- Haskell-Cafe - Haskell.org

Key-Parametrized Lookup Table
by Alexander Foremny 02 Aug '12

02 Aug '12

Hello list, I am currently thinking that a problem of mine would best be solved if there was a Map-like data structure in which the value returned is parametrized over the lookup type. I wonder is this makes sense and if such a data structure exists or if it could be created while still being well typed. I essentially want to statically define a scope of Key values and dynamically define a list of keys. > -- Scope of possible keys. > type Label = String > data Key a where > KeyStr :: Label -> Key String > KeyInt :: Label -> Key Int > KeyChoice :: Label -> [a] -> Key a > -- Some key values, to be extended at runtime. > strKey "Some String" > strKey' "Another String" > intKey "Some integer" > choiceKey "Chose one" [ "a", "b", "c" ] :: KeyChoice String Now I need a data structure to possibly associate a value to the key. > data MapG = ... > type Value a = a > insert :: Key a -> Value a -> MapG Key Value -> MapG Key Value > lookup :: Key a -> MapG Key Value -> Maybe (Value a) I tried implementing this with multiple Map k a's. I tried adding a phantom type on some storage type of to implement KeyChoice as of type Key Int, but I ran into troubles with this approach. I wonder if Dynamic or Type Families could achieve this, but I am quite at a loss and would like to hear your opinion. I did try to search for this a bit, but I don't quite know how to phrase my problem. I'd like to apologize in advance if this question has been asked already. Regards, Alexander Foremny

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Haskell Platform - BSD License?
by Sai Hemanth K 02 Aug '12

02 Aug '12

Hi, I am trying to use haskell for building a tool (in a commercial setting). I am trying to figure out what all licenses are involved here. Is there a single license for the entire haskell platform (and the runtime) or is it that I need to look at the individual licenses of all the libraries and tools that make up the platform and point to them separately? The wikipedia page on haskell platform [0] says Haskell Platform is BSD licensed. But I do not find any such info elsewhere. Any pointers on this would be greatly appreciated, Thanks, Hemanth K [0] Haskell platform on wikipedia : http://en.wikipedia.org/wiki/Haskell_Platform

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What Haskell Records Need
by Jonathan Geddes 02 Aug '12

02 Aug '12

Greetings, tl;dr - What Haskell Records need are semantic editor combinators for free. I know this is yet another Record proposal among many, but none of them out there strike me as being exactly what I want in Haskell. Take the following types from a contrived example. >type Salary = Integer > >data Job = Job > { title :: String > , salary :: Salary > } > >data Person = Person > { name :: String > , job :: Job > } Since I've used record syntax, I get getter/accessor functions (title, salary, name, job) for free. Now suppose I want to create an aggregate getter function: return the salary of a given person. Piece of cake, it's just function composition >getSalary :: Person -> Salary >getSalary = salary . job Done! Now suppose I want to write a setter/mutator function for the same nested field >setSalaryMessy :: Salary -> Person -> Person >setSalaryMessy newSalary person = > person { > job = (job person) { > salary = newSalary > } > } Ouch! And that's not even very deeply nested. Imagine 4 or 5 levels deep. It really makes Haskell feel clunky next to `a.b.c.d = val` that you see in other languages. Of course immutability means that the semantics of Haskell are quite different (we're creating new values here, not updating old ones) but it's still common to model change using these kinds of updates. What if along with the free getters that the compiler generates when we use record syntax, we also got semantic editor combinator (SEC) functions[0] that could be used as follows? >setSalary newSalary = job' $ salary' (const newSalary) > >giveRaise amount = job' $ salary' (+amount) > >givePercentRaise percent = job' $ salary' (*(1+percent)) For each field x, the compiler generates a function x' (the tic is mnemonic for change). These little functions aren't hard to write, but they're classic boilerplate. >job' :: (Job -> Job) -> Person -> Person >job' f person = person {job = f $ job person} >salary' :: (Salary -> Salary) -> Job -> Job >salary' f job = job { salary = f $ salary job} These type of utility functions are a dream when working with any reference type or State Monad. > modify $ givePercentRaise 0.25 The compiler could also generate polymorphic SEC functions for polymorphic fields. Further, the compiler could disallow using old-style update syntax for fields whose SEC update function is not in scope, giving us fine-grained control over access and update. On the other hand we currently have to create new functions to achieve this (exporting the getter means exporting the ability to update as well, currently). Of course this doesn't address the namespacing issues with records, but it is likely nicely orthogonal to other proposals which do. Also note that there's a package on hackage [1] that will generate SEC functions using TH. It's nice, but I prefer the style of field names used above for updaters (field' vs editField). Let me know what you think. I'll write up an official proposal if there's a bit of general interest around this. Thanks for reading, --Jonathan [0] - http://conal.net/blog/posts/semantic-editor-combinators [1] - http://hackage.haskell.org/packages/archive/sec/0.0.1/doc/html/Data-Semanti…

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Proposal: Technique to handle package dependency gridlock
by Tom Murphy 02 Aug '12

02 Aug '12

We've got a problem with dependencies: http://cdsmith.wordpress.com/2011/01/21/a-recap-about-cabal-and-haskell-lib… http://cdsmith.wordpress.com/2011/01/17/the-butterfly-effect-in-cabal/ http://www.reddit.com/r/haskell/comments/x4knd/what_is_the_reason_for_haske… I'd like to present a proto-proposal for another arrow in our quiver. First, a few... Principles: This problem isn't uniquely Haskell's ...Although it may be uniquely Haskell's to solve. Lots of languages have a problem managing their package dependencies. To quote Chris Smith, "it’s fair to say that perhaps Haskell is one of the few environments where we’ve got a prayer at solving the problem." It's a magnitude problem The trick is to not find one perfect solution; it's to use enough effective solutions that the remaining tricky cases can be swept away individually. Solving the problem will most likely involve several techniques (good versioning policy, a little work on the part of package maintainers, smarter Cabal, new tools, etc.). Once we get the problem down to a manageable level, then it's a problem that can be dealt with package-by-package. Haskell packages are not black boxes Inheriting - from the imperative world - the idea of packages as indivisible units may be a mistake. A language like Ruby may have to import a full library, because it's nearly-impossible to reason about the behavior of part of it in isolation from the rest of it. Haskell's not like that, though! Referential transparency gives us awesome powers to reason about a pure function's behavior in nearly all cases. We could define package dependency in terms of providing functions-that-behave-like-the-ones-we-used. I mean, for god's sake, we can write QuickCheck properties like `quickSort == bubbleSort` and test it with a very high degree of assurance. The ability to pass the same suite of QuickCheck tests, or even the ability to pass a test like `fmap-3.1.2 == fmap 3.1.3`, could be strong enough proof - in the vast majority of cases - of functions' equivalence and therefore interchangeability. This could ease a significant number of package version constraints. Call it Property-Based Versioning - where your packages dependencies are based on which other versions of the package can provide the same functionality *for the functions and types that your package uses*. Programmer time is precious Thus far, the package dependency issue has been more or less a tug-of-war between package maintainers not wanting to do more work, and package users not wanting to do more work. Users just want to use the packages. Maintainers have a potentially-endless list of versions to check compatibility for ("I'm using foo-2.5.3. I assume anything 2.3 or greater is fine. Should I check through and see if 2.4 works? 2.3? 2.2? [...]"). Not to mention they can't know what the future will hold for later versions of the dependent packages, and they have to make a usually-pretty-uneducated guess about the future. Instead of maintainers and users, maybe computers should lift some of the load. Huge swaths of the most-used packages on Hackage are pure functions and data types. These are relatively easy to reason about, and if we could come close to reducing our dependency problem by, say, 30-50%, that would in my view put us within striking distance of having a very smoothly-running package ecosystem. These are decisions the community has to make There are lots of decisions to make and tradeoffs to balance. The main reason that I'm presenting such an embryonic description of this technique is to see what the Haskell community values most. In my view the ideal would be to be able to write a dependency description like "This package needs functions that behave like the ones I used in foo-3.1.2". A potentially less hair-shirty way to get some of the same functionality is for package authors to have a cabal-install or pre-cabal-install tool that determines which functions and data types are used from a package, finds equivalent ones in other packages, and write a dependency spec based on its findings (this wouldn't have anything to say about compatibility of not-yet-written packages, of course). Tom

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Haskell Implementors' Workshop 2012: Call for Participation
by Johan Tibell 02 Aug '12

02 Aug '12

===================================================================== Call for Participation ACM SIGPLAN Haskell Implementors' Workshop http://haskell.org/haskellwiki/HaskellImplementorsWorkshop/2012 Copenhagen, Denmark, September 14th, 2012 The workshop will be held in conjunction with ICFP 2012 http://www.icfpconference.org/icfp2012/ ===================================================================== There are less than nine days left until the early registration expires! The Haskell Implementors' Workshop is to be held alongside ICFP 2012 this year in Copenhagen, Denmark. There will be no proceedings; it is an informal gathering of people involved in the design and development of Haskell implementations, tools, libraries, and supporting infrastructure. This relatively new workshop reflects the growth of the user community: there is a clear need for a well-supported tool chain for the development, distribution, deployment, and configuration of Haskell software. The aim is for this workshop to give the people involved with building the infrastructure behind this ecosystem an opportunity to bat around ideas, share experiences, and ask for feedback from fellow experts. We intend the workshop to have an informal and interactive feel, with a flexible timetable and plenty of room for ad-hoc discussion, demos, and impromptu short talks. * Program: http://www.haskell.org/haskellwiki/HaskellImplementorsWorkshop/2012 * Local arrangements (including travel and accommodation): http://icfpconference.org/icfp2012/local.html * Registration is available via (workshop abbreviation: HIM): http://icfpconference.org/icfp2012/registration.html Workshop organizers: * Workshop Co-Chairs: Gregory Collins, Google Johan Tibell, Google * Program Committee: Lennart Augustsson, Standard Chartered Bank Manuel M T Chakravarty, University of New South Wales Simon Marlow, Microsoft Research David Terei, Stanford University =====================================================================

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Haskell Weekly News: Issue 238
by Daniel Santa Cruz 02 Aug '12

02 Aug '12

Welcome to issue 238 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of July 22 to 28, 2012. Quotes of the Week * clsmith: i find programming in haskell is mostly about reinventing things and then discovering them in Prelude * EvanR: ruby, cobol of the 2000s * monochrom: "customers who used == also liked: foldr, map, Ord, continuation passing style" * jmcarthur: we calmly talk trolls to death Top Reddit Stories * Haskell logo QR code Domain: self.haskell, Score: 44, Comments: 5 On Reddit: [1] http://goo.gl/DuSSU Original: [2] http://goo.gl/DuSSU * In Praise of EitherT Domain: ocharles.org.uk, Score: 43, Comments: 9 On Reddit: [3] http://goo.gl/V2R9B Original: [4] http://goo.gl/M7rTb * The must-know list of packages on hackage? Domain: self.haskell, Score: 39, Comments: 29 On Reddit: [5] http://goo.gl/N7i6J Original: [6] http://goo.gl/N7i6J * SmartCheck: a better QuickCheck for testing algebraic data and generalizing counterexamples found Domain: leepike.wordpress.com, Score: 39, Comments: 6 On Reddit: [7] http://goo.gl/ENFxH Original: [8] http://goo.gl/SZMCG * Compile Haskell (Fay) to Javascript using Javascript compiled with GHCJS Domain: ghcjs.github.com, Score: 36, Comments: 8 On Reddit: [9] http://goo.gl/YOSuG Original: [10] http://goo.gl/d7Af4 * Programming in Fay - Simple example Domain: youtube.com, Score: 30, Comments: 17 On Reddit: [11] http://goo.gl/Ieswf Original: [12] http://goo.gl/4DcLd * Flexibly-Phased Constraints in Haskell Domain: jonmsterling.com, Score: 28, Comments: 19 On Reddit: [13] http://goo.gl/uvYY4 Original: [14] http://goo.gl/3cRD4 * Do you use Haskell in business? What do you use it for? Domain: self.haskell, Score: 24, Comments: 57 On Reddit: [15] http://goo.gl/AcGlL Original: [16] http://goo.gl/AcGlL * ANN: CmdTheLine 0.2 released -- with new tutorial. Domain: elifrey.com, Score: 23, Comments: 11 On Reddit: [17] http://goo.gl/yezpc Original: [18] http://goo.gl/cM9kZ * State of the Shelly (Haskell scripting lib) Domain: yesodweb.com, Score: 23, Comments: 1 On Reddit: [19] http://goo.gl/siupj Original: [20] http://goo.gl/FyrDC * Hackage b0rked for cabal 0.10.2 Domain: self.haskell, Score: 22, Comments: 4 On Reddit: [21] http://goo.gl/HzgX8 Original: [22] http://goo.gl/HzgX8 * Pipes to Conduits part 0: Combining Functors Domain: unknownparallel.wordpress.com, Score: 22, Comments: 18 On Reddit: [23] http://goo.gl/sF95u Original: [24] http://goo.gl/Jepr5 * Deconstraining DSLs [PDF] Domain: doc.ic.ac.uk, Score: 19, Comments: 2 On Reddit: [25] http://goo.gl/Ur7Lz Original: [26] http://goo.gl/E2DhL * What is the reason for Haskell's cabal package problems? Domain: self.haskell, Score: 17, Comments: 41 On Reddit: [27] http://goo.gl/O0x9z Original: [28] http://goo.gl/O0x9z * Pipes to Conduits part 4: Recovering from Abort Domain: unknownparallel.wordpress.com, Score: 16, Comments: On Reddit: [29] http://goo.gl/tr9Jy Original: [30] http://goo.gl/XbAUR * Pipes to Conduits part 1: Yield and Await Domain: unknownparallel.wordpress.com, Score: 15, Comments: 5 On Reddit: [31] http://goo.gl/KV0io Original: [32] http://goo.gl/GuFqX * Pipes to Conduits part 2: Upstream Results Domain: unknownparallel.wordpress.com, Score: 15, Comments: 9 On Reddit: [33] http://goo.gl/ONYzN Original: [34] http://goo.gl/Tf3Si * Instance Templates: a proposed feature to change type class hierarchies without breaking compatibility Domain: github.com, Score: 15, Comments: 6 On Reddit: [35] http://goo.gl/7Jx8c Original: [36] http://goo.gl/LG1YK * Categories over pairs of types Domain: twanvl.nl, Score: 15, Comments: 1 On Reddit: [37] http://goo.gl/8ee77 Original: [38] http://goo.gl/jxLPR * Pipes to Conduits part 3: Abort Domain: unknownparallel.wordpress.com, Score: 14, Comments: 12 On Reddit: [39] http://goo.gl/IZbij Original: [40] http://goo.gl/2kgLg Top StackOverflow Questions * What does “Pure” mean, in the context of programming languages and paradigms? votes: 21, answers: 2 Read on SO: [41] http://goo.gl/a0IUe * Can a thunk be duplicated to improve memory performance? votes: 17, answers: 3 Read on SO: [42] http://goo.gl/rznKJ * What's the current status of restricted monads? votes: 14, answers: 1 Read on SO: [43] http://goo.gl/UnMbE * Is it good style to use a case expression on an Unit value just to use guards? votes: 13, answers: 3 Read on SO: [44] http://goo.gl/E8xn3 * Temporary namespace/context in Haskell votes: 12, answers: 3 Read on SO: [45] http://goo.gl/AWg4O * Using the LogicT Monad Transformer in Haskell votes: 12, answers: 1 Read on SO: [46] http://goo.gl/DOw0H * Irrefutable pattern does not leak memory in recursion, but why? votes: 11, answers: 1 Read on SO: [47] http://goo.gl/qK7rx * How to determine type of Haskell functions? votes: 11, answers: 3 Read on SO: [48] http://goo.gl/Rn5F7 * Haskell: map runST votes: 9, answers: 3 Read on SO: [49] http://goo.gl/mWnvL * How to play with Control.Monad.Writer in haskell? votes: 9, answers: 1 Read on SO: [50] http://goo.gl/GLTJc Until next time, Daniel Santa Cruz References 1. http://www.reddit.com/r/haskell/comments/xb3ig/haskell_logo_qr_code/ 2. http://www.reddit.com/r/haskell/comments/xb3ig/haskell_logo_qr_code/ 3. http://ocharles.org.uk/blog/posts/2012-07-24-in-praise-of-EitherT.html 4. http://www.reddit.com/r/haskell/comments/x3ikr/in_praise_of_eithert/ 5. http://www.reddit.com/r/haskell/comments/x4g18/the_mustknow_list_of_package… 6. http://www.reddit.com/r/haskell/comments/x4g18/the_mustknow_list_of_package… 7. http://leepike.wordpress.com/2012/07/26/smartcheck/ 8. http://www.reddit.com/r/haskell/comments/x7fyt/smartcheck_a_better_quickche… 9. http://ghcjs.github.com/fay-hello-js.jsexe/ 10. http://www.reddit.com/r/haskell/comments/x43nd/compile_haskell_fay_to_javas… 11. https://www.youtube.com/watch?v=dhFZ8vx9byc 12. http://www.reddit.com/r/haskell/comments/xbihk/programming_in_fay_simple_ex… 13. http://www.jonmsterling.com/posts/2012-07-22-flexibly-phased-constraints-in… 14. http://www.reddit.com/r/haskell/comments/wzwda/flexiblyphased_constraints_i… 15. http://www.reddit.com/r/haskell/comments/x8qnh/do_you_use_haskell_in_busine… 16. http://www.reddit.com/r/haskell/comments/x8qnh/do_you_use_haskell_in_busine… 17. http://elifrey.com/2012/07/23/CmdTheLine-Tutorial/ 18. http://www.reddit.com/r/haskell/comments/x0xda/ann_cmdtheline_02_released_w… 19. http://www.yesodweb.com/blog/2012/07/shelly-update 20. http://www.reddit.com/r/haskell/comments/x9etz/state_of_the_shelly_haskell_… 21. http://www.reddit.com/r/haskell/comments/x16h7/hackage_b0rked_for_cabal_010… 22. http://www.reddit.com/r/haskell/comments/x16h7/hackage_b0rked_for_cabal_010… 23. http://unknownparallel.wordpress.com/2012/07/24/pipes-to-conduits-part-0-co… 24. http://www.reddit.com/r/haskell/comments/x2lao/pipes_to_conduits_part_0_com… 25. http://www.doc.ic.ac.uk/~wlj05/files/Deconstraining.pdf 26. http://www.reddit.com/r/haskell/comments/x3bta/deconstraining_dsls_pdf/ 27. http://www.reddit.com/r/haskell/comments/x4knd/what_is_the_reason_for_haske… 28. http://www.reddit.com/r/haskell/comments/x4knd/what_is_the_reason_for_haske… 29. http://unknownparallel.wordpress.com/2012/07/28/pipes-to-conduits-part-4-re… 30. http://www.reddit.com/r/haskell/comments/xar9r/pipes_to_conduits_part_4_rec… 31. http://unknownparallel.wordpress.com/2012/07/25/pipes-to-conduits-part-1-yi… 32. http://www.reddit.com/r/haskell/comments/x4si1/pipes_to_conduits_part_1_yie… 33. http://unknownparallel.wordpress.com/2012/07/26/pipes-and-conduits-part-2-u… 34. http://www.reddit.com/r/haskell/comments/x6w9k/pipes_to_conduits_part_2_ups… 35. https://github.com/mgsloan/instance-templates 36. http://www.reddit.com/r/haskell/comments/x71ll/instance_templates_a_propose… 37. http://twanvl.nl/blog/haskell/categories-over-pairs-of-types 38. http://www.reddit.com/r/haskell/comments/xab88/categories_over_pairs_of_typ… 39. http://unknownparallel.wordpress.com/2012/07/27/pipes-to-conduits-part-3-ab… 40. http://www.reddit.com/r/haskell/comments/x8yx2/pipes_to_conduits_part_3_abo… 41. http://stackoverflow.com/questions/11607927/what-does-pure-mean-in-the-cont… 42. http://stackoverflow.com/questions/11675807/can-a-thunk-be-duplicated-to-im… 43. http://stackoverflow.com/questions/11599685/whats-the-current-status-of-res… 44. http://stackoverflow.com/questions/11620649/is-it-good-style-to-use-a-case-… 45. http://stackoverflow.com/questions/11661963/temporary-namespace-context-in-… 46. http://stackoverflow.com/questions/11695961/using-the-logict-monad-transfor… 47. http://stackoverflow.com/questions/11625840/irrefutable-pattern-does-not-le… 48. http://stackoverflow.com/questions/11638232/how-to-determine-type-of-haskel… 49. http://stackoverflow.com/questions/11662696/haskell-map-runst 50. http://stackoverflow.com/questions/11684321/how-to-play-with-control-monad-…

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GHC rendering of non-ASCII characters configurable?
by Richard Cobbe 01 Aug '12

01 Aug '12

I'm working on an application that involves processing a lot of Unicode data, and I'm finding the built-in Show implementation for Char to be really inconvenient. Specifically, it renders all characters at U+0080 and above with decimal escapes: Prelude> '\x80' '\128' This is annoying because all of the Unicode charts give the code points in hex, and indeed the charts are split into different PDFs at numbers that are nice and round in hex but not in decimal. So in order to figure out which character I'm looking at, I have to convert back to hex and then look it up in the charts. Is there any way to ask GHC to render super-ASCII characters with their hexadecimal escapes, instead? I'm perfectly happy to write my own custom Show instance, but I don't know how to hook that into ghci's REPL (or, for that matter, the routines that HUnit uses to generate the messages on failed tests, etc.). I'm using GHC 7.4.1 on MacOS 10.7.4. Thanks, Richard

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Re: [Haskell-cafe] specifying using type class
by Patrick Browne 01 Aug '12

01 Aug '12

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Re: [Haskell-cafe] The following is supposed to be initializing a 2D array but it doesn't seem to work.
by KC 31 Jul '12

31 Jul '12

That was it! On Tue, Jul 31, 2012 at 3:34 PM, Jan-Willem Maessen <jmaessen(a)alum.mit.edu>wrote: > > > On Tue, Jul 31, 2012 at 5:52 PM, KC <kc1956(a)gmail.com> wrote: > >> All I am getting is this: >> >> array ((1,5),(1,5)) [((1,5),1.0)] >> >> Maybe the behaviour of ghc was changed since the article was written. >> > > I think you've made a common mistake here. Array bounds are (lower in all > dimensions, upper in all dimensions). So perhaps you intended to use the > bounds ((1,1), (5,5)) in your code? > > I make this mistake frequently. > > -Jan-Willem Maessen > -- -- Regards, KC

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CRIP: the Curiously Reoccuring Instance Pattern
by oleg＠okmij.org 31 Jul '12

31 Jul '12

Ryan Ingram wrote: > I've been seeing this pattern in a surprising number of instance > definitions lately: > > instance (a ~ ar, b ~ br) => Mcomp a ar b br [1] > instance (b ~ c, CanFilterFunc b a) => CanFilter (b -> c) a [2] And here are a few more earlier instances of the same occurrence: http://okmij.org/ftp/Haskell/typecast.html > What I'm wondering is--are there many cases where you really want the > non-constraint-generating behavior? It seems like (aside from contrived, > ahem, instances) whenever you have instance CLASS A B where A and B share > some type variables, that there aren't any valid instances of the same > class where they don't share the types in that way. Instances of the form class C a b class C a a class C a b are very characteristic of (emulation) of disequality constraints. Such instances occur, in a hidden form, all the time in HList -- when checking for membership in a type-level list, for example. There are naturally two cases: when the sought type is at the head of the list, or it is (probably) at the tail of the list. class Member (h :: *) (t :: List *) instance Member h (Cons h t) instance Member h t => Member h (Cons h' t) Of course instances above are overlapping. And when we add functional dependencies (since we really want type-functions rather type relations), they stop working at all. We had to employ work-arounds, which are described in detail in the HList paper (which is 8 years old already).

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