February 2013 - Haskell-Cafe

[ANNOUNCE] London Haskell user group - Being Lazy - Wed 27-Feb-13
by Derek Wright 17 Feb '13

17 Feb '13

Wednesday 27-Feb-2013 6-45pm (prompt) City University, College Building, St John Street, London. EC1V 4PB http://www.meetup.com/London-HUG/events/103545552/ Being Lazy Speaker: Peter Marks This talk will explore lazy evaluation in Haskell. After a brief introduction to non-strictness, and establishing a basic conceptual model of evaluation in Haskell, the primary focus will be on how we can utilize laziness to allow us to write programs differently to how we would in an eagerly evaluated language. The session will be rich on code with a worked example of a complete program. We'll also touch on some of the pitfalls of laziness that we run into in real world situations. Peter Marks has been developing software for over thirty years and has been using Haskell commercially for four years. He currently leads the FPF team at Barclays and is an active member of the London Haskell community, running the Hoodlums meetup group. N.B. The start time is slightly later than previously, to make sure everybody has arrived by the time the talk starts. Doors open 6-30pm, try not to be late!

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ANNOUNCE: hackage-proxy 0.1.0.0
by Michael Snoyman 17 Feb '13

17 Feb '13

I'd like to announce the first release of a new tool called hackage-proxy. The purpose is to provide a local proxy for a Hackage server, which somehow modifies files in transport. The motivating case for this was getting more meaningful error output from Stackage when compiling against GHC HEAD. When compiling against actual Hackage, cabal will simply refuse to try to build packages which have upper version bounds such as "base < 4.7". This introduces a big dilemma: * Package authors do not want to bump the version bounds on their packages until they've tested against that version. * It's very difficult to do meaningful tests of GHC until packages on Hackage have been updated. Hopefully this package can help resolve the dilemma. Instead of requiring authors to upload new versions of their packages in order to test them, this proxy will modify the cabal files it downloads and strip off the version bounds of specified packages. Then, you can test with a newer version of GHC and find actual compilation errors instead of version bound constraints. ## Example Usage 1. cabal install hackage-proxy 2. Run hackage-proxy. By default, it will use the official Hackage server as the source, drop bounds on the packages base, process, directory, template-haskell, and Cabal, and serve from port 4200. All of this can be modified via command-line options. 3. Edit your ~/.cabal/config file. Comment out the hackage.haskell.orglines, and add in something like the following: remote-repo: hackage-proxy:http://localhost:4200 4. cabal update 5. cabal install your-package-list-here I think this can be a very valuable tool for anyone wanting to test out newer versions of GHC. In addition, as part of my normal Stackage work, I'm now collecting fairly detailed error logs of a number of packages. If this would be useful for the GHC team or anyone else, let me know and I can try and provide the logs somehow.

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Netwire, keyboard events, and games
by Patrick Hurst 16 Feb '13

16 Feb '13

I'm using netwire to build a game; one of the things the player can do is move around using WASD. I want to use key *events* as the 'basis' of my wires, not the entire state vector of the keyboard so that, for example, a press event on D increments the velocity by (200, 0) and a release event decrements it by that much. However, this has the problem that I can't actually get the velocity unless I have an event to feed into the wires, which means I can only update the screen when the user presses or releases a key. While this might make for interesting gameplay, it's not what I want. is the right thing to do to make the input something like a Maybe KeyEvent instead, and pass in Nothing whenever I want to get output without an event for, e.g., a render?

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What magic has the new IO manager done to improve performance ?
by yi huang 16 Feb '13

16 Feb '13

I' m curious about the design and trade offs behind the new IO manager. I see two changes from the code: 1. Run IO manager thread on each capability. 2. Use ONESHOT flag to save a system call. Is there other interesting things to know? Is it possible to use epoll's ET mode to save even more system calls? -- http://yi-programmer.com/

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Text.JSON and utf8
by Martin Hilbig 16 Feb '13

16 Feb '13

hi, tl;dr: i propose this patch to Text/JSON/String.hs and would like to know why it is needed: @@ -375,7 +375,7 @@ where go s1 = case s1 of - (x :xs) | x < '\x20' || x > '\x7e' -> '\\' : encControl x (go xs) + (x :xs) | x < '\x20' -> '\\' : encControl x (go xs) ('"' :xs) -> '\\' : '"' : go xs ('\\':xs) -> '\\' : '\\' : go xs (x :xs) -> x : go xs i recently stumbled upon CouchDB telling me i'm sending invalid json. i basically read lines from a utf8 file with german umlauts and send them to CouchDB using Text.JSON and Database.CouchDB. $ file lines.txt lines.txt: UTF-8 Unicode text lets take 'ö' as an example. i use LANG=de_DE.utf8 ghci tells > 'ö' '\246' > putChar '\246' ö > putChar 'ö' ö > :m + Text.JSON Database.CouchDB > runCouchDB' $ newNamedDoc (db "foo") (doc "bar") (showJSON $ toJSObject [("test","ö")]) *** Exception: HTTP/1.1 400 Bad Request Server: CouchDB/1.2.1 (Erlang OTP/R15B03) Date: Mon, 11 Feb 2013 13:24:49 GMT Content-Type: text/plain; charset=utf-8 Content-Length: 48 Cache-Control: must-revalidate couchdb log says: Invalid JSON: {{error,{10,"lexical error: invalid bytes in UTF8 string.\n"}},<<"{\"test\":\"<F6>\"}">>} this is indeed hex ö: > :m + Numeric > putChar $ toEnum $ fst $ head $ readHex "f6" ö if i apply the above patch and reinstall JSON and CouchDB the doc creation works: > runCouchDB' $ newNamedDoc (db "db") (doc "foo") (showJSON $ toJSObject [("test", "ö")]) Right someRev but i dont get back the ö i expected: > Just (_,_,x) <-runCouchDB' $ getDoc (db "foo") (doc "bar") :: IO (Maybe (Doc,Rev,JSObject String)) > let Ok y = valFromObj "test" =<< readJSON x :: Result String > y "\195\188" > putStrLn y Ã¼ apperently with curl everything works fine: $ curl localhost:5984/db/foo -XPUT -d '{"test": "ö"}' {"ok":true,"id":"foo","rev":"someOtherRev"} $ curl localhost:5984/db/foo {"_id":"bars","_rev":"someOtherRev","test":"ö"} so how can i get my precious ö back? what am i doing wrong or does Text.JSON need another patch? another question: why does encControl in Text/JSON/String.hs handle the cases x < '\x100' and x < '\x1000' even though they can never be reached with the old predicate in encJSString (x < '\x20') finally: is '\x7e' the right literal for the job? thanks for reading have fun martin

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What magic has the new parallel io manager done to improve performance?
by yi huang 16 Feb '13

16 Feb '13

I'm curious about the design and trade offs in IO manager, I can point out two changes that should boost performance: 1. Run an IO manager thread on each capability. 2. Use ONESHOT flag to save a system call. I wonder is there anything else? It must be interesting. -- http://yi-programmer.com/

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how to release (on hackage) a package that needs foreign libs?
by Johannes Waldmann 16 Feb '13

16 Feb '13

What is the recommended approach for publishing (on hackage) a package that depends on external libraries? In my case, the current version of http://hackage.haskell.org/package/satchmo (sources at https://github.com/jwaldmann/satchmo) depends on minisat-haskell-bindings (in Haskell), minisat-c-bindings (in C), and minisat (C++), all from https://github.com/niklasso/ My guess is that this would require that the external libraries are packaged (deb, rpm?), then hackage gods install them, and only then, my package will be built (and docs will be shown)? - J.W.

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optimization of recursive functions
by Andrew Polonsky 15 Feb '13

15 Feb '13

Hello, Is there any difference in efficiency between these two functions, when compiled with all optimizations? map f [] = [] map f (a:as) = f a : map f as and map f x = map' x where map' [] = [] map' (a:as) = f a : map' as Thanks, Andrew

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ANN: rss2irc-1.0, announces RSS/Atom feed updates to IRC
by Simon Michael 15 Feb '13

15 Feb '13

I'm pleased to announce a new release of rss2irc, the software behind hackagebot on #haskell. rss2irc is an IRC bot that polls an RSS or Atom feed and announces updates to an IRC channel, with options for customizing output and behavior. It aims to be an easy-to-use, reliable, well-behaved bot. Release notes: 1.0 (2013/2/15) New: * more robust item detection and duplicate announcement protection, with simpler options * easier irc address syntax, drop -p/--port option * can poll urls with semicolon parameter separator (eg darcsweb's) * can poll https feeds * can poll from stdin (-) * can poll a file containing multiple copies of a feed (eg for testing) * can announce item urls containing percent * `--cache-control` option sets a HTTP Cache-Control header * `--use-actions` announces with CTCP ACTIONs (like the /me command) Fixed: * updated for GHC 7.6 & current libs * initialises http properly on microsoft windows * builds threaded and optimised by default * thread and error handling is more robust, eg don't ignore exceptions in the irc writer thread * no longer adds stray "upload:" to IRC messages * renamed --dupe-descriptions to `--allow-duplicates` * dropped --debug flag * new item detection and announcing is more robust * announcements on console are clearer * a simulated irc connection is not logged unless --debug-irc is used Best, -Simon

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(+) on two lists ?
by sheng chen 15 Feb '13

15 Feb '13

Hi, I was puzzled by the following little program. sum' [] = [] sum' (x:xs) = x + sum' xs I thought the GHC type checker will report a type error. However, the type checker accepts this program and gives the type Num [a] => [[a]] -> [a] When I add type annotation to the program sum' :: Num [a] => [[a]] -> [a] sum' [] = [] sum' (x:xs) = x + sum' xs The GHC asks me to add FlexibleContexts language extension. I would appreciate explanation on this issue. Thanks, Sheng

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