May 2010 - Haskell-Cafe

Calculating a value as side effect of converting list of strings into list of records.
by Eugeny N Dzhurinsky 24 May '10

24 May '10

Hello, All! I need some help to solve the following task: ================================================================================================= import Data.Map as M import Data.List as L data Pair = Pair { name, value :: String } type IxPair = (Int, String, String) type IxPairParser = Pair -> Maybe IxPair type RecordUpdateF r = String -> String -> r -> r convertPairsToRecords :: RecordUpdateF r -> IxPairParser -> r -> [Pair] -> [r] convertPairsToRecords updateRecF parsePairF initRec pairs = M.elems $ L.foldl' processWithPairs initMap pairs where initMap = M.empty processWithPairs resMap = maybe resMap (updateMapF resMap) . parsePairF updateMapF res (idx, key, value) | idx `M.member` res = M.adjust (updateRecF key value) idx res | otherwise = M.insert idx (updateRecF key value initRec) res ================================================================================================= In short, input looks like name=value pairs: pair_1_username=user pair_1_password=pass pair_1_fullname=vasya poopking The parser IxPairParser takes the pair of name=value, and extracts ordering number of the current pair, making it a triple like below (1, username, user) then convertPairsToRecords takes the triple and performs lookup in the Map for a record with given ID. Then it updates the record and places it back into the map. If no record is found - then new one is created, updated and inserted into the Map. Now, I've got the case when input contains pairs like below pair_1_lastlogin=2010-05-24 12:23 ... pair_2_lastlogin=2009-12-20 10:30 ... pair_2_lastlogin=2010-04-06 18:20 At this point I need to update the record and keep the latest date somewhere, so it will be possible to extract the latest login date among all such pairs. I can think about post-processing list [r], extract lastlogin field and simply use fold to get the oldest one. But I'd like to calculate this information during parsing of the input. And I am not allowed to change the interface of convertPairsToRecords. Can somebody please suggest how to solve the task? Probably with Writer monad, or may be State? Thank you in advance! -- Eugene Dzhurinsky

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Best way to instance Fix?
by Sam Martin 24 May '10

24 May '10

Hi! I'm trying to work out the best way to generate (ideally derive) instances for the Fix type. Here's a cut down example: data Greet x = AlloAllo x x | AuRevoir deriving Show newtype Fix f = In { out :: f (Fix f) } -- deriving Show -- DOESN'T COMPILE -- workaround instance Show (Fix Greet) where show (In i) = "In " ++ show i In other words, given a number of parametised types that I can derive, say, Ord, Eq and Show for, how should I go about getting the instances for the Fix-d version of them as well? I've tried a few things, but no luck so far. Any clues? Cheers, Sam

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ANNOUNCE: yesod 0.2.0
by Michael Snoyman 24 May '10

24 May '10

Hi all, I'm very happy to announce the release of yesod 0.2.0. Yesod is a Haskell web framework for type-safe, RESTful web applications. I won't bore you all with the full release announcement here, please see the blog[1]. Installing yesod should be as simple as: cabal update && cabal install yesod. Please note that due to usage of type families in Template Haskell code, this release requires GHC 6.12. To get started with Yesod, I recommend going through the tutorials on the documentation site[2]. Please let me know if anything is unclear, or if you find any bugs. The next feature I'm targeting for Yesod is persistence. If anyone has any thoughts to share on this topic, please send me an e-mail. Michael [1] http://www.snoyman.com/blog/entry/yesod-0-2-0-released/ [2] http://docs.yesodweb.com/yesod/

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Functional Parsers in GHCI
by Amiruddin Nagri 24 May '10

24 May '10

Hi All, I am a newbie to Haskell, and have been using Programming in Haskell along with Eric Meijer videos covering the book syllabus. I am currently on Chapter 8 Functional Parsers, and trying to implement few of the examples given in the book. Following is the example : item = \inp -> case inp of [] -> [] (x:xs) -> [(x,xs)] p = do x <- item y <- item z <- item return (x,y,z) ========= When I compile it using GHCI, I get the following error : [1 of 1] Compiling Main ( parser.hs, interpreted ) parser.hs:5:8: No instance for (Monad ((->) [t])) arising from a do statement at parser.hs:5:8-16 Possible fix: add an instance declaration for (Monad ((->) [t])) In a stmt of a 'do' expression: x <- item In the expression: do x <- item y <- item z <- item return (x, y, z) In the definition of `p': p = do x <- item y <- item z <- item .... Failed, modules loaded: none. ========= I tried googling for samples, above error message etc. But not getting to solution. Can anyone guide me as to what am I doing wrong and how can I fix this. Regards, Amiruddin Nagri, India GTalk : amir.nagri(a)gmail.com

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[ANNOUNCE] First Public Release of the Snap Framework
by Gregory Collins 24 May '10

24 May '10

Hello all, To coincide with Hac Phi 2010 (http://www.haskell.org/haskellwiki/Hac_%CF%86) the Snap team is happy to announce the first public release of the Snap Framework, a simple and fast Haskell web programming server and library for unix systems. For installation instructions, documentation, and more information, see our website at http://snapframework.com/. Snap is well-documented and has a test suite with a high level of code coverage, but it is early-stage software with still-evolving interfaces. Snap is therefore most likely to be of interest to early adopters and potential contributors. Snap is BSD-licensed and currently only runs on Unix platforms; it has been developed and tested on Linux and Mac OSX Snow Leopard. Snap Features: * A simple and clean monad for web programming, similar to happstack's but simpler. * A *fast* HTTP server library with an optional high-concurrency backend (using libev). * An XML-based templating system for generating xhtml that allows you to bind Haskell functionality to XML tags in your templates. * Some useful utilities for web handlers, including gzip compression and fileServe. * Iteratee-based I/O, allowing composable streaming in O(1) space without any of the unpredictable consequences of lazy I/O. If you have questions or comments, please contact us on our mailing list (http://mailman-mail5.webfaction.com/listinfo/snap) or in the #snapframework channel on the freenode IRC network. Cheers, G -- Gregory Collins <greg(a)gregorycollins.net>

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ANN: hledger 0.10 released
by Simon Michael 23 May '10

23 May '10

hledger 0.10 is released, with installation and bug fixes and api improvements. Best, -Simon home: http://hledger.org Release notes: 2010/05/23 hledger 0.10 ........................ * fix too-loose testpack dependency, missing safe dependency * fix ghc 6.12 compatibility with -fweb * fix handling of non-ascii arguments with ghc 6.12 * fix "0.8" in --version output * fix an occasional stack overflow error due to infinite recursion in Posting/Transaction equality tests * the -fwebhappstack build flag is gone for now, to avoid a cabal problem * parsing: if there is no description, don't require a space after the transaction date * parsing: balance balanced-virtual postings separately, allow them to have an implicit amount * parsing: timelog entries now generate balanced transactions, using virtual postings * parsing: simpler high-level parse error message * parsing: clearer bad date errors * add: fix wrongful program exit on bad dates * print: negative account patterns now exclude transactions containing any posting to a matched account * vty: rename the ui command to vty for consistency * vty: fix restricted account scope when backing up to top level * web: fix non-ascii handling with ghc 6.12 * web: fix a bug possibly affecting reload-on-change * consolidate module namespace under Hledger, api cleanups Stats: 44 days, 81 commits since last release. Now at 4904 lines of code including tests, 144 tests, 53% coverage.

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ANN: atom-1.0.4
by Tom Hawkins 23 May '10

23 May '10

Atom [1] is an Haskell DSL for hard realtime embedded programming. This release adds 'exactPhase' to precisely control the phasing of Atom rule executions. This patch was contributed by Lee Pike. BTW, I created a google group [2] for discussions related to the use for functional programming in embedded systems. -Tom [1] http://hackage.haskell.org/package/atom [2] http://groups.google.com/group/fp-embedded

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An online forum for Haskell questions
by Don Stewart 23 May '10

23 May '10

Hey all, I thought I might just draw attention to a relatively new forum for Haskell information on the interwebs, that hasn't been mentioned here before. The Stack Overflow Haskell questions forum: http://stackoverflow.com/questions/tagged/haskell We've had more than 1000 questions asked and answered about Haskell there now, and for many readers of haskell-cafe@ it might be a more efficient way to get quick questions answered. Norman Ramsey is waiting patiently to help you! :-) For in-depth, or open questions, haskell-cafe exploration is more appropriate, in my opinion -- and we continue to have special communities for particular projects (e.g. libraries@ , glasgow-haskell-users@, happstack@ xmonad@) . But if you have a problem you know others might have solved already, consider the "haskell" tag on SO. -- Don

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Clean proof -- correction
by R J 23 May '10

23 May '10

Correction: the theorem is h . either (f, g) = either (h . f, h . g) (Thanks to Lennart for pointing out the typo.) From: rj248842(a)hotmail.com To: haskell-cafe(a)haskell.org Subject: Clean proof? Date: Sun, 23 May 2010 15:41:20 +0000 Given the following definition of "either", from the prelude: either :: (a -> c, b -> c) -> Either a b -> c either (f, g) (Left x) = f x either (f, g) (Right x) = g x what's a clean proof that: h . either (f, g) = either (h . f, g . h)? The only proof I can think of requires the introduction of an anonymous function of z, with case analysis on z (Case 1: z = Left x, Case 2: z = Right y), but the use of anonymous functions and case analysis is ugly, and I'm not sure how to tie up the two cases neatly at the end. For example here's the "Left" case: h . either (f, g) = {definition of "\"} \z -> (h . either (f, g)) z = {definition of "."} \z -> (h (either (f, g) z) = {definition of "either" in case z = Left x} \z -> (h (f x)) = {definition of "."} \z -> (h . f) x = {definition of "."} h . f Thanks. The New Busy is not the too busy. Combine all your e-mail accounts with Hotmail. Get busy. _________________________________________________________________ The New Busy is not the old busy. Search, chat and e-mail from your inbox. http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:…

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Clean proof?
by R J 23 May '10

23 May '10

Given the following definition of "either", from the prelude: either :: (a -> c, b -> c) -> Either a b -> c either (f, g) (Left x) = f x either (f, g) (Right x) = g x what's a clean proof that: h . either (f, g) = either (h . f, g . h)? The only proof I can think of requires the introduction of an anonymous function of z, with case analysis on z (Case 1: z = Left x, Case 2: z = Right y), but the use of anonymous functions and case analysis is ugly, and I'm not sure how to tie up the two cases neatly at the end. For example here's the "Left" case: h . either (f, g) = {definition of "\"} \z -> (h . either (f, g)) z = {definition of "."} \z -> (h (either (f, g) z) = {definition of "either" in case z = Left x} \z -> (h (f x)) = {definition of "."} \z -> (h . f) x = {definition of "."} h . f Thanks. _________________________________________________________________ The New Busy is not the too busy. Combine all your e-mail accounts with Hotmail. http://www.windowslive.com/campaign/thenewbusy?tile=multiaccount&ocid=PID28…

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