- Haskell-Cafe - Haskell.org

[solved] forall disappear from type signature
by Takayuki Muranushi 02 Nov '12

02 Nov '12

Today, I encountered a strange trouble with higher-rank polymorphism. It was finally solved by nominal typing. Was it a bug in type checker? lack of power in type inference? or lack of my understanding? I'll submit this in hope that it will at least result in better GHC error message some day: My following code https://github.com/sugoi/sugoi/blob/0aec19b29274ddb7c5ae46b9b733e91bae0790b… > runDB :: Lens NetworkState RIB > runDB = lens (f::NetworkState -> RIB) (\x s -> s { _runDB = x }) > where f :: NetworkState -> RIB Didn't typecheck with following error: Sugoi/Types.hs:38:15: Couldn't match expected type `RIB' with actual type `DB.DBMT (Maybe Int) IO a0 -> IO (StM (DB.DBMT (Maybe Int) IO) a0)' Expected type: NetworkState -> RIB Actual type: NetworkState -> DB.DBMT (Maybe Int) IO a0 -> IO (StM (DB.DBMT (Maybe Int) IO) a0) In the first argument of `lens', namely `(f :: NetworkState -> RIB)' In the expression: lens (f :: NetworkState -> RIB) (\ x s -> s {_runDB = x}) How come an expression `(f :: NetworkState -> RIB)' is not of type NetworkState -> RIB ? When I changed the definition of runDB as follows, > runDB = lens (\NetworkState{_runDB = x} -> x) (\x s -> s { _runDB = x }) The source of error was more clear: Expected type: DB.DBMT (Maybe Int) IO a1 -> IO (StM (DB.DBMT (Maybe Int) IO) a1) Actual type: DB.DBMT (Maybe Int) IO a -> IO (StM (DB.DBMT (Maybe Int) IO) a) So, ghc couldn't unify these two types. By using the nominal typing, namely > - type RIB = RunInBase (DB.DBMT (Maybe Int) IO) IO > + newtype RIB = RIB (RunInBase (DB.DBMT (Maybe Int) IO) IO) the problem was solved. Always grateful to haskell-cafe from being there, -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

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Haskell Weekly News: Issue 249
by Daniel Santa Cruz 02 Nov '12

02 Nov '12

Welcome to issue 249 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of October 21 to October 27, 2012. Quotes of the Week * johnw: ah terminology, just when you think you know something, they redefined "know"... * lispy: The crappy thing about using partial fun*** Exception: Pattern match failure * shachaf: This Useful Information information brought to you by: ghci, and: the letter t * edwardk: i should have picked a better name to minimize confusion cmccann: huh. that's not something I ever expected to hear edwardk say * beaky: why did they settle on bitshiftrightassign (>>=) for monadic bind? Top Reddit Stories * Why is this simple text processing program so much slower than in dynamic languages? Domain: honza.ca, Score: 64, Comments: 189 On Reddit: [1] http://goo.gl/jW5ow Original: [2] http://goo.gl/pPzIb * Fay slides Domain: fay-lang.org, Score: 50, Comments: 42 On Reddit: [3] http://goo.gl/0UM4T Original: [4] http://goo.gl/h8iZE * Haskell for all: "Hello, core!" Domain: haskellforall.com, Score: 48, Comments: 6 On Reddit: [5] http://goo.gl/qXRd2 Original: [6] http://goo.gl/9cBeo * Haskell job opening at Functor AB Domain: alpmestan.com, Score: 35, Comments: 5 On Reddit: [7] http://goo.gl/hQmoa Original: [8] http://goo.gl/ASA9c * Enemy of the Mutable State Domain: ozonehouse.com, Score: 32, Comments: 10 On Reddit: [9] http://goo.gl/Wn3Yv Original: [10] http://goo.gl/r18by * Gloss(ish) and Fay: An Example Domain: zelda.designacourse.com, Score: 29, Comments: 7 On Reddit: [11] http://goo.gl/rtR9Y Original: [12] http://goo.gl/1cF0z * Yesodweb: Future work to improve the performance of Warp Domain: yesodweb.com, Score: 25, Comments: On Reddit: [13] http://goo.gl/0YF64 Original: [14] http://goo.gl/fq7ds * (a,b,c,d) as sugar for (a,(b,(c,d))) ? Domain: self.haskell, Score: 23, Comments: 37 On Reddit: [15] http://goo.gl/cc30R Original: [16] http://goo.gl/cc30R * Using Cloud Haskell in HPC Cluster Domain: malcodigo.blogspot.com.es, Score: 23, Comments: On Reddit: [17] http://goo.gl/LxmqY Original: [18] http://goo.gl/ycmZr * Haskell Platform 2012.4.0.0 release candidates Domain: trac.haskell.org, Score: 23, Comments: 3 On Reddit: [19] http://goo.gl/B4MNc Original: [20] http://goo.gl/Q4fHP Top StackOverflow Questions * Concrete example showing that monads are not closed under composition (with proof)? votes: 24, answers: 3 Read on SO: [21] http://goo.gl/sys7F * Why is there no IO transformer in Haskell? votes: 15, answers: 2 Read on SO: [22] http://goo.gl/04Xc7 * Is it possible to make GHC optimize (deforest) generic functions such as catamorphisms? votes: 14, answers: 1 Read on SO: [23] http://goo.gl/f4ZlU * Confused by the meaning of the 'Alternative' type class and its relationship to other type classes votes: 13, answers: 5 Read on SO: [24] http://goo.gl/RXNuV * How to add fields that only cache something to ADT? votes: 13, answers: 0 Read on SO: [25] http://goo.gl/FjLlX * How to reliably compare runtime of Haskell and C? votes: 10, answers: 1 Read on SO: [26] http://goo.gl/R7whA * Currying Product Types votes: 10, answers: 3 Read on SO: [27] http://goo.gl/7gxIS * Modular Program Design - Combining Monad Transformers in Monad Agnostic functions votes: 9, answers: 2 Read on SO: [28] http://goo.gl/8rh1B * Is there a nicer way to apply a function to both elements of a pair in a list than a list comprehension? votes: 9, answers: 3 Read on SO: [29] http://goo.gl/JF3p5 * How unpacking strict fields goes together with polymorphism? votes: 9, answers: 1 Read on SO: [30] http://goo.gl/58MuT Until next time, Daniel Santa Cruz References 1. http://honza.ca/2012/10/haskell-strings 2. http://www.reddit.com/r/haskell/comments/120h6i/why_is_this_simple_text_pro… 3. http://fay-lang.org/fay-talk/ 4. http://www.reddit.com/r/haskell/comments/11yrpi/fay_slides/ 5. http://www.haskellforall.com/2012/10/hello-core.html 6. http://www.reddit.com/r/haskell/comments/11vuzz/haskell_for_all_hello_core/ 7. http://alpmestan.com/posts/2012-10-23-haskell-job-opening-at-functor.html 8. http://www.reddit.com/r/haskell/comments/11yf8c/haskell_job_opening_at_func… 9. http://www.ozonehouse.com/mark/enemy/ 10. http://www.reddit.com/r/haskell/comments/125zt0/enemy_of_the_mutable_state/ 11. http://zelda.designacourse.com/codeworld.html 12. http://www.reddit.com/r/haskell/comments/126vdk/glossish_and_fay_an_example/ 13. http://www.yesodweb.com/blog/2012/10/future-work-warp 14. http://www.reddit.com/r/haskell/comments/11w7wv/yesodweb_future_work_to_imp… 15. http://www.reddit.com/r/haskell/comments/11ysa5/abcd_as_sugar_for_abcd/ 16. http://www.reddit.com/r/haskell/comments/11ysa5/abcd_as_sugar_for_abcd/ 17. http://malcodigo.blogspot.com.es/2012/10/using-cloud-haskell-in-hpc-cluster… 18. http://www.reddit.com/r/haskell/comments/122m6y/using_cloud_haskell_in_hpc_… 19. http://trac.haskell.org/haskell-platform/wiki/ReleaseCandidates 20. http://www.reddit.com/r/haskell/comments/124kje/haskell_platform_2012400_re… 21. http://stackoverflow.com/questions/13034229/concrete-example-showing-that-m… 22. http://stackoverflow.com/questions/13056663/why-is-there-no-io-transformer-… 23. http://stackoverflow.com/questions/13099203/is-it-possible-to-make-ghc-opti… 24. http://stackoverflow.com/questions/13080606/confused-by-the-meaning-of-the-… 25. http://stackoverflow.com/questions/13098448/how-to-add-fields-that-only-cac… 26. http://stackoverflow.com/questions/13009728/how-to-reliably-compare-runtime… 27. http://stackoverflow.com/questions/13080074/currying-product-types 28. http://stackoverflow.com/questions/13007123/modular-program-design-combinin… 29. http://stackoverflow.com/questions/13022720/is-there-a-nicer-way-to-apply-a… 30. http://stackoverflow.com/questions/13098375/how-unpacking-strict-fields-goe…

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Comparing Haskell and Darcs
by Henk-Jan van Tuyl 01 Nov '12

01 Nov '12

L.S., I just found a site that compares o.a. Haskell to other languages[0] and Darcs to other version control systems[1]: Hammer principle. Haskell scores quite good, Darcs could do better. Regards, Henk-Jan van Tuyl [0] http://hammerprinciple.com/therighttool/items/haskell [1] http://hammerprinciple.com/versioncontrol/items/darcs -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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Deriving settings from command line, environment, and files
by David Thomas 01 Nov '12

01 Nov '12

Is there a library that provides a near-complete solution for this? I looked around a bit and found many (many!) partial solutions on hackage, but nothing that really does it all. In coding it up for my own projects, however, I can't help but feel like I must be reinventing the wheel. What I want is something that will process command line options (this seems to be where most packages are targetted), environment variables, and settings files (possibly specified in options), and override/default appropriately. Did I miss something? If not, I'd like to see about fixing the situation, but very much want to avoid the whole "There were X libraries that didn't quite do it right; now there's X+1!" thing. Are there any opinions as to which of the many libraries is likely the best starting point if I'm going to contribute to one of them?

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Looking for advice on modelling a 3-way merge (with varying strategies)
by Oliver Charles 01 Nov '12

01 Nov '12

Hello, I'm currently trying to implement a way 3 way merge, that is dispatched on type. Essentially, I want to be able to say "I have this large data type which is a product of various other types - here's how to merge it". By 3 way merge, I mean that I have a 'new' value, a 'current' value and a 'common ancestor' value. So far I have: import Control.Applicative import Data.Monoid newtype Merge a = Merge { runMerge :: Either [String] a } instance Functor Merge where fmap f a = Merge $ fmap f $ runMerge a instance Applicative Merge where pure = Merge . Right (Merge (Right f)) <*> (Merge (Right a)) = Merge $ Right $ f a (Merge (Right _)) <*> (Merge (Left c)) = Merge $ Left $ c (Merge (Left c)) <*> (Merge (Left c')) = Merge $ Left $ c <> c' (Merge (Left c)) <*> _ = Merge $ Left $ c ok :: a -> Merge a ok = pure failMerge :: String -> Merge a failMerge x = Merge $ Left [x] So far so good, so I can add my first merge strategy: mergeEq :: Eq a => String -> a -> a -> a -> Merge a mergeEq lbl new current ancestor | current == new = ok new | current == ancestor = ok new | new == ancestor = ok current | otherwise = failMerge lbl And we can make use of this to build more complicated merge strategies: data Person = Person { name :: String, surname :: String } deriving (Show) mergeEqOn :: Eq a => (b -> a) -> String -> b -> b -> b -> Merge a mergeEqOn l lbl n c a = mergeEq lbl (l n) (l c) (l a) mergePerson :: Person -> Person -> Person -> Merge Person mergePerson new current ancestor = Person <$> mergeEqOn name "name" new current ancestor <*> mergeEqOn surname "surname" new current ancestor >>> runMerge $ mergePerson Person { name="Steve", surname="Bobman" } Person { name="Joe", surname="Obman" } Person { name="oe", surname="Obman" } Left ["name"] >>> runMerge $ mergePerson Person { name="Steve", surname="Bobman" } Person { name="Joe", surname="Obman" } Person { name="Joe", surname="Obman" } Right (Person {name = "Steve", surname = "Bobman"}) Everything does exactly what I want, but it doesn't smell as good as I expect from Haskell. Firstly, all my primitives or operations take 3 arguments that have to be threaded in a very specific pattern. If I accidently call mergeEq ancestor current new, I've ran things in the wrong order and I'm Gonna Have a Bad Time. So that's no good. Secondly, is there a better way of labelled parts of a merge? I suspect not, as at some point I need a human readable display, but I did wonder about using a lens as a label, and then mapping lenses to human readable names later -- but that will require a bit of newtyping in order to provide the Eq instance, or maybe i make a HumanLens type which is a lens and a name?). Finally, I'm currently a little obsessed with the idea of building applicatives from the composition or product of other applicatives - can I use this trick here? I initially used 'Compose ((,) [String]) Maybe a' but this means it's possible to have (["Conflicts"], Just "But success!"), which is nonsense. I'd love to hear your thoughts! - ocharles

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Haskell User Group Hamburg (Germany) - Meeting Thursday November 8th 2012
by Stefan Schmidt 31 Oct '12

31 Oct '12

Hello cafe, there's an upcomming meeting of the Haskell user group Hamburg next week on Thursday, November 8th. We'll start 19:00 at the fortytools GmbH in Hamburg (Georgsplatz 10, 20099 Hamburg). We don't have a program or talk yet. Maybe we'll discuss our experiences from the last ICFP and the Haskell Symposium. If you are interested to join us, have a look at our doodle (only in german) http://www.doodle.com/gvdsbsyfz7a7427g Be careful: the Google Maps link in the Doodle may direct you to the old address. Georgsplatz 10, 20099 Hamburg is the new one. Further announcements will be made via Twitter, so you may want to follow https://twitter.com/hug_hh.

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ANNOUNCE: btree-concurrent
by Johan Brinch 31 Oct '12

31 Oct '12

Greetings Haskellers, We are pleased to announce this new package, featuring a concurrent b-tree developed as part of our master's thesis on backup systems. The package features code related to STM, caching, QuickCheck and of course concurrency and parallelism. And it's also on Hackage: http://hackage.haskell.org/package/btree-concurrent In case the build fails on Hackage and the documentation doesn't work, it's also available here: http://hindsight.dk/doc/html/btree-concurrent Credits to Thomas Conway of Melbourne University, for discussions related to the implementation, and to our supervisor Ken Friis Larsen of University of Copenhagen. And now, the description of the package: ===== A backend agnostic, concurrent BTree with relaxed balance[1] written in Haskell using a mix of IO operations and pure STM. Although the code does work, it is neither production-ready nor complete. Features include: * Caching: While nodes are periodically saved on persistent storage (e.g. disk) they are cached in-memory during operations. * Live flushing: It is possible to save the current version of the tree to disk and run modifying operations at the same time. The nodes are updated buttom-up to ensure a valid tree in the case of a crash. * Backend agnosticism: A simple API is used as an abstraction for the persistent storage. * Verification: The test-suite uses QuickCheck to compare the trees behaviour to that of Data.Map. Deficients include: * Memory greedy. Nodes are not stored in a compact fashion and are constantly being serialised/deserialised. * findMin and findMax are incomplete and may fail (see the TODO for details). * The implementation does not parallelise well. [1] B-trees with relaxed balance, K. S. Larsen & R. Fagerberg, Parallel Processing Symposium, 1995. Proceedings., 9th International -- Johan Brinch & Morten Brøns-Pedersen

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ANNOUNCE: plat
by Miguel Mitrofanov 31 Oct '12

31 Oct '12

My current project at work has a web interface. Therefore I needed an HTML templating library. I've tried several and found them all unsatisfactory. Most of them generate Haskell code from templates (with Template Haskell or a separate utility), and I don't like metaprogramming; and recompiling the program every time you make a small change in the template kinda defeats the purpose. Other libraries make it PITA to implement basic things, like iterating over an array. So, I'm proud to announce the first release of another template library: plat-0.1.0.1 (http://hackage.haskell.org/package/plat) An example of what it does. If your data looks like do "name" =: "Mac's tools" "staff" =: [ do "index" =: "1" "name" =: "Alice" , do "name" =: "Bob" "index" =: "2" "bad" =: True , do "index" =: "3" "name" =: "Nicolás" ] and your template is List of employees at @name: @staff[person @person.index@.. @person.name@{@!person.bad (going to be fired)@|@}@ @]@# There won't be an empty line in the result Then the result you get would be this: List of employees at Mac's tools: 1. Alice 2. Bob (going to be fired) 3. Nicolás Keep in mind, that it's actually a string template library, and the resulting string doesn't have to be valid HTML, or any HTML at all. I haven't benchmarked this library - since I intended to use it for web interface in an program which won't be released outside, speed was not in my priorities; but I'll work on that in future versions. Also, templates are supposed to be in UTF-8, as I've successfully forgot all about other encodings. So, if you want binary templates, I'm afraid, this library is not for you.

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Safe lens?
by Greg Fitzgerald 31 Oct '12

31 Oct '12

Why are getters from the 'lens' package unsafe? Is there a subset like Data.Label.Pure from 'fclabels' that can be imported safely? $ cat a.hs {-# LANGUAGE Safe #-} import Control.Lens.Getter main = print 123 $ runghc a.hs a.hs:3:1: Control.Lens.Getter: Can't be safely imported! The module itself isn't safe. Thanks, Greg

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ANNOUNCE: network-house
by Niklas Hambüchen 30 Oct '12

30 Oct '12

In absence of a library that easily parses network packages from recorded raw data (not sockets), I have taken the network stack from the House project and put it into a separate package, network-house. http://hackage.haskell.org/package/network-house This library allows you to parse Ethernet, TCP, UDP, IPv4, IPv6, ICMP, DHCP and TFTP packages from raw data (e.g. recorded traffic). I have left the source code almost unmodified (apart from the relocation of one module and a few necessary LANGUAGE pragmas). As House development itself seems dormant, I invite you to collaborate on the network stack on https://github.com/nh2/network-house. If you would like to contribute, these are the things it definitely needs: * A ByteString interface (it currently works on UArrays). If you know what the most performant way is to convert a UArray to a ByteString, please let me know. * Some examples how to use it * Haddock documentation and code cleanup * Somebody should ask the original authors if they would like to relax the license from GPL to BSD. Cheers Niklas

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