants to work remotely from Hawaii; I all want to work remotely from Hawaii too), but we are willing to sponsor work visas for new hires. So whether you've just moved to Australia or have been here for generations, give us a shout next time you're in Sydney. AfC Sydney -- Andrew Frederick Cowie Head of Engineering Anchor Systems https://twitter.com/afcowie https://github.com/anchor/ https://www.anchor.com.au/

1 0

cabal sandbox: Can't load .so/.DLL for...
by Joel Hermanns 08 Oct '14

08 Oct '14

Hi all, I have some trouble with cabal sandboxes. I used `cabal sandbox add-source` to add a library. When I add the library as a dependency for my project and try to build I’m always getting <command line>: can't load .so/.DLL for: /Users/joel/workspace/haskell/bookingsystem/.cabal-sandbox/lib/x86_64-osx-ghc-7.8.3/google-maps-0.1.0.0/libHSgoogle-maps-0.1.0.0-ghc7.8.3.dylib (dlopen(/Users/joel/workspace/haskell/bookingsystem/.cabal-sandbox/lib/x86_64-osx-ghc-7.8.3/google-maps-0.1.0.0/libHSgoogle-maps-0.1.0.0-ghc7.8.3.dylib, 9): Symbol not found: _googlezmmapszm0zi1zi0zi0_WebziGoogleziMapsziInternal_queryAPI_closure Referenced from: /Users/joel/workspace/haskell/bookingsystem/.cabal-sandbox/lib/x86_64-osx-ghc-7.8.3/google-maps-0.1.0.0/libHSgoogle-maps-0.1.0.0-ghc7.8.3.dylib Expected in: flat namespace in /Users/joel/workspace/haskell/bookingsystem/.cabal-sandbox/lib/x86_64-osx-ghc-7.8.3/google-maps-0.1.0.0/libHSgoogle-maps-0.1.0.0-ghc7.8.3.dylib) I couldn’t find a solution so far. Anyone can help me? I’m using ghc-7.8.3 on OS X 10.9.5 and cabal-install version 1.20.0.3 using version 1.20.0.1 of the Cabal library. Thanks, Joel

2 3

type families with non-type indices
by Doug McIlroy 08 Oct '14

08 Oct '14

Is there an analog or variant of type families that is indexed by values rather than by types? An example of such a family is integers mod p. One wants to define the operations once for all p, so that a type for any particular p can be introduced by a one-line declaration. Of course this can be done with p being a run-time parameter: data Modp = Modp Integer Int instance Num (Modp) where (Modp x p) + (Modp y q) = if p==q then Modp ((x+y) 'mod' p) p else error "unequal moduli in Modp operation" But it would be better to catch the error at compile time and not have to check at every operation. Doug

2 1

IO Exceptions through monad transformers stack
by Nicola Gigante 08 Oct '14

08 Oct '14

Hi, I’m not sure if this is the right list for “base” questions like this, if there are any more specific lists please let me know. I’m writing a parser and it works in a stack of monad transformers defined in this way: type Parser = StateT ParserState (ExceptT ParserError IO) The base monad is IO because the parse could have the need to load other files included by the file that’s being parsed. The ExceptT transformer make it easy for the lexer and the parser to signal errors. At the end I have a runParser function that compose runExceptT and evalStateT and provides me with a nice Either that contains the parsed result or a value of type ParserError. The ParserError data type is the following: data ParserError = LexingError <fields> | ParsingError <fields> Currently, whenever the parser encounter an error I simply do throwError $ ParsingError something…, for example. What I would like to do is to report in this way also the possible IO errors that can occur, by adding a IOError constructor to the datatype above and having runParser return such a value if any operation on the underlying IO monad throws an exception. How can I do something like that? The root of the problem is that I still have to grasp how the IO exceptions relate to all the other ExceptT, ErrorT, MonadError, types and how all the functions like throwError, catchError, catch, try, and so on work together, in a stack of transformers. Any clarification or pointer would be very appreciated Thank you very much, Nicola

3 3

ANN: islink 0.1.0.0: check if an HTML element is a link (useful for web scraping)
by Marios Titas 07 Oct '14

07 Oct '14

Hello everybody, I'd like to announce the first public release of islink. It's library that basically provides a list of combinations of HTML tag names and attributes that correspond to links to external resources. This includes things like ("a", "href"), ("img", "src"), ("script", "src") etc. It also comes with a convenience function to check if a particular pair (tag, attribute) corresponds to a link. This can be useful for web scraping. Here's an example how to use it to extract all (external) links from an HTML document (with the help of hxt): {-# LANGUAGE Arrows #-} import Text.Html.IsLink import Text.XML.HXT.Core -- returns a list of tuples containing the tag name, attribute name, -- attribute value of all links getAllLinks :: FilePath -> IO [(String, String, String)] getAllLinks path = runX $ doc >>> multi getLink where doc = readDocument [withParseHTML yes, withWarnings no] path getLink :: ArrowXml a => a XmlTree (String, String, String) getLink = proc node -> do tag <- getName -< node attrbNode <- getAttrl -< node attrb <- getName -< attrbNode val <- xshow getChildren -< attrbNode isLinkA -< (tag, attrb, val) where isLinkA = isLink `guardsP` this isLink (tag, attrb, _) = isLinkAttr tag attrb

1 0