- Haskell-Cafe - Haskell.org

Data instance for a GADT
by Ozgur Akgun 25 May '10

25 May '10

Cafe, How can I provide a Data instance for a GADT? I am trying to TH on it, and Uniplate requires Data. I tried StandaloneDeriving, but it seems not to work. Best, -- Ozgur Akgun

3 4

Re: [Haskell-cafe] Writing a web service client
by Ionut G. Stan 25 May '10

25 May '10

Hello, Thanks for all the links. It seems that I have a lot to study before I produce a decent library :) The Network.HTTP package looks nice too. It seems to be at the right level of abstraction compared to Network.Browser. I have to get used with searching the Hackage repository... :) On 5/25/10 3:12 PM, Christopher Done wrote: > Hello Ionut, > > On 25 May 2010 13:26, Ionut G. Stan<ionut.g.stan(a)gmail.com> wrote: >> The first step that I want to undertake is to write a client library for the >> GitHub API, so my questions would be: >> >> 1. Is there any such library? > > I don't know of nor have seen such a library on Hackage or Github. > There was some brief talk on StackOverflow[1], but nothing about a > full library. I would be interested in using such a library as I am an > enthusiastic user of Github. > > [1]: http://stackoverflow.com/questions/1816092/using-the-github-gist-api-from-h… > >> 2. What other client library should I consider as an API model? Or maybe >> there's some paper/blog post on how do design a Haskell API? > > There are some good examples of Haskell code that uses REST-ish > APIs.[1][2][3][4] > > Regarding designing a Haskell API, Real World Haskell has some > chapters about that.[5] > > My personal advice would be at least to ensure you make it > cabal-install-able, so that entails having a proper cabal file, > documented with Haddock format, with QuickCheck test cases if > appropriate, and provide one or two examples in the documentation, see > Don's guide about writing a Haskell program[8]. > > [1]: http://hackage.haskell.org/package/flickr > [2]: http://hackage.haskell.org/package/twitter > [3]: http://hackage.haskell.org/package/feed-cli > [4]: http://hackage.haskell.org/package/Bitly > [5]: http://book.realworldhaskell.org/read/writing-a-library-working-with-json-d… > [6]: http://book.realworldhaskell.org/read/io-case-study-a-library-for-searching… > [7]: http://book.realworldhaskell.org/read/advanced-library-design-building-a-bl… > [8]: http://www.haskell.org/haskellwiki/How_to_write_a_Haskell_program > >> 3. What network package do people use for accessing simple, rest-like web >> services? Network.Curl? If not, which other? > > Network.Curl is OK but it relies on a C library which is another > dependency and it's maybe preferable to use the HTTP library or > similar library written in pure Haskell, that you can find on Hackage. > -- Ionuț G. Stan | http://igstan.ro

2 1

[ANNOUNCE] SyntaxMacros-0.1
by Marcos Viera 25 May '10

25 May '10

We are pleased to announce the first release of SyntaxMacros [1]. SyntaxMacros is an EDSL for constructing compilers out of a collection of pre-compiled, statically type-checked, possibly mutually dependent ``language-definition fragments''. This way of constructing a compiler brings syntax macros for free. This approach is based on a collection of techniques we have previously developed: - transformation of typed abstract syntax trees [2] makes it possible to construct parsers on the fly in a type-safe way - parser combinators [3] make it possible to construct parsers dynamically - first-class attribute grammars [4] make it possible to define semantics in a compositional way More information can be found at: http://www.cs.uu.nl/wiki/Center/SyntaxMacrosForFree Regards, Marcos Viera, Doaitse Swierstra, Atze Dijkstra and Arthur Baars [1]- http://hackage.haskell.org/package/SyntaxMacros [2]- http://www.cs.uu.nl/wiki/Center/TTTAS [3]- http://www.cs.uu.nl/wiki/bin/view/HUT/ParserCombinators [4]- http://www.cs.uu.nl/wiki/Center/AspectAG

1 0

Writing a web service client
by Ionut G. Stan 25 May '10

25 May '10

Hi, café I've begun learning Haskell for some time already, and although I don't really feel confident writing real world Haskell, I have a little idea that I want to materialize in Haskell code so I'm pretty eager to do something instead of just reading books and blog posts. The first step that I want to undertake is to write a client library for the GitHub API, so my questions would be: 1. Is there any such library? 2. What other client library should I consider as an API model? Or maybe there's some paper/blog post on how do design a Haskell API? 3. What network package do people use for accessing simple, rest-like web services? Network.Curl? If not, which other? Thanks, -- Ionuț G. Stan | http://igstan.ro

2 1

Re: [Haskell-cafe] TDD in Haskell
by Liam O'Connor 25 May '10

25 May '10

QuickCheck is great for TDD. I have used it for such purposes. You literally encode the contract of the function as quickcheck properties. It's very lovely. Cheers. ~Liam > > > > On 25 May 2010 21:36, Ionut G. Stan <ionut.g.stan(a)gmail.com> wrote: >> Hi, >> >> I'm doing TDD in pretty much all of the languages that I know, and I want to >> introduce it early in my Haskell learning process. I wonder though, if >> there's some established process regarding TDD, not unit testing. >> >> I've heard of QuickCheck and HUnit, but I've also read that QuickCheck is >> used mostly for testing pure code, while HUnit for impure code? >> >> What framework lends itself better for writing tests before the actual >> production code? Can you point out to some resources regarding this? >> >> >> Oh, and a small off-topic question? Is it considered a good practice to use >> implicit imports in Haskell? I'm trying to learn from existing packages, but >> all those "import all" statements drive me crazy. >> >> Thanks, >> -- >> Ionuț G. Stan | http://igstan.ro >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >

1 0

How to deal with huge text file?
by Magicloud Magiclouds 25 May '10

25 May '10

Hi, I have a file including some operation logs, in the format of the following. And I have some code to analyze it. Well, it ate all my memories. --- log: Log for item A =========== 09:10 read accountA 09:20 read accountB Log for item B .... --- code: file <- U.readFile filename mapM_ (\text -> let ope = parseOPE text in if findMissingOperation ope then U.putStrLn $ show $ fst ope else return () ) $ seperateOutput file --This function separates the input text file into paragraph by "Log for". -- 竹密岂妨流水过山高哪阻野云飞

4 11

ANNOUNCE: Packages for computer vision, robotics, and FRP plus demo video of haskell-controlled robot
by Noam Lewis 25 May '10

25 May '10

I'm pleased to announce the first public release, and a new version (still in "alpha" stage) of the following packages on hackage: - cv-combinators <http://hackage.haskell.org/package/cv-combinators>: Functional Combinators for Computer Vision (currently mostly a high level functional wrapper for HOpenCV) - Includes, among others, high-level code to get images from a webcam, detect faces, and show results in a window. - HOpenCV <http://hackage.haskell.org/package/HOpenCV>: Low-level bindings for the OpenCV computer vision library (for now, quite minimal but includes among others haar detection that can be used for face detection) - allocated-processor<http://hackage.haskell.org/package/allocated-processor>: Functional combinators for monadic actions that require allocation and de-allocation, and (building on that) a basic FRP framework that includes a generalized operator for expressing strictly causal systems - RMP <http://hackage.haskell.org/package/RMP>: Binding to code that controls a Segway RMP. Both RMP and cv-combinators include demo programs (under /src). Reading the code is highly recommended for people who are interested in using the packages. The demo program in the RMP package was used to control the robot in the following video: http://www.youtube.com/watch?v=2ZblWWxeLsY&feature=player_embedded More information can be found at: http://www.ee.bgu.ac.il/~noamle/ <http://www.ee.bgu.ac.il/%7Enoamle/> Enjoy, Noam Lewis

1 0

Re: [Haskell-cafe] ANN: atom-1.0.4
by Tom Hawkins 25 May '10

25 May '10

On Mon, May 24, 2010 at 2:34 AM, Graham Klyne <GK-lists(a)ninebynine.org> wrote: > I think this looks like an interesting idea... can you provide a pointer to > a description of the DSL/API itself? Unfortunately, there's not much aside from the Haddock documentation [1] [2] and some misc links on my webpage [3]. [1] http://hackage.haskell.org/packages/archive/atom/1.0.4/doc/html/Language-At… [2] http://hackage.haskell.org/packages/archive/atom/1.0.4/doc/html/Language-At… [3] http://tomahawkins.org/

1 0

Type families vs. functional dependencies -- how to express something
by Tomáš Janoušek 25 May '10

25 May '10

Hello all, for the past few hours I've been struggling to express a certain idea using type families and I really can't get it to typecheck. It all works fine using functional dependencies, but it could be more readable with TFs. From those papers about TFs I got this feeling that they should be as expressive as FDs, and we should use them (some people even occasionally mentioning that FDs may be deprecated in favor of them). Can someone help me make the following work, please? The working code with functional dependencies: > {-# LANGUAGE FlexibleInstances #-} > {-# LANGUAGE MultiParamTypeClasses #-} > {-# LANGUAGE FunctionalDependencies #-} > {-# LANGUAGE TypeFamilies #-} > {-# LANGUAGE UndecidableInstances #-} > {-# LANGUAGE ScopedTypeVariables #-} > > __ = undefined > > data HNil = HNil > data HCons a b = HCons a b > > data A a = A a > data B a = B a > > class X a l p | a -> l p where > ll :: a -> l > pp :: a -> p > > instance X (B l) HNil (B l) where > ll _ = HNil > pp p = p > > instance (X x l p) => X (A t -> x) (HCons t l) p where > ll _ = HCons __ (ll (__ :: x)) > pp p = pp (p (A (__ :: t))) > > -- (inferred) > -- testX :: (X a l (B l)) => a -> B l > testX x = pp x `asTypeOf` B (ll x) The motivation here is that A a represents a variable of type a, B b represents a computation with state b, and functions of type A a -> B b represent a computations with state b that use the first parameter as an accessor for a variable (or, more precisely, state component) of type a. Now, I want testX to take such function (with n parameters) and provide it with those accessors, fixing the type b to contain components of the corresponding accessors only. (The example uses dummy types and undefineds for simplicity.) This pretty much works: testX (B __) :: B HNil testX (\(A _) -> B __) :: B (HCons t HNil) testX (\(A True) -> B __) :: B (HCons Bool HNil) testX (\(A _) (A _) -> B __) :: B (HCons t (HCons t1 HNil)) testX (\(A _) (A _) (A _) -> B __) :: B (HCons t (HCons t1 (HCons t2 HNil))) testX (\(A _) -> B HNil) :: error This is my attempt to rephrase it using type families: > class X' a where > type L a > type P a > ll' :: a -> L a > pp' :: a -> P a > > instance X' (B l) where > type L (B l) = HNil > type P (B l) = B l > ll' _ = HNil > pp' p = p > > instance (X' x) => X' (A t -> x) where > type L (A t -> x) = HCons t (L x) > type P (A t -> x) = P x > ll' _ = HCons __ (ll' (__ :: x)) > pp' p = pp' (p (A (__ :: t))) > > -- (inferred) > -- testX' :: (B (L a) ~ P a, X' a) => a -> P a > testX' x = pp' x `asTypeOf` B (ll' x) Only the X' (B l) instance works, the other produces a strange error: testX' (B __) :: P (B HNil) :: B HNil testX' (\(A _) -> B __) :: error Couldn't match expected type `a' against inferred type `Main.R:L(->) t (B a)' Expected type: P (A t -> B a) Inferred type: B (L (A t -> B a)) Unifying those two types by hand, I get: P (A t -> B a) ~> P (B a) ~> B a B (L (A t -> B a)) ~> B (HCons t (L (B a))) ~> B (HCons t HNil) Hence, a = HCons t HNil. But GHC doesn't get that. I'm using GHC 6.12.1. Thanks for any hints. Best regards, -- Tomáš Janoušek, a.k.a. Liskni_si, http://work.lisk.in/

3 3

Good US Grad schools for functional languages?
by Job Vranish 25 May '10

25 May '10

Anybody know of a good grad school in the US for functional languages? (good = has Ph.D. program that covers functional languages, type systems, correctness proofs, etc...) So far Indiana University is the only one I've found that has a strong showing in this area. A way to get into one of the awesome UK schools for free would work too :D - Job

7 9

Re: [Haskell] ANNOUNCE: Introducing Sifflet, visual functional programming language
by Henning Thielemann 24 May '10

24 May '10

Gregory D. Weber schrieb: > Introducing Sifflet -- version 0.1.5, first public release! > > Sifflet is a visual, functional programming language. Cool - when do we get something like Gem Cutter? :-) http://resources.businessobjects.com/labs/cal/gemcutter-techpaper.pdf

2 1

Re: [Haskell-cafe] Haskell SVG path parser?
by Tillmann Vogt 24 May '10

24 May '10

Am 24.05.2010 14:21, schrieb Dimitry Golubovsky: > Hi, > > Does there exist any Haskell package/library to parse the syntax of > SVG elements, esp. PATH? Hi Dimitry, If you search on hackage for svg, you find my library SVGFonts. I first thought SVGFont is a special syntax similar to SVG, but it seems to be a subset of SVG. In my opinion it would be OK to split the SVG part out into a new library. I have made some improvements to SVGFonts which I haven't uploaded (i.e. points from bezier curves lying in a raster of a certain resolution). From a short look at Hennings code I guess my code may be little bit more beginner style, but parsing of the d attribute is implemeted more fully. I have spent quite some time on a tricky function "commandsToPoints" in ReadFont.hs which is about parsing the d field and translating it into outline points. Are you looking for something like that? > > That is, the syntax of the "d" attribute (e. g. M 100 100 L 300 100 L > 200 300 z)? > > Hackage doesn't seem to have any, and Google search yields very broad results. > > Thanks. > > PS It should not be hard to write such parser, I just don't want to > reivent the wheel ;)

2 1

Haskell SVG path parser?
by Dimitry Golubovsky 24 May '10

24 May '10

Hi, Does there exist any Haskell package/library to parse the syntax of SVG elements, esp. PATH? That is, the syntax of the "d" attribute (e. g. M 100 100 L 300 100 L 200 300 z)? Hackage doesn't seem to have any, and Google search yields very broad results. Thanks. PS It should not be hard to write such parser, I just don't want to reivent the wheel ;) -- Dimitry Golubovsky Anywhere on the Web

2 3

problem with regex replace with back references
by Juan Maiz 24 May '10

24 May '10

I'm trying use subRegex to replace using back references just like the docs says: http://hackage.haskell.org/packages/archive/regex-compat/0.92/doc/html/Text… But when i try to replace with \1 i got \SOH and not "e". Can anyone help? subRegex (mkRegex "e") "hello" "\1" => "h\SOHllo" Thanks in advance -- Juan Maiz Lulkin Flores da Cunha ------------------------------------------------- === @ Softa === http://mailee.me === http://hi.im/joaomilho ------------------------------------------------- “The most exciting breakthroughs of the 21st century will not occur because of technology but because of an expanding concept of what it means to be human” John Naisbitt

2 1

A most inelegant approach to "extensible records"
by Günther Schmidt 24 May '10

24 May '10

Hi everyone, as I'm trying to design a "query language" the one thing that causes the most grieve is the apparent requirement for "extensible records". There are by now a number of solutions in Haskell for this, most prominently HList. But at the end of the day even for HList to work one needs to define singleton types, something like: data FirstName = FirstName data LastName = LastName data BirthDate = BirthDate Now this isn't much of a nit and at least it works. But overall review of "extensible records" indicates that any solution/implementation requires a tremendous amount of type-level trickery. I short, any approach I've seen so far uses elaborate type class hierarchies, functional dependencies, etc., all coding on the *type* level. I have here a very, very inelegant alternative, of which I wonder if it offers a new angle to pursue the whole thing. 1. Initial "Record" names = \fn -> fn "firstName" "lastName" "birthDate" "zipCode" Please ignore for now that all "fields" are of type String. 2. "Extension" of Record namesCity = \fn -> names fn "residence" The record (1.) gets "extended" by the field "residence" 3. Selection nameZip n _ _ _ z = \fn -> fn n z basically here we "extract" the fields firstName and residence. 4. Test toList a b = [a, b] test = (namesCity nameZip) toList Now I know this is all very very inelegant and at 1st sight totally unfeasible. But maybe we can use Conal Eliots semantic editor combinators to smoothen this out? Günther

1 0

Proof question -- (==) over Bool
by R J 24 May '10

24 May '10

I'm trying to prove that (==) is reflexive, symmetric, and transitive over the Bools, given this definition: (==) :: Bool -> Bool -> Boolx == y = (x && y) || (not x && not y) My question is: are the proofs below for reflexivity and symmetricity rigorous, and what is the proof of transitivity, which eludes me? Thanks. Theorem (reflexivity): For all x `elem` Bool, x == x. Proof: x == x = {definition of "=="} (x && x) || (not x && not x) = {logic (law of the excluded middle)} True Theorem (symmetricity): For all x, y `elem` Bool, if x == y, then y == x. Proof: x == y = {definition of "=="} (x && y) || (not x && not y) = {lemma: "(&&)" is commutative} (y && x) || (not x && not y) = {lemma: "(&&)" is commutative} (y && x) || (not y && not x) = {definition of "=="} y == x Theorem (transitivity): For all x, y, z `elem` Bool, if x == y, and y == z,then x == z. Proof: ? _________________________________________________________________ Hotmail has tools for the New Busy. Search, chat and e-mail from your inbox. http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:…

8 13

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

1 0

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

3 3

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/

1 0

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

3 5

[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>

4 7

ANN: hledger 0.10 released
by Simon Michael 24 May '10

24 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.

1 0

ANN: atom-1.0.4
by Tom Hawkins 24 May '10

24 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

1 0

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

1 0

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:…

5 4

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…

2 1

Haskell and scripting
by Limestraël 23 May '10

23 May '10

Hello Café, I don't know if you know conky<http://en.wikipedia.org/wiki/Conky_%28software%29>. It's a well-known open-source system monitor (a software that displays information on the desktop, like CPU frequency, disk usage, network rate, etc.). It is quite good, but it's very descriptive, and even if you can call shell commands it's clearly not made for being scripted. What I would do is to make a similar system monitor, which base would be compiled Haskell code, but that would be scriptable with some DSL, or already existing interpreted language. I've thought about a Lisp/Scheme language, since those languages are functional, dynamically typed and simple (so enable a quick scripting) and I'm not very keen on making my own DSL What I would like to know is: 1) If you have other solutions 2) How do haskellers usually script their applications

21 47

Mechanics of type-level proxies through branding?
by Dave Neuer 23 May '10

23 May '10

Hi. I'm a Haskell newbie, and I've been reading Oleg's work about lightweight dependent types in Haskell, and I've been trying to figure out if I understand how branding works (warning bells already, I know). At http://okmij.org/ftp/Computation/lightweight-dependent-typing.html#Branding, he states "A language with higher-rank types or existential type quantification lets us introduce type proxies for the values. We can associate a value with some type in such a way that type equality entails value equality..." Then, in the code for eliminating array bounds checking at http://okmij.org/ftp/Haskell/eliminating-array-bound-check.lhs, the following example of branding is given: >> brand:: (Ix i, Integral i) => Array i e >> -> (forall s. (BArray s i e, BIndex s i, BIndex s i) -> w) >> -> w -> w >> brand (a::Array i e) k kempty = >> let (l,h) = bounds a >> in if l <= h then k ((BArray a)::BArray () i e, BIndex l, BIndex h) >> else kempty > ... > > The function brand has a higher-rank type. It is the existential > quantification of 's' as well as the absence of BArray constructor > elsewhere guarantee that the same brand entails the same bounds. I think I understand that the fact that the type variable 's' is shared between BArray, and BIndex's type constructors in the type annotation for "brand", that the array and indices share the same brand. I also think I understand that since 's' is existentially quantified and a phantom type, it's unique and cannot ever be unified w/ any other type? Additionally, it seems that this is all only within the scope of the continuation? That is, "type equality entails value equality" but not "value equality entails type equality", e.g. if I call brand two times with arrays of the same bounds, I'd get two different brands? Finally, I'm confused why the unit type in the explicit type expression at BArray's value constructor application doesn't foil the branding, i.e. why it doesn't destroy the uniqueness/opaqueness of 's'? Thanks for anyone who can explain this! Dave

2 1

How to construct a lazy list of eagerly-evaluated items?
by Vladimir Ch. 23 May '10

23 May '10

I'm using Project Euler to learn Haskell. In particular, I'm writing a program for Problem 18: http://projecteuler.net/index.php?section=problems&id=18. As a solution, I'm constructing a list, containing maximum sums of values on a path from top of the triangle to a given item. In this list, item value equals value of an item from an input list plus maximum of values of the "top neighbours", taken from the same list we're constructing. Here's the program: BEGIN CODE triangle18 = [75...etc - definition of the input triangle sqrti n = truncate (sqrt (fromIntegral n)) -- returns true if a number is an exact square isSquare n = n == r * r where r = sqrti n -- returns true if a number is a triangular numberx isTriangular n = isSquare (8 * n + 1) -- returns index of the top-right neighbour prevRight i = i - (sqrti (8 * i + 1) - 1) `div` 2 -- returns index of the top-left neighbour prevLeft i = prevRight i - 1 -- returns the list of top neighbours (at most 2) prev i | i == 0 = [] | isTriangular i = [prevRight i] | isTriangular (i + 1) = [prevLeft i] | otherwise = [prevLeft i, prevRight i] -- returns the list of 'path sums' for a given list of input numbers maxPaths vals@(v:vs) = v : zipWith mpath [1..] vs where mpath i val = val + maximum [(maxPaths vals) !! pi| pi <- prev i] result18 = maximum (maxPaths triangle18) END CODE The program works, but consumes obscene amount of memory. I suspect that it's because of lazy evaluation of items in the list we're constructing (and also using during construction). How can I force eager evaluation of items, put into the list? Any other comments and suggestions are also very welcome. I know index-based item lookup in a list is not efficient, but I'll take care of this later (I don't know much about arrays yet). For now, I'm more concerned with the memory usage.

2 2

making the GHC Api not write to stderr
by Phyx 23 May '10

23 May '10

I was wondering how to forcibly quiet down the API. I have a custom handler in place, but when I call the function on failure both my handler gets called and somewhere somehow errors get printed to the stderr, which I really need to avoid. My current code looks like getModInfo :: Bool -> String -> String -> IO (ApiResults ModuleInfo) getModInfo qual file path = handleSourceError processErrors $ runGhc (Just libdir) $ do dflags <- getSessionDynFlags setSessionDynFlags $ configureDynFlags dflags target <- guessTarget file Nothing addTarget target setSessionDynFlags $ dflags { importPaths = [path] } load LoadAllTargets graph <- depanal [] False let modifier = moduleName . ms_mod modName = modifier $ head graph includes = includePaths dflags imports = importPaths dflags dflags' <- Debug.trace (moduleNameString modName) getSessionDynFlags setSessionDynFlags $ dflags' { includePaths = path:includes , importPaths = path:imports } parsed <- parse modName checked <- typecheckModule parsed

4 7

ANN: has-0.4 Entity based records
by HASHIMOTO, Yusaku 23 May '10

23 May '10

Hello, I'm pleased to announce the release of my new library, named "has", written to aim to ease pain at inconvinience of Haskell's build-in records. With the has, You can reuse accessors over records to write generic function, combine records with another. Repository is at GitHub: http://github.com/nonowarn/has Uploaded on Hackage: http://hackage.haskell.org/package/has So you can install this by "cabal install has" You can use the has in three steps (without counting installation). 1. Write {-# OPTIONS_GHC -fglasgow-exts #-} top of your code, import Data.Has module. > {-# OPTIONS_GHC -fglasgow-exts #-} > import Data.Has 2. Define entities. "Entity" is data to index field in records. You can define an entity in one line. > data Foo = Foo; type instance TypeOf Foo = Int (I lied) Before semicolon, declares entity. After semicolon, specifies the type to which the entity points. Define some entities for later examples. > data Bar = Bar; type instance TypeOf Bar = Double > data Baz = Baz; type instance TypeOf Baz = String > data Quux = Quux; type instance TypeOf Quux = Bool 3. Define Records by concatinating fields of entities. > type MyRecord = FieldOf Foo :&: FieldOf Bar :&: FieldOf Baz This is almost same as writing < data MyRecord = MyRecord { foo :: Int < , bar :: Double < , baz :: String < } To construct a value of record, remove colons and replace entities in record with values, and uncapitalize some words. > aRecord :: MyRecord > aRecord = fieldOf 42 & fieldOf 3.14 & fieldOf "string" And you can play with it. To read/write/modify a value of field in records, you can use functions with names stealed from data-accessor. But uses value-level entities instead of accessors. < Foo ^. aRecord -- Reading < Foo ^= 4649 $ aRecord -- Writing < Foo ^: (*2) $ aRecord -- Modifying If we have another record type contains Foo field, You can still access the field in the same way. > type AnotherRecord = FieldOf Bar :&: FieldOf Foo > anotherRecord :: AnotherRecord > anotherRecord = fieldOf 2.71 & fieldOf 31 < Foo ^. anotherRecord -- And this also works Using these functions and Has constraint, You can write generic functions over records. > fooIsGreaterThan :: (Has Foo r) => r -> Int -> Bool > fooIsGreaterThan r x = (Foo ^. r) > x < aRecord `fooIsGreaterThan` 40 -- evaluated to True < anotherRecord `fooIsGreaterThan` 40 -- evaluated To False Even if you defined another record by combining records by (:&:), you can still access the field, and apply to generic functions. > type MoreRecord = FieldOf Baz :&: FieldOf Quux > type CombinedRecord = AnotherRecord :&: MoreRecord > combinedRecord :: CombinedRecord > combinedRecord = (fieldOf 1.618 & fieldOf 39) & (fieldOf "sowaka" & fieldOf True) > -- We can omit parentheses > -- (even place parens anyware in record) < combinedRecord `fooIsGreaterThan` 40 -- This yet works The Has constraint provides not only genericity but also safety. If the record doesn't satisfy the constraint, the type checker rejects it. > predicateOnRecords :: (Has Foo r, Has Quux r) => r -> Bool > predicateOnRecords r = fooIsGreaterThan r 30 && (Quux ^. r) < predicateOnRecords combinedRecord -- This is OK < predicateOnRecords aRecord -- This yields compile error More examples included in package[1] [1]: http://github.com/nonowarn/has/tree/master/examples/ This library is inspired by HList[2], and interfaces are stealed from data-accessors[3]. And lenses[4], fclabels[5], and records[6] devote themselves to similar purposes. [2]: http://hackage.haskell.org/package/HList [3]: http://hackage.haskell.org/package/data-accessor [4]: http://hackage.haskell.org/package/lenses [5]: http://hackage.haskell.org/package/fclabels [6]: http://hackage.haskell.org/package/records Enjoy! -nwn

3 9

Performance Issue
by Richard Warburton 22 May '10

22 May '10

A colleague of mine pointed out that ghc wasn't performing as he expected when optimising some code. I wonder if anyone could offer any insight as to why its not noting this common subexpression: main = print $ newton 4 24 newton a 0 = a newton a n = ((newton a (n-1))^2 + a)/(2*(newton a (n-1))) Compiled with 'ghc -O3 --make perf.hs', results in: real 0m5.544s user 0m5.492s sys 0m0.008s However if we factor out the repeated call to the newton method: main = print $ newton2 4 24 newton2 a 0 = a newton2 a n = (x^2 + a)/(2*x) where x = newton2 a (n-1) real 0m0.004s user 0m0.004s sys 0m0.004s It looks to me like Referential transparency should make this a sound optimisation to apply, but ghc isn't doing it even on -O3. regards, Richard

9 13

Re: [Haskell-cafe] Enum instantiation
by Antoine Latter 22 May '10

22 May '10

On Fri, May 21, 2010 at 8:59 PM, R J <rj248842(a)hotmail.com> wrote: > If I type "toEnum 5," the error I get is: > <interactive>:1:0: > Ambiguous type variable `a' in the constraint: > `Enum a' arising from a use of `toEnum' at <interactive>:1:0-7 > Probable fix: add a type signature that fixes these type variable(s) > *Main> Here the problem is not in your instance of Enum, but that GHCi doesn't know which instance of enum to use. In most code this isn't a problem, because the surrounding context often gives enough information for the type-checker to work with. But when there isn't enough context, you'll need to provide a type signature. Does "toEnum 5 :: Day" work? Antoine

1 0

Enum instantiation
by R J 22 May '10

22 May '10

I'd like to make "Day" an instance of class "Enum," but the definition of toEnum below seems to be completely wrong, because integers seem not permit pattern matching. How is toEnum defined? Thanks. data Day = Sunday | Monday | Tuesday | Wednesday | Thursday | Friday | Saturday deriving (Eq, Ord, Show) instance Enum Day where fromEnum Sunday = 0 fromEnum Monday = 1 fromEnum Tuesday = 2 fromEnum Wednesday = 3 fromEnum Thursday = 4 fromEnum Friday = 5 fromEnum Saturday = 6 toEnum 0 = Sunday toEnum 1 = Monday toEnum 2 = Tuesday toEnum 3 = Wednesday toEnum 4 = Thursday toEnum 5 = Friday toEnum 6 = Saturday _________________________________________________________________ Hotmail is redefining busy with tools for the New Busy. Get more from your inbox. http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:…

3 2

ambiguous type variable problem when using forall, multiparameter type classes, and constraints on polymorphic values (and syb-with-class)
by Jeremy Shaw 21 May '10

21 May '10

Hello, I am trying to understand why I am getting an ambigious type variable error, and what I can do to work around it. The problem is occurring while trying to use syb-with-class, but I have stripped it down to it's bare essentials, so the following code is self-contained, and does not require an understanding of syb-with-class. > {-# LANGUAGE RankNTypes, KindSignatures, MultiParamTypeClasses, FlexibleInstances, ScopedTypeVariables, FlexibleContexts #-} > module Main where A Proxy data type used to pass some extra type info around in gmapQ. > data Proxy (a :: * -> *) = Proxy A simple multiple parameter type class. > class Data (ctx :: * -> *) a gmapQ takes a function, forall a. (Data ctx a) => a -> r, applies the function to all the immediate sub-types of 'a' and collects the results as a list. This is possible because the 'a' is forall'd, and (in the real implementation) the Data class provides the mechanisms for accessing all the immediate sub-types of 'a' and doing useful things with them. > gmapQ :: Proxy ctx -> (forall a. (Data ctx a) => a -> r) -> (forall a. (Data ctx a) => a -> [r]) > gmapQ = undefined Now we create a context. > data Foo m a = Foo { unFoo :: m a } And create a Data instance for it. > instance Data (Foo m) a Here is an example that attempts to recursively apply itself to all the subtrees. (I believe this function will ultimately return 0, since it tries to sum the results of the children. When you reach types with no more sub-types, I think you will get an empty list, and sum [] == 0. Though it's not really important to the issue at hand). > bar1 :: forall a m r. (Data (Foo m) a, Num r) => a -> r > bar1 x = sum $ gmapQ (undefined :: Proxy (Foo m)) bar1 x The problem I have is when I try to add an additional constraint on 'm', such as (Monad m) => > -- bar2 :: forall a m r. (Monad m, Data (Foo m) a, Num r) => a -> r > -- bar2 x = sum $ gmapQ (undefined :: Proxy (Foo m)) (bar2 :: forall b. (Monad m, Data (Foo m) b, Num r) => b -> r) x I get the error: /tmp/a.lhs:48:54: Ambiguous type variable `m' in the constraint: `Monad m' arising from a use of `bar2' at /tmp/a.lhs:48:54-57 Probable fix: add a type signature that fixes these type variable(s) Failed, modules loaded: none. Adding all the scoped type variable stuff does not seem to help. Alas, I can not figure out if this is a limitation of the type-checker, or something that is fundamentally impossible. Nor can I figure out how to work around the issue. In my real code I need to define the data instance like: > -- instance (Monad m) => Data (Foo m) a Which, by itself is fine. But that results in my needing to add (Monad m) to the 'bar' function. And that is what I can't figure out how to do.. Thanks! - jeremy

2 1

FW: Why does this Ord-class instance crash?
by R J 21 May '10

21 May '10

Why does the following, trivial code snippet below hang GHCi when I type"Scalene > Failure", and what's the fix? data Triangle = Failure | Equilateral | Isosceles | Scalene deriving (Eq, Show) instance Ord Triangle where Failure < Failure = False Failure < _ = True Equilateral < Failure = False Equilateral < Equilateral = False Equilateral < _ = True Isosceles < Scalene = True Isosceles < _ = False Scalene < _ = False (I tried submitting this to beginners(a)haskell.org, but even though I've signed up for that mailing list, I got a bounce-back saying that I needed admin approval to submit anything to that list, and I haven't heard from an admin, so I'm posting it here.) _________________________________________________________________ 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…

4 3

Tools to make tidy HTML
by Dmitri O.Kondratiev 21 May '10

21 May '10

Hello! Please advise haskell libraries similar to convert real-world HTML to well-formed XML. I need something similat to "HTML Tidy" library: http://tidy.sourceforge.net/ Thanks! Dmitri

1 0

Register now for: Haskell in Leipzig (Germany), June 4.
by Johannes Waldmann 21 May '10

21 May '10

Schedule and registration for the "Haskell in Leipzig" meeting on June 4 are now on-line: http://www.iba-cg.de/hal5.html It's our fifth meeting, and here are five good reasons to go: * learn Haskell: attend tutorials for beginners * get first-hand information: authors of Leksah, HXT, Hawk give tutorials/talks * share your expert knowledge: at lambda camp * see beyond Haskell: learn dependently typed programming in Agda * relax at BBQ party and listen to strange sounds: unsafePerformance The meeting is presented by http://www.fit-leipzig.de/ , in co-operation with http://www.imn.htwk-leipzig.de/ , http://www.leipziger-medienstiftung.de/, http://www.iba-cg.de/ Best regards, Johannes Waldmann (for the Hal5 program committee)

1 0

Недвижимость во Франции, аренда и продажа вилл
by moreletto 21 May '10

21 May '10

Офис нашей компании находится в Ницце - в самом сердце Французской Ривьеры, что дает нам непосредственную возможность предложить виллы для аренды и продажи во всем их многообразии, а также сориентировать наших клиентов на самые интересные и значимые мероприятия в богатой культурной и светской жизни Лазурного берега. Мы предоставляем нашим клиентам широкий выбор вилл класса люкс по всей протяжённости Лазурного берега Франции - от Сан Тропе до Монако, оказываем визовую поддержку, а также организуем визиты на приглянувшиеся Вам объекты при заключении контрактов по аренде или покупке вилл и другой недвижимости <http://www.property-serviceazur.com/> средиземноморского побережья. Уважаемые партнеры, мы предлагаем Вам сотрудничество в приобретении Вашими клиентами *недвижимости во Франции, на Лазурном берегу *, который всегда ценился поклонниками искусства, знатоками истории, любителями изысканной кухни и живописных пейзажей.

2 1

String rewriting
by Roly Perera 21 May '10

21 May '10

Hi, I'm looking for a simple way to rewrite strings according to simple composable rules like: replace "_" by "\$\\hole\$" replace "-n" where n matches an integer by "^{n}" so that I can import some pretty-printed output into a LaTeX alltt environment. I'm guessing that this nice functional stream transformation problem has been solved thousands of times. Could anyone point me to a simple package that would do this for me? many thanks, Roly

5 4

Intuitive function given type signature
by R J 21 May '10

21 May '10

What are some simple functions that would naturally have the following type signatures: f :: (Integer -> Integer) -> Integer g :: (Integer -> Integer) -> (Integer -> Integer) (Bird problem 1.4.5) _________________________________________________________________ Hotmail has tools for the New Busy. Search, chat and e-mail from your inbox. http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:…

6 8

Type famillies & Lifting IO
by oleg＠okmij.org 21 May '10

21 May '10

Maciej Piechotka wrote: > class (Monad m, Monad (IO' m)) => MonadIO m where > type IO' m :: * -> * > liftIO :: IO a -> IO' m a > liftM :: m a -> IO' m a The signature for liftIO betrays a problem. Since liftIO is a member of a type class, when liftIO is used in code, the type checker has to find the appropriate instance (i.e., resolve the overloading). Suppose we have this piece of code: tt :: FooIO () tt = liftIO (return () :: IO ()) How to find the appropriate instance of MonadIO and resolve the liftIO overloading? The argument type of liftIO does not mention the type class parameter m at all, and so it is of no guidance for overloading resolution. We know that the return type of liftIO in the tt code is FooIO (). Thus we have to find such instance of the MonadIO class that its associated type IO' m is equal to FooIO. In other words, we have to find m as a solution of the equation IO' m = FooIO Or, we have to invert the type function IO' m. Alas, type functions are not generally injective and hence not invertible. Indeed, nothing prevents the user from defining two instances of MonadIO, for types m1 and m2, such that IO' m1 = IO' m2 = FooIO.

1 0

ANN: threads-0.1.0.1
by Bas van Dijk 21 May '10

21 May '10

Dear all, I uploaded threads-0.1.0.1 to hackage. Threads is a small package that lets you fork threads and wait for their result. The basic interface is simply: module Control.Concurrent.Thread where data ThreadId α forkIO ∷ IO α → IO (ThreadId α) wait ∷ ThreadId α → IO (Either SomeException α) threadId ∷ ThreadId α → Control.Concurrent.ThreadId I hope the names and types speak for themselves. Additionally the following module is provided that lets you wait for a group of threads to terminate: module Control.Concurrent.Thread.Group where data ThreadGroup new ∷ IO ThreadGroup forkIO ∷ ThreadGroup → IO α → IO (ThreadId α) wait ∷ ThreadGroup → IO () Installation: $ cabal update $ cabal install threads API docs: http://hackage.haskell.org/package/threads-0.1.0.1 Development: darcs get http://code.haskell.org/~basvandijk/code/threads Regards, Bas

1 0

Retrospective type-class extension
by Tony Morris 21 May '10

21 May '10

We all know that "class (Functor f) => Monad f" is preferable but its absence is a historical mistake. We've all probably tried once: instance (Functor f) => Monad f where ... However, is there a type system extension (even proposed but not implemented) that allows me to retrospectively apply such a notion? Ideally something like this would be handy if it could somehow be retrospectively applied: Monad <- Applicative <- Pointed <- Functor -- Tony Morris http://tmorris.net/

7 9

RE: [Haskell-cafe] making the GHC Api not write to stderr
by Phyx 21 May '10

21 May '10

I've thought about that, but my specific problem is that I'm calling the Haskell code compiled to a shared lib and then hPutStr is failing, presumable because there is no valid stderr (and I would image also no stdin or out). So in order for me to use that method I would have to create a new file handle and direct stderr to that. Or somehow create a fake Handle and pass it that. Or another alternative is on the side where I call the lib, create and attach some hidden console to the process and have it write there. But that all seems like a hackish way out, It would be best if the API just allowed me to turn off this behavior of writing to stderr. From: Jean-Denis Koeck [mailto:jdkoeck@gmail.com] Sent: Thursday, May 20, 2010 21:59 To: Phyx Subject: Re: [Haskell-cafe] making the GHC Api not write to stderr Maybe this <http://nominolo.blogspot.com/2010/04/haskell-tip-redirect-stdout-in-haskell .html> could help you ? 2010/5/20 Phyx <lonetiger(a)gmail.com> I was wondering how to forcibly quiet down the API. I have a custom handler in place, but when I call the function on failure both my handler gets called and somewhere somehow errors get printed to the stderr, which I really need to avoid. My current code looks like getModInfo :: Bool -> String -> String -> IO (ApiResults ModuleInfo) getModInfo qual file path = handleSourceError processErrors $ runGhc (Just libdir) $ do dflags <- getSessionDynFlags setSessionDynFlags $ configureDynFlags dflags target <- guessTarget file Nothing addTarget target setSessionDynFlags $ dflags { importPaths = [path] } load LoadAllTargets graph <- depanal [] False let modifier = moduleName . ms_mod modName = modifier $ head graph includes = includePaths dflags imports = importPaths dflags dflags' <- Debug.trace (moduleNameString modName) getSessionDynFlags setSessionDynFlags $ dflags' { includePaths = path:includes , importPaths = path:imports } parsed <- parse modName checked <- typecheckModule parsed _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

1 0

No copy XML parser (rough idea only)
by Joachim Breitner 20 May '10

20 May '10

Hi, an idea recently crossed my mind that describes a (possibly?) useful way of accessing XML data from Haskell that is very memory efficient – at least as long as you only read the XML; for XML processing and generation, other existing libraries are probably well suited. Given a (possibly very large) XML file as a ByteString, a „normal“ XML parser would have an algebraic data type for node etc., parsing the ByteString into a tree of haskell values. Simplified a bit, the data structure could look like this: > data Node = Node String [Attrib] [Node] | CData String The in-memory representation of such a a data structure is presumably very large, compared to the XML ByteString. Additionally, all the content is copied in memory and I evaluated a part of the Node (e.g. the tag name), this is retained by the constructor as long as the Node, and thus the whole document tree, is retained. My idea is to copy as little as possible, and refer to the existing ByteString as much as possible. Remember that a substring of a ByteString points at the same memory region, just with a different offset and length. So, in my case, the datatype looks like > newtype Node = Node ByteString > newtype CData = CData ByteString > newtype Attrib = Attrib ByteString where the constructors would not be exported. Instead, accessor methods would be provided: > tagName :: Node -> String > tagAttribs :: Node -> [Attrib] > tagContent :: Node -> [Either Node CData] > cDataContent :: CData -> ByteString So when searching for a certain tag, the library would only have to generate (and retain) pointers to the ByteString, but the tag name themselves do not have to be retained after the comparision. If it turns out to be too expensive to parse the fragment that is referenced by Node on every invocation of tagContent, this could be made a field of the constructor. Due to lazy evaluation, it will only be evaluated when needed, but then kept in memory: > data Node = Node ByteString [Either Node CData] About cDataContent: This method cannot just return the fragment of the document unaltered, unfortunately, as XML entities will have to be replaced. But if it returns a lazy ByteString, everything between the entities can be a chunk refering to the original data, again avoiding the creation of a copy. Now this is just a quick idea I wanted to share. I have not tried it yet, so I might be overlooking something which prevents the whole thing from working. It might also be the case that, due to lazy evaluation, the “usual” approach is actually well enough (at least if it uses ByteString instead of String). I’m curious what you think, Joachim Breitner -- Joachim Breitner e-Mail: mail(a)joachim-breitner.de Homepage: http://www.joachim-breitner.de ICQ#: 74513189 Jabber-ID: nomeata(a)joachim-breitner.de

4 4

Devices and Webcams, The Basics
by Eitan Goldshtrom 20 May '10

20 May '10

Hi everyone, I would like to start working on a program that requires access to a camera attached to the computer probably via USB or otherwise internally. Unfortunately I don't know anything about using devices in haskell. I tried looking up how to access the microphone one too and had little success. Could someone just point me in the direction of tutorials for learning the basics of both devices in general as well as the webcam more specifically? The webcam is a bit more of a priority. -Eitan

4 4

Bird problem 1.6.2 -- is there an easier method?
by R J 20 May '10

20 May '10

Bird problem 1.6.2 is: If f :: (a, b) -> c, then define a function "swap" such that: flip (curry f) = curry (f . swap). I'd very much appreciate if someone could tell me whether there's a rigorous solution simpler than mine, which is: Since (.) :: (q -> r) -> (p -> q) -> (p -> r), we have f :: q -> r and swap :: p -> q. Type unification of f requires q = (a, b) and r = c. Since f :: (a, b) -> c and curry :: ((l, m) -> n) -> (l -> m -> n), typeunification requires l = a, b = m, and n = c. Therefore,curry :: ((a, b) -> c) -> (a -> b -> c), and (curry f) :: a -> b -> c. Since flip :: (s -> t -> u) -> t -> s -> u, type unification requiress = a, t = b, and u = c. Therefore, flip :: (a -> b -> c) -> b -> a -> c,and flip (curry f) :: b -> a -> c. Therefore, curry (f . swap) :: b -> a -> c, and p :: b -> a. Therefore,swap :: b -> a -> (a, b), and: swap :: b -> a -> (a, b)swap x y = (y, x) _________________________________________________________________ Hotmail has tools for the New Busy. Search, chat and e-mail from your inbox. http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:…

2 1

Deducing a type signature
by R J 20 May '10

20 May '10

Bird 1.6.3 requires deducing type signatures for the functions "strange" and "stranger." Are my solutions below correct? (i) strange f g = g (f g) Assume g :: a -> b. Then f :: (a -> b) -> c. But since g :: a -> b,f g :: a, so c = a. Therefore, f :: (a -> b) -> a, and g (f g) :: a.Therefore, strange :: ((a -> b) -> a) -> (a -> b) -> a. (ii) stranger f = f f Assume f :: a -> b. Since "f f" is well-typed, type unification requiresa = b. Therefore, f :: a -> a, and stranger :: (a -> a) -> a. _________________________________________________________________ Hotmail is redefining busy with tools for the New Busy. Get more from your inbox. http://www.windowslive.com/campaign/thenewbusy?ocid=PID28326::T:WLMTAGL:ON:…

3 2

Extensible Records and Functional References
by Günther Schmidt 20 May '10

20 May '10

Hi all, I just read "Functional References are a cheap and cheerful technique for working with the existing (non-extensible) record system, and may be of interest to extensible record implementers. A good implementation can be found on ..." on http://hackage.haskell.org/trac/ghc/wiki/ExtensibleRecords Does this mean that there is a way to implement extensible records via functional references? Günther

1 0