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Failure to install Network.CGI
by william murphy 18 Jul '11

18 Jul '11

Hi all, I tried to install the Network.CGI module by inputting: cabal install CGI into the terminal but got this error: Resolving dependencies... Configuring cgi-3001.1.8.2... Preprocessing library cgi-3001.1.8.2... Building cgi-3001.1.8.2... <command line>: cannot satisfy -package-id network-2.3.0.2-24fdc6b92867c7236e81708f93cae7d0 (use -v for more information) cabal: Error: some packages failed to install: cgi-3001.1.8.2 failed during the building phase. The exception was: ExitFailure 1 I'm almost positive I have the latest version of cabal, and have tried many things, but can't seem to see what I'm leaving out. Thanks, William

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How to use Yesod OAuth
by Haisheng Wu 18 Jul '11

18 Jul '11

Hello, I'm trying to use Yesod oAuth plugin to a SNS site which is very similar to Twitter. I add oAuth into the authPlugin and I'm able to see the auth URL at login page. I click the URL and it forwards me to a page ask me input that SNS site account. I fill in my account then the browser navigate to the authorize page display a authorised code (8 digits). Then, how I suppose to continue? (I think this is the 2nd phase of the oauth process) I expect there will be a text box in my App where I can fill in the authorised code. Or even the oAuth plugin automatically get the authorised code and continue to the next oauth phase. Appreciate your help! Haisheng

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Trying to make a calculator
by Emanuil Boyanov 18 Jul '11

18 Jul '11

Hi guys, I'm new to Haskell, I learn it for couple of months now and we've learned only simple functions like map, zip etc. But now my assignment is a lot tougher than I thought so I'll need some guidelines. I've googled my assignment and I saw a person with the exact assignment so I am gonna copy the task then I'm gonna show you what I've done so far. Here it is: Make calculator function. The function argument is a list of strings and also form such list, as each string of the argument made definite action. - If the string has the form of an arithmetic expression - calculate this expression. The string result becomes part of the list- result. If the expression contains a variable which is not assigned value, the result is displayed "undefined". - If the string has the form Name = value calculated from the last expression is assigned to the variable with the corresponding name in the list, and in the result list is formed a string with type name = ... where the site stands blank corresponding value. If there is not a calculated expression to be assigned to form a string "no value". - If the string is non-blank, but there is a species different from the above two case, form the string "error". - If the string is empty, incl. when it contains only spaces, in the result there is not form a string. Expressions consist of integers without sign variables, operations + (Addition), - (subtraction), * (multiplication) and / (divide) In the beginning, end or between the elements of each row can have spaces - they are irrelevant to its correctness. Example: the list-argument ["3 +7 / 2", "2 + x", "= s", "2 * s +4", "", "2 + +4 / 5"] function should provide a result-list ["6", "undefined", "s = 6", "16", "error"]. ========= So what I've done so far is: calculator [] = [] calculator [" "] = [] calculator [c] = [c] calculator (c:" ":cs) = c:calculator cs calculator (" ":cs) = calculator cs calculator (c:" ":" ":cs) = calculator (c:" ":cs) calculator (c:cs) | (head c `elem` ['a'..'z']) && (head(tail c) `elem` ['+','-','*','/']) && (last(take (length c-2)c) `elem` ['1','2','3','4','5','6','7','8','9']) = calculator cs | otherwise = "error":calculator cs I try at first to use pattern matching for the blank strings in the first list, so that I can get rid off them in the second list. Am I even on the right way here? I found the "elem" function and I'm trying to check if the first char of the string is a letter or a number, then if the second char is one of the operations and so on. Problem is that I have no idea how can I check if the variable is defined already or how to save a variable and then give that variable the new value - is that even possible? :] And another thing `elem` [1..9] isn't working - how can I make a single char of a string to hold an integer.. Could you give me some guidelines or at least tell me if I'm on the right way here so I can move forward and improve my program. Thanks a lot!

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ANNOUNCE: docidx-1.0.0
by Andy Gimblett 18 Jul '11

18 Jul '11

Hi all. I'd like to announce docidx, a new tool for Haskell documentation: http://hackage.haskell.org/package/docidx http://github.com/gimbo/docidx.hs docidx is a program which creates a static HTML page indexing your installed packages, with links to your local haddock docs and to each package's hackage page. It covers global and user packages, and handles multiple installed versions sensibly. Here's how the output looks: http://github.com/gimbo/docidx.hs/raw/master/examples/example.png The idea is to complement the "index by module name" which Cabal creates and maintains. Sometimes you want to find things by package. :-) (But note that unlike Cabal's index, docidx's isn't automatically updated when you install a new package; so, I run it once an hour from cron.) Please see the github page for more details, including customisation options. Hopefully somebody will find this useful. Maybe one day it could be part of cabal-install? :-) Thanks! -Andy PS: A bit of history/due credit: Martijn van Steenbergen did something similar in PHP in early 2009: http://thread.gmane.org/gmane.comp.lang.haskell.cafe/53531/focus=53572 ; then I wrote a static version in Python later that year: http://gimbo.org.uk/blog/2009/09/23/ ; then Andy Price ported that to Haskell: https://github.com/andyprice/docidx.hs ; finally, I rewrote that to build the index via Cabal rather than walking the filesystem directly - and here we are. -- Andy Gimblett haskell(a)gimbo.org.uk

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Network.Html simpleTable fonts
by william murphy 18 Jul '11

18 Jul '11

How does one change the font and size of text in a cells in a simpleTable? As in, to change the height, one could state: x = simpleTable [] [height "5"] exampleTable Thanks, Will

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Why aren't files flushed at exit?
by Paul Johnson 18 Jul '11

18 Jul '11

If you open a file for writing and then exit with output unflushed, then Haskell does not flush the file for you. In ghci the program seems to work, but then when you compile it in ghc it mysteriously fails. I've just been bitten by this, but when I went to the bug tracker I found http://hackage.haskell.org/trac/ghc/ticket/4119 ticket 4119, which describes this behaviour and was resolved as "invalid". So presumably this behaviour is by design. Given that most environments get this right, why doesn't Haskell? Paul.

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Contacting Planet Haskell admins
by Michael Snoyman 17 Jul '11

17 Jul '11

Hi, I just sent an email to the Planet Haskell admins, and it bounced back. Apparently it's still forwarding to Don's old email address. Anyone able to update it? Michael

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Re: [Haskell-cafe] Haskell and Databases
by Greg Weber 17 Jul '11

17 Jul '11

Hi Tobias, Have you seen DSH [1]? You might also be interested in Persistent [2], but it sounds like it has different goals than what you are after. [1] http://hackage.haskell.org/package/DSH [2] http://www.yesodweb.com/book/persistent

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Constructor discipline and dependent types.
by Patrick Browne 17 Jul '11

17 Jul '11

Hi, My understanding of the constructor discipline (CD) is that it is a restriction on the form of an equation that guarantees that the equation represents an executable function. The CD restricts the LHS of an equation to consist of a function name, constructors, and variables. Also there should be no new variables on the RHS. So CD permits function to be defined and prohibits general assertions which are more in the domain of a specification language. Question 1: Is the above a reasonable understanding of CD? Question 2: I have read that the lack of dependent types (DT) prevents writing general assertions in Haskell. Is CD related to DT? If so how are they related? Regards, Pat This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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Animas/Yampa: ArrowChoice?
by Ertugrul Soeylemez 16 Jul '11

16 Jul '11

Hello all, I really like the way Animas (fork of Yampa) represents reactive systems, and I would love to write some of my simulations using it. Unfortunately most of what I want to do requires dynamic systems, which can be boiled down to me believing to need an ArrowChoice instance for the SF arrow, because I have to choose between different signal paths depending on the input signal or events. Animas appears to be only suitable for robot-like systems with specific, predefined actors. Is there anything I can do about it? Is it difficult to write the ArrowChoice instance? Or is there a different solution, which I overlooked? Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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urWeb, Haskell and dependent types ....
by Vasili I. Galchin 16 Jul '11

16 Jul '11

Hello, Here is probably a good paper to get people up to speed on dependent types (ironically written by two contributors to this mailing list??): okmij.org/ftp/papers/lightweight-static-capabilities.pdf Vasili

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How to determine minimal dependency of package
by yi huang 16 Jul '11

16 Jul '11

I'm writing my first haskell package, how do i determine the minimal dependency of it, for example, it use Data.Data, how do i know which version of base package first introduce Data.Data module, i can't find the answer with google. -- http://www.yi-programmer.com/blog/

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Domain-Specific Languages: Call for Participation
by Chung-chieh Shan 15 Jul '11

15 Jul '11

DSL 2011: IFIP Working Conference on Domain-Specific Languages 6-8 September 2011, Bordeaux, France **** Call for Participation: Online registration deadline July 30, 2011 **** Details of the program and accommodation are available at http://dsl2011.bordeaux.inria.fr. == Invited Speakers == - Jeremy Gibbons - University of Oxford, UK. - Claude Kirchner - INRIA, France. == Distilled Tutorials on Domain-Specific Languages == The purpose of these tutorials are not to give a general overview, but to make the attendees aware of one point and to make them master it. - Jerzy Karczmarczuk. Specific "scientific" data structures, and their processing. - Oleg Kiselyov. Implementing explicit and finding implicit sharing in embedded DSL. - Keiko Nakata. A total interpreter for While with interactive I/O. - Josef Svenningsson. Combining deep and shallow embeddings for EDSLs. - Walid Taha. Accurate programming: thinking about programs in terms of properties. - William Cook. Build your own partial evaluator in 90 minutes. == DSL Technical Papers == - Basile Starynkevitch. MELT a Translated Domain Specific Language Embedded in the GCC Compiler. - Azer Bestavros and Assaf Kfoury. A Domain-Specific Language for the Incremental and Modular Design of Large-Scale Verifiably-Safe Flow Networks. - Tiark Rompf, Kevin J. Brown, Hassan Chafi, Hyoukjoong Lee, Arvind K. Sujeeth, Martin Odersky and Kunle Olukotun. Building-Blocks for Performance Oriented DSLs. - Dominic Orchard and Alan Mycroft. Efficient and Correct Stencil Computation via Pattern Matching and Static Typing. - Tim Bauer, Martin Erwig, Alan Fern and Jervis Pinto. Adaptation-Based Programming in Haskell. - Eric Walkingshaw and Martin Erwig. A DSEL for Studying and Explaining Causation. - Lucas Beyak and Jacques Carette. SAGA: A DSL for story management.

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GHC API output files options problem
by Marianna Rapoport 15 Jul '11

15 Jul '11

Hi all, we are working on a Haskell support plugin for IntelliJ IDEA, and need to provide correct compilation of Haskell programs. We compile the latter using GHC API, with options specifying .hi and .o paths. For some reason, unlike a standalone GHC compilation, GHC API ignores the values of the outputFile and outputHi flags. Please find attached a minimal example where "err_test.hs" uses GHC API to compile "src/A.hs". Thus, "test_ghc.sh" and "test_my.sh" should do the same, but the first works correctly while the latter puts "use.o" and "use.hi" to the "src" folder. To launch "test_my.sh", please change the value of lib in "err_test.hs" according to your GHC installation path. Thank you very much in advance! Marianna Rapoport, amaurremi(a)gmail.com Oleg Sobolev, sobolev_ov(a)mail.ru

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HaskellDB DB Layout Description
by Tom Murphy 15 Jul '11

15 Jul '11

Hi, I've found good explanations of the HaskellDB combinators, but I can't find good information about how to correctly define the database layout. Can anyone point me to a resource, or give a quick example? Thanks! Tom

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Request for comments: dbus-core 0.9
by John Millikin 15 Jul '11

15 Jul '11

I think I'm pretty close to releasing the next version of my D-Bus library, dbus-core. This is a big release that changes a lot of the API, so I'd like to see if anybody 1) has any problems with the new APIs or 2) has any suggested improvements. haddock: https://john-millikin.com/temp/dbus-core_0.9/ tarball: https://john-millikin.com/temp/dbus-core_0.9.tar.gz pdf: https://john-millikin.com/temp/dbus-core_0.9.pdf examples: https://john-millikin.com/temp/dbus-core_0.9/examples/ Notable improvements in this release are: • Merges dbus-core and dbus-client. The modules "DBus.Client" and "DBus.Client.Simple" provide a very nice, high-level API to common D-Bus operations. Note: I've reverted to the dbus-client_0.3 API for this, since not many users liked the monad-based API introduced in 0.4. • The class Variable was split into IsAtom, IsValue, and IsVariant. Together these let the rest of the type conversion API go away -- you no longer have to worry about Array, Structure, or Dictionary (except in a few, rare cases). • Support for custom transports (eg, a pipe or in-memory buffer). • Improved support for custom authentication mechanisms. Also, while the docs are obviously not finished, I'm interested to know if anyone finds any parts of the library confusing -- those'll get extra doc love.

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Call for GUI examples - Functional Reactive Programming
by Heinrich Apfelmus 14 Jul '11

14 Jul '11

Dear Haskellers, Can GUI programming be liberated from the IO monad? Functional Reactive Programming (FRP) promises as much, and I'm trying to make this dream a reality with my [reactive-banana][] library. Having released version 0.4.0.0, I am now looking for example programs to direct the future evolution of the library. Do you know any *small GUI programs* that you would *like* to see *implemented with Functional Reactive Programming?* I'm sure that some of you * are interested in FRP and would like to learn from a few simple but beautiful examples * or have written small GUI programs that they are unhappy with because they were not purely functional * or have thought about FRP before and concocted a few examples that are very tricky to implement with FRP * or have written a nice little GUI application that simply makes a great example. I would love to hear your examples, so that I can try to convert them to FRP style and test my library against them! Strictly speaking, it doesn't have to be an example with a graphical interface, I'm also interested in audio and animation examples, though I'm currently focusing on GUIs. Here a few examples of what I am looking for: * I think that Tim Docker's minimal step sequencer [hbeat][] simply makes a great example. I hope that wxHaskell offers a platform-independent way to play sound. * While editing the [Haskell Weekly News][hwn], Brent Yorgey wrote a little command-line [program to gather newsworthy items][zipedit]. But wouldn't a GUI be great? Since I'm only interested in the GUI, someone would have to supply the feed parser for this example to be viable. Maybe the current editor, Daniel Santa Cruz, might be interested? * Notes of a musical performance can be modeled as event streams (MIDI), as Henning Thielemann has [done with great effect][midi streams]. Surely, reactive-banana should be up to the task, but writing an arpeggiator seems impossible at the moment. [reactive-banana]: http://hackage.haskell.org/package/reactive-banana [zipedit]: http://byorgey.wordpress.com/2008/06/21/zipedit/ [hbeat]: http://dockerz.net/twd/hBeat [midi streams]: http://hackage.haskell.org/package/streamed [hwn]: http://www.haskell.org/haskellwiki/Haskell_Weekly_News Best regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com

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Re: [Haskell-cafe] FFI for a beginner
by Donn Cave 14 Jul '11

14 Jul '11

>> The docs <http://www.haskell.org/ghc/docs/latest/html/users_guide/ffi-ghc.html#glasgo…> >> say that -#include pragmas no longer work, but fail to explain how to >> load code without them. Suffice to say I have no recourse but trial >> and error." > > Ah, now that is a GHC documentation question. Can you tell us what > should the docs should say instead? Then we can fix the docs. - I believe I would say (agreeing with M Parès) that hsc2hs serves this purpose - put "#include <termios.h>" in the .hsc file and use hsc2hs features like #const and (#peek x, y). The termios example could cover a lot more ground by operating on the termios struct and calling tcsetattr directly from Haskell, using hsc2hs # macros (I append an example implementation), but you'd also want to explain that hsc2hs is only needed for these # macros, not for FFI in general. But hsc2hs as of GHC 7.0.3 generates a "{-# INCLUDE" pragma, which of course is cause for complaint from ghc. So there might be more to be fixed, beyond just the documentation. That line probably has no actual purpose other than to annoy ghc, and as a workaround it can simply be removed from the .hs file. - My impression is that Greencard isn't the best way to get started with FFI and hasn't been for a decade or so, so it should probably be purged from docs. - The "root" page needs some attention from someone who understands what "safe" and "unsafe" mean. Currently (under "Improving efficiency") it cites the conventional interpretation that a function must be "safe" if it calls back into the runtime, but neglects to mention that it must also be "safe" if it is not to block execution of other threads, including bound OS threads - so related to efficiency issues you might want to make any slow I/O operation "safe" to avoid this. There may be other points about "unsafe" that need to be documented. Donn -------- {-# LANGUAGE ForeignFunctionInterface #-} module TTY (clearICANON) where import Data.Word (Word32) import Foreign import Foreign.C #include <termios.h> type TCFlag = (#type tcflag_t) type Speed = (#type speed_t) data Termios = Termios { termios_c_iflag :: TCFlag , termios_c_oflag :: TCFlag , termios_c_cflag :: TCFlag , termios_c_lflag :: TCFlag , termios_c_cc :: [CChar] , termios_c_ispeed :: Speed , termios_c_ospeed :: Speed } deriving Show instance Storable Termios where sizeOf _ = #size struct termios alignment _ = alignment (undefined::CDouble) peek a = do iflag <- (#peek struct termios, c_iflag) a oflag <- (#peek struct termios, c_oflag) a cflag <- (#peek struct termios, c_cflag) a lflag <- (#peek struct termios, c_lflag) a cc <- peekArray 20 ((#ptr struct termios, c_cc) a) ispeed <- (#peek struct termios, c_ispeed) a ospeed <- (#peek struct termios, c_ospeed) a return (Termios iflag oflag cflag lflag cc ispeed ospeed) poke a (Termios iflag oflag cflag lflag cc ispeed ospeed) = do (#poke struct termios, c_iflag) a iflag (#poke struct termios, c_oflag) a oflag (#poke struct termios, c_cflag) a cflag (#poke struct termios, c_lflag) a lflag pokeArray ((#ptr struct termios, c_cc) a) (take 20 (cc ++ repeat 0)) (#poke struct termios, c_ispeed) a ispeed (#poke struct termios, c_ospeed) a ospeed foreign import ccall "tcgetattr" tcgetattr :: CInt -> Ptr Termios -> IO CInt foreign import ccall "tcsetattr" tcsetattr :: CInt -> CInt -> Ptr Termios -> IO CInt setLFlag :: TCFlag -> Termios -> Termios setLFlag c a = a { termios_c_lflag = ((termios_c_lflag a) .|. c) } clearLFlag :: TCFlag -> Termios -> Termios clearLFlag c a = a { termios_c_lflag = ((termios_c_lflag a) .&. (complement c)) } setTTYAttr :: CInt -> (Termios -> Termios) -> IO () setTTYAttr fd fn = alloca $ \ termiosp -> do status <- tcgetattr fd termiosp if status == 0 then do termios <- peek termiosp poke termiosp (fn termios) status <- tcsetattr fd (#const TCSAFLUSH) termiosp if status == 0 then return () else throwErrno "tcgetattr" else throwErrno "tcgetattr" clearICANON = setTTYAttr 0 (clearLFlag (#const ICANON))

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[iteratee] empty chunk as special case of input
by Sergey Mironov 14 Jul '11

14 Jul '11

Hi community, hi John. I find myself reading bindIteratee[1] function for a several days.. there is something that keeps me away from completely understanding of the concept. The most noticeble thing is \nullC\ guard in the definition. To demonstate the consequences of this solution, let me define an iterator like myI = Iteratee $ \onDone _ -> onDone 'a' (Chunk "xyz") It is a bit unusial, since myI substitutes real stream with a fake one (xyz). Now lets define two actions producing different results in unusual manner: printI i = enumPure1Chunk ['a'..'g'] i >>= run >>= print i1 = (return 'b' >> myI >> I.head) -- myI substitutes the stream, last /I.head/ produces 'x', OK i2 = (I.head >> myI >> I.head) -- produces 'b'! I expected another 'x' here but myI's stream was ignored by >>= Well, I understand that this is probably an expected behaviour, but what is it for? Why we can't handle null input like non-null? Iterator may just stay in it's current state in that case. Thanks in advance Sergey -- [1] - bindIteratee (basically, >>=) code from Data.Iteratee.Base.hs bindIteratee :: (Monad m, Nullable s) => Iteratee s m a -> (a -> Iteratee s m b) -> Iteratee s m b bindIteratee = self where self m f = Iteratee $ \onDone onCont -> let m_done a (Chunk s) | nullC s = runIter (f a) onDone onCont m_done a stream = runIter (f a) (const . flip onDone stream) f_cont where f_cont k Nothing = runIter (k stream) onDone onCont f_cont k e = onCont k e in runIter m m_done (onCont . (flip self f .))

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Compiler support for orphan instances
by Henning Thielemann 14 Jul '11

14 Jul '11

Yesterday I tried to install fix-imports and ran into another example of problems with changing transitive closures of orphan instance propagation: $ cabal install fix-imports Resolving dependencies... Configuring fix-imports-0.1.2... Preprocessing executables for fix-imports-0.1.2... Building fix-imports-0.1.2... [1 of 6] Compiling Types ( src/Types.hs, dist/build/FixImports/FixImports-tmp/Types.o ) [2 of 6] Compiling Util ( src/Util.hs, dist/build/FixImports/FixImports-tmp/Util.o ) [3 of 6] Compiling Index ( src/Index.hs, dist/build/FixImports/FixImports-tmp/Index.o ) src/Index.hs:48:15: No instance for (Monad (Either [Char])) arising from a use of `sequence' at src/Index.hs:48:15-40 Possible fix: add an instance declaration for (Monad (Either [Char])) ... It is a repeating problem that you use an instance without noting it (here Monad (Either [Char])) and import it without noting it (you import a module, that possibly by accident imported and thus re-exported that instance). If the modules you import, decide to no longer use the Monad Either instance, then your imports will ignore the instance. I thought that some compiler support could reduce this problem. Proposal 1 GHC might warn, if you use an orphan instance but import it from a module, that does not define it. Example: module A where instance Msg s => Monad (Either s) ... module B where import A module C where {- B does not define instance Monad Either, but propagates that instance from A. -} import B test :: Either String Int test = do fail "bla" return 5 {- Here GHC should emit a warning, since the Monad Either instance is imported from B but not directly from A. If B drops the 'import A' this will no longer work. -} Proposal 2 Sometimes a module is actually intended for re-exporting an instance. Sometimes an instance is moved from module A to module Z, but A still re-exports the instance in order to maintain compatibility. In these cases proposal 1 would yield false alarms. So maybe there should be a pragma for tagging instances that are intentionally re-exported. Problem: How to identify instances? This is certainly related to earlier ideas of explicit named instance imports. module A where instance Msg s => Monad (Either s) ... module B where import A {- Explicitly propagate the instance from A, which is as good as defining it here. If the instance is not in scope, then compilation should fail at this pragma. This way it is asserted, that the instance is really exported. -} {-# INSTANCE-EXPORT Monad (Either s) #-} module C where import B test :: Either String Int test = do fail "bla" return 5 {- No warning here, because B intentionally propagated the Monad Either instance. -} Proposal 3 Forget about all those proposals and implement named instances. However the proposals 1 and 2 let you write code that works on all Haskell 98 compilers. Compilers may just not warn or ignore the INSTANCE-EXPORT pragma. Proposal 3 requires a language extension and using it will work only on the compilers that support that extension. Proposal 4 Forget about flexible, undecidable, overlapping, orphan and other instances and create a system, where classes are replaced by explicit method dictionaries and some rules for inserting these dictionaries, where the rules can be declared by the package author in the modules.

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Re: [Haskell-cafe] FFI for a beginner
by Jason Dagit 14 Jul '11

14 Jul '11

I don't know how to make ghc load them without using either hsc2hs or c2hs. I've had better experiences with hsc, but your mileage may vary. On Apr 8, 2011 3:34 AM, "Andrew Pennebaker" <andrew.pennebaker(a)gmail.com> wrote: ncurses is proving too difficult to setup, so I'm working on a new library called charm. The C code works by itself, but I can't compile a Haskell wrapper for it. While the tutorials at HaskellWiki<http://www.haskell.org/haskellwiki/FFI_complete_examples>are helpful, they're outdated. Argh! The docs<http://www.haskell.org/ghc/docs/latest/html/users_guide/ffi-ghc.html#glasgo…>say that -#include pragmas no longer work, but fail to explain how to load code without them. Suffice to say I have no recourse but trial and error. GitHub: charm <https://github.com/mcandre/charm> and hscharm<https://github.com/mcandre/hscharm> <https://github.com/mcandre/charm>$ make cp /usr/include/charm.c . ghc --make -fforce-recomp -o hellocharm hellocharm.hs charm.hs charm.c -I/usr/include -dylib-install-name /usr/lib/libcharm.dynlib [1 of 2] Compiling Charm ( charm.hs, charm.o ) [2 of 2] Compiling Main ( hellocharm.hs, hellocharm.o ) Linking hellocharm ... ld: duplicate symbol _Charm_getWidth_info in charm.o and charm.o collect2: ld returned 1 exit status make: *** [hellocharm] Error 1 Cheers, Andrew Pennebaker www.yellosoft.us _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Haskell Weekly News: Issue 190
by Daniel Santa Cruz 14 Jul '11

14 Jul '11

Welcome to issue 190 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of July 3 to 9, 2011. You can find the HTML version of this issue at: http://contemplatecode.blogspot.com/2011/07/haskell-weekly-news-issue-190.h… Announcements Karel Gardas announced that he, Stephen Blackheath, and David Terei are "working together on ARM registerised port of GHC. The port is using LLVM as a code generator and in kind of working already." Testing is much appreciated! [1] http://goo.gl/TVvGD Matthias Fischmann inveted people to the HacPhi satellite in Berlin on July 29-31. See the post for details. Admission is free, but there is limited space, so don't wait too long! [2] http://goo.gl/XqdvI Quotes of the Week * chrisdone: [on hpaste as motivational tool] When I added hlint I was like 'hey Neil, check this', and he was all 'Oh noes! I must fix all the bugs!' * *amacleod started excising GOTOs from his BASIC code when he was 10. *roconnor started excising GOTOs from his Pascal code when he was 11. *luite used goto's in his C code last year. * AlanPerlis: One can't proceed from the informal to the formal by formal means. * Axman6: well, if C++ is anything to go by, the next Haskell will have lambdas and easy concurrency! * Cale: Constructing a definition from a type is formally the same thing as constructing a proof from a theorem. (Just with different logical systems in place :) * xplat: im in ur Sierpinski space, classifyin' ur subobjects * cmccann: in ContT, flow controls you * [Kron] What is the meaning of life? D: [joe6] learning agda? * camccann: Applying readFile to a file path doesn't return a String value any more than putting a steak next to a meat grinder magically turns them into a hamburger. * blackdog: the type system makes you strong, makes you hard. Top Reddit Stories * GHC on ARM Domain: ghcarm.wordpress.com, Score: 35, Comments: 4 On Reddit: [3] http://goo.gl/idJ7s Original: [4] http://goo.gl/D5zjE * The Monad.Reader Issue 18 Domain: themonadreader.wordpress.com, Score: 32, Comments: 3 On Reddit: [5] http://goo.gl/XA67w Original: [6] http://goo.gl/tzVX2 * Haskell vs. ML in the context of "Worse is Better" Domain: news.ycombinator.com, Score: 31, Comments: 25 On Reddit: [7] http://goo.gl/LpTbP Original: [8] http://goo.gl/bNU4w * Do (side) effects really need a total order? Domain: pchiusano.blogspot.com, Score: 31, Comments: 8 On Reddit: [9] http://goo.gl/jfxWl Original: [10] http://goo.gl/5tmUZ * Call for GUI examples - Any small GUI applications that you would like to see implemented with Functional Reactive Programming? Domain: apfelmus.nfshost.com, Score: 30, Comments: 13 On Reddit: [11] http://goo.gl/MMZJb Original: [12] http://goo.gl/X7IXP * Impredicative Polymorphism - A Use Case Domain: augustss.blogspot.com, Score: 23, Comments: 6 On Reddit: [13] http://goo.gl/QCwpt Original: [14] http://goo.gl/owsJT * Yesod :: Testimonials from a developer and designer Domain: yesodweb.com, Score: 19, Comments: 9 On Reddit: [15] http://goo.gl/5HOKo Original: [16] http://goo.gl/Zad2d * Parallelizing a nonogram solver Domain: jaspervdj.be, Score: 14, Comments: 2 On Reddit: [17] http://goo.gl/5LFlF Original: [18] http://goo.gl/BUbUA * A Haskell newbie's guide to Snap, Part 2 Domain: learnmeahaskell.blogspot.com, Score: 13, Comments: 0 On Reddit: [19] http://goo.gl/KTW50 Original: [20] http://goo.gl/rF1B1 * Losing my mind: Is Rhask (R language+Haskell) a real entity? Domain: self.haskell, Score: 11, Comments: 8 On Reddit: [21] http://goo.gl/O95KN Original: [22] http://goo.gl/O95KN Top StackOverflow Questions * How to gain control of a 5GB heap in Haskell? votes: 27, answers: 1 Read on SO: [23] http://goo.gl/OdZIG * Using higher-order Haskell types in C# votes: 16, answers: 2 Read on SO: [24] http://goo.gl/qzvok * Why applicative functors in Haskell and functional programming votes: 16, answers: 6 Read on SO: [25] http://goo.gl/72kE7 * Is there OS written in Haskell? votes: 16, answers: 3 Read on SO: [26] http://goo.gl/PgOeO * A good way to avoid “sharing”? votes: 15, answers: 1 Read on SO: [27] http://goo.gl/9GLzu * Hard Time Learning Haskell [migrated] votes: 13, answers: 0 Read on SO: [28] http://goo.gl/hhonY * Creating a Haskell REPL within a Haskell application votes: 11, answers: 1 Read on SO: [29] http://goo.gl/jZU7I About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz References 1. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/90890 2. http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/90922 3. http://www.reddit.com/r/haskell/comments/ija21/ghc_on_arm/ 4. http://ghcarm.wordpress.com/2011/07/06/armv7-thumb-vfpv3-support-and-github… 5. http://www.reddit.com/r/haskell/comments/ihz6h/the_monadreader_issue_18/ 6. http://themonadreader.wordpress.com/2011/07/05/issue-18/ 7. http://www.reddit.com/r/haskell/comments/ig4yk/haskell_vs_ml_in_the_context… 8. http://news.ycombinator.com/item?id=2725386 9. http://www.reddit.com/r/haskell/comments/iihrl/do_side_effects_really_need_… 10. http://pchiusano.blogspot.com/2011/07/do-side-effects-really-need-total-ord… 11. http://www.reddit.com/r/haskell/comments/ijsnv/call_for_gui_examples_any_sm… 12. http://apfelmus.nfshost.com/blog/2011/07/08-call-for-gui-examples.html 13. http://www.reddit.com/r/haskell/comments/il5g5/impredicative_polymorphism_a… 14. http://augustss.blogspot.com/2011/07/impredicative-polymorphism-use-case-in… 15. http://www.reddit.com/r/haskell/comments/iitr1/yesod_testimonials_from_a_de… 16. http://www.yesodweb.com/page/testimonials 17. http://www.reddit.com/r/haskell/comments/ihdbh/parallelizing_a_nonogram_sol… 18. http://jaspervdj.be/posts/2011-07-05-parallelizing-a-nonogram-solver.html 19. http://www.reddit.com/r/haskell/comments/iiksr/a_haskell_newbies_guide_to_s… 20. http://learnmeahaskell.blogspot.com/2011/07/haskell-newbies-guide-to-snap-p… 21. http://www.reddit.com/r/haskell/comments/ig5mk/losing_my_mind_is_rhask_r_la… 22. http://www.reddit.com/r/haskell/comments/ig5mk/losing_my_mind_is_rhask_r_la… 23. http://stackoverflow.com/questions/6623391/how-to-gain-control-of-a-5gb-hea… 24. http://stackoverflow.com/questions/6562378/using-higher-order-haskell-types… 25. http://stackoverflow.com/questions/6570779/why-applicative-functors-in-hask… 26. http://stackoverflow.com/questions/6638080/is-there-os-written-in-haskell 27. http://stackoverflow.com/questions/6614023/a-good-way-to-avoid-sharing 28. http://stackoverflow.com/questions/6618081/hard-time-learning-haskell-migra… 29. http://stackoverflow.com/questions/6588999/creating-a-haskell-repl-within-a…

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ANN: fix-imports-0.1.2
by Evan Laforge 14 Jul '11

14 Jul '11

Not sure if anyone else is using this, but I've fixed some bugs and bumped fix-imports by a few more versions. Here's copy paste from the package description: A small standalone program to manage the import block of a haskell program. It will try to add import lines for qualified names with no corresponding import, remove unused import lines, and sort the import block according to some heuristic you can define. This only works for qualified imports! Unqualified imports are left untouched. It's most convenient if bound to an editor key. Changes since last announcement: Mon Jul 11 23:00:19 PDT 2011 qdunkan(a)gmail.com * version 0.1.2 Mon Jul 11 22:58:56 PDT 2011 qdunkan(a)gmail.com * fix but where multiple comments above an import are lost Fri Jul 1 15:16:40 PDT 2011 qdunkan(a)gmail.com * don't reload file if no changes were made Fri Jul 1 14:43:02 PDT 2011 qdunkan(a)gmail.com * sort and format the import block even if imports weren't added or removed Wed Jun 15 12:39:12 PDT 2011 qdunkan(a)gmail.com * version 0.1.1 Wed Jun 15 12:14:31 PDT 2011 qdunkan(a)gmail.com * upgrade to haskell-src-exts 1.11, which removes the mandatory fixities Wed Jun 15 12:11:27 PDT 2011 qdunkan(a)gmail.com * fix bug causing qualified vs. non-qualified imports to be sorted inconsistently Thu Jun 2 12:10:08 PDT 2011 qdunkan(a)gmail.com * more specific versions for base

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Paid work available in functional web programming
by Adam Chlipala 13 Jul '11

13 Jul '11

First, I apologize in advance if this post is deemed off-topic. It's about Ur/Web, a statically typed, pure functional programming languages that is much influenced by Haskell. For instance, Ur/Web includes monadic IO and type classes. I write to mention briefly that I'm looking for people interested in writing Ur/Web programs for pay. Ur/Web is a DSL for building modern web applications, and I believe it is truly a secret weapon for that domain, and one that should appeal to many Haskell fans. I have one customer now for whom I'm leading a project to develop a particular web application, and I'd like to have more. The current project would benefit from more programming help, and I would also like to develop a network of people interested in future projects. More information on Ur/Web can be found here: http://www.impredicative.com/ur/ So, are you interested in being paid at least market rates to develop real web applications in a cool Haskell-like language, thus helping to push functional programming further into the real world? Might you have any leads for possible customers for this kind of work, possibly including start-ups that want to do in-house development using a cool language with close support from the language creator (which happens to be me!)? I would love to receive reply e-mails with positive answers to either question!

5 5

Splitting Hackage Packages and re-exporting entire modules (with same module name)
by Thomas DuBuisson 13 Jul '11

13 Jul '11

All, I have decided it would be beneficial to split System.Crypto.Random and the rest of crypto-api into different packages. Is there I way I can create a package, "entropy", with System.Crypto.Random but continue to expose that module from crypto-api (allowing people who use that module some time to move)? If so, how? If not, does anyone else see value in this and how it can be added to the infrastructure? Cheers, Thomas

3 3

Seeking Don Stewart
by wren ng thornton 13 Jul '11

13 Jul '11

Hello all, Sorry for the spam. I'm trying to get ahold of Don Stewart, but it looks like there's some hiccup at Galois. When I mail him I get: <dons(a)galois.com>: host mail.galois.com[69.30.63.196] said: 550 5.1.1 <dons(a)galois.com>... User unknown (in reply to RCPT TO command) Does anyone know what's up, or if there's another (non-emergency) way to get in touch with him? -- Live well, ~wren

6 5

Baffling Hackage build log
by wren ng thornton 13 Jul '11

13 Jul '11

As a dutiful package maintainer I just checked to make sure unification-fd built correctly on Hackage. Unfortunately it didn't, and it gives this inscrutable error log: cabal: There is no package named unification-fd. Perhaps you need to run 'cabal update' first? Something awry on Hackage? -- Live well, ~wren

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Re: [Haskell-cafe] Type checking oddity -- maybe my own confusion
by oleg＠okmij.org 13 Jul '11

13 Jul '11

Ryan Newton wrote: > The desired goal was that everywhere I construct a value using the Assign > constructor, that the resulting value's type to be tainted by the AssignCap > constraint. Your code essentially accomplishes the goal: > data E m where > Assign :: AssignCap m => V -> E m -> E m -> E m > Varref :: V -> E m > -- ... Every time you construct the value of the type (E m) with the Assign constructor, you have to constructively prove -- provide evidence for -- that the type m is a member of AssignCap. That evidence is stored within the E m value and so can be reused whenever it is needed later. Let us modify the example slightly: > class AssignCap m > class MutateCap m > > data E m where > Assign :: AssignCap m => V -> E m -> E m -> E m > Mutate :: MutateCap m => V -> E m -> E m -> E m > Varref :: V -> E m > -- ... > > type V = String > > -- test1 :: E t -> E t > test1 (Assign v x1 x2) = Assign v x1 x2 > > -- test2 :: MutateCap t => E t -> E t > test2 (Assign v x1 x2) = Mutate v x1 x2 To use the constructor Assign, we need the evidence that AssignCap m holds. The deconstructed value (Assign v x1 x2) provides such an evidence (as a `hidden' field of the data type). Therefore, the inferred type for test1 has no constraints. In contrast, in test2 nothing provides Mutate with the evidence that MutateCap m holds. Therefore, the inferred type has the MutateCap m constraint. If storing the evidence is not desired, then one should use ordinary ADT with smart constructors: > -- Data constructors should not be exported > data E1 m = Assign1 V (E1 m) (E1 m) | Varref1 V > isAssign (Assign1 v x1 x2) = Just (v,x1,x2) > isAssign _ = Nothing > > assign :: AssignCap m => V -> E1 m -> E1 m -> E1 m > assign = Assign1 > > -- test3 :: AssignCap m => E1 m -> E1 m > test3 e | Just (v,x1,x2) <- isAssign e = assign v x1 x2 OCaml has a so-called private constructors, which permit deconstruction but prohibit construction. They are quite handy, saving us trouble writing the views like isAssign. > Actually... to go a bit further, I thought there was some way to arrange > this so that, upon GHC type-checking the case statement, it would realize > that certain cases are forbidden based on the type, and would consider the > pattern match complete without them (or issue an error if they are present). The key word in the phrase ``certain cases are forbidden based on the type'' is _type_ -- rather than a type class. If you wish to use this special GADT feature, you have to `invert' your design. Rather than specifying what is _required_ to construct an (E m) value, you should specify what is _provided_. > data E2 m where > Assign2 :: V -> E2 m -> E2 m -> E2 IOT > Varref2 :: V -> E2 PureT > > test4 :: E2 IOT -> E2 IOT > test4 (Assign2 v x1 x2) = Assign2 v x1 x2 > test4 (Varref2 _) = undefined The compiler complaints Couldn't match type `IOT' with `PureT' Inaccessible code in a pattern with constructor Varref2 :: V -> E2 PureT, in an equation for `test4' In the pattern: Varref2 _ In an equation for `test4': test4 (Varref2 _) = undefined as desired.

1 0

Type checking oddity -- maybe my own confusion
by Ryan Newton 13 Jul '11

13 Jul '11

Hi all, Is there something wrong with the code below? My anticipation was that the type of "test" would include the class constraint, because it uses the Assign constructor. But if you load this code in GHCI you can see that the inferred type was "test :: E m -> E m". Thanks, -Ryan {-# LANGUAGE GADTs #-} class AssignCap m data PureT data IOT instance AssignCap IOT data E m where Assign :: AssignCap m => V -> E m -> E m -> E m Varref :: V -> E m -- ... type V = String -- I expected the following type but am not getting it: -- test :: AssignCap m => E m -> E m test x = case x of Assign v e1 e2 -> Assign v e1 e2 -- And this is the same: Assign v e1 e2 -> x

3 3

Why is (+++) not a functor?
by Felipe Almeida Lessa 13 Jul '11

13 Jul '11

Hey! =) ArrowChoice's (+++) documentation [1] says that "this is in general not a functor" [1]. What does it mean for (+++) to be or not to be a functor? Cheers, [1] http://hackage.haskell.org/packages/archive/base/4.3.1.0/doc/html/Control-A… -- Felipe.

2 1

Re: [Haskell-cafe] How to ensure code executes in the context of a specific OS thread?
by Jason Dagit 12 Jul '11

12 Jul '11

On Jul 5, 2011 1:04 PM, "Jason Dagit" <dagitj(a)gmail.com> wrote: > > On Tue, Jul 5, 2011 at 12:33 PM, Ian Lynagh <igloo(a)earth.li> wrote: > > On Tue, Jul 05, 2011 at 08:11:21PM +0100, Simon Marlow wrote: > >> > >> In GHCi it's a different matter, because the main thread is running > >> GHCi itself, and all the expressions/statements typed at the prompt > >> are run in forkIO'd threads (a new one for each statement, in fact). > >> If you want a way to run command-line operations in the main thread, > >> please submit a feature request. I'm not sure it can be done, but > >> I'll look into it. > > > > We already have a way: -fno-ghci-sandbox > > I've removed all my explicit attempts to forkIO/forkOS and passed the > command line flag you mention. I just tried this but it doesn't > change the behavior in my example. I tried it again and discovered that due to an argument parsing bug in cabal-dev that the flag was not passed correctly. I explicitly passed it and verified that it works. Thanks for the workaround. By the way, I did look at the user guide for options like this and didn't see it. Which part of the manual is it in? Can I still make a feature request for a function to make code run on the original thread? My reasoning is that the code which needs to run on the main thread may appear in a library in which case the developer has no control over how ghc is invoked. The alternative appears to be context specific workarounds. I think that a general solution, if possible, will make gui libraries easier to develop for Ghc. I'm hooping it's as simple as exposing a pthreads call. Thanks, Jason

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The Typeable class is changing
by Yitzchak Gale 12 Jul '11

12 Jul '11

Simon Marlow has announced[1] on the Haskell Libraries list that the Typeable class is changing. The standard way to create a Typeable instance is just to derive it. If you do that, you will not be affected by this change. But it seems that many packages create Typeable instances by explicitly using mkTyCon. If your package does this, it will eventually break, after a deprecation period. Please respond to this thread if you own a package that will be affected by this change. Can someone who has quick access to the entire contents of Hackage please do a grep and find out exactly which packages on Hackage will be affected? Thanks. -Yitz [1] http://www.haskell.org/pipermail/libraries/2011-July/016546.html

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Fixing Performance Leaks at the Type Level
by Gershom Bazerman 12 Jul '11

12 Jul '11

This post is in literate Haskell. It describes how certain performance leaks are introduced in type level programming. These leaks do not affect program runtimes, but can cause compile times to grow drastically. They exist both with Functional Dependencies and Type Families, but are currently worse with the former, and have grown worse with the new constraint solver in GHC 7. It is intended both as a guide to those encountering these issues, and as a motivation for the GHC development team to address such issues as the constraint solver is developed and improved. > {-# OPTIONS_GHC -fcontext-stack=1000 #-} > {-# LANGUAGE FlexibleContexts, FlexibleInstances, FunctionalDependencies, MultiParamTypeClasses, OverlappingInstances, TypeSynonymInstances, TypeOperators, UndecidableInstances, TypeFamilies #-} > module TypePerformance where Our running example, for simplicity's sake, is a type-level map of a single function. For reference, here is the code for a simple value-level map of a single function. > vfoo = id > mapfoo (x : xs) = vfoo x : mapfoo xs > mapfoo [] = [] Because Haskell is a lazy language, this runs in O(n) time and constant stack. We now lift map to the type level, to operate over HLists. First, the basic HList types > infixr 3 :* > data x :* xs = x :* xs deriving Show > data HNil = HNil deriving Show Next, a large boring HList > -- Adds ten cells > addData x = i :* i :* d :* d :* s :* > i :* i :* d :* d :* s :* > x > where i = 1 :: Int > d = 1 :: Double > s = "" > > -- Has 70 cells. > sampleData = addData $ addData $ addData $ addData $ addData $ > addData $ addData $ > HNil Next, a simple polymorphic function to map > class Foo x y | x -> y > where foo :: x -> y > foo = undefined > instance Foo Int Double > instance Foo Double Int > instance Foo String String Now, our map > class HMapFoo1 as bs | as -> bs where > hMapFoo1 :: as -> bs > > instance (Foo a b, HMapFoo1 as bs) => HMapFoo1 (a :* as) (b :* bs) where > hMapFoo1 (x :* xs) = foo x :* hMapFoo1 xs > > instance HMapFoo1 HNil HNil where > hMapFoo1 _ = HNil If we enable the following line, compilation time is ~ 9 seconds. > testHMapFoo1 = hMapFoo1 sampleData Furthermore, playing with the size of sampleData, we see that the time spent in compilation is superlinear -- each additional cell costs a greater amount of time. This is because while Haskell is lazy at the value level, it is strict at the type level. Therefore, just as in a strict language, HMapFoo1's cost grows O(n^2) because even as we induct over the as, we build up a stack of bs. Just as in a strict language, the solution is to make hMapFoo tail recursive through introducing an accumulator. This also reverses the hlist, but never mind that. > class HMapFoo2 acc as bs | acc as -> bs where > hMapFoo2 :: acc -> as -> bs > > instance (Foo a b, HMapFoo2 (b :* bs) as res) => HMapFoo2 bs (a :* as) res where > hMapFoo2 acc (x :* xs) = hMapFoo2 (foo x :* acc) xs > > instance HMapFoo2 acc HNil acc where > hMapFoo2 acc _ = acc If we enable the following line, compilation time is a much more satisfying ~0.5s. > testHMapFoo2 = hMapFoo2 HNil sampleData But wait, there's trouble on the horizon! Consider the following version: > class HMapFoo3 acc as bs | acc as -> bs where > hMapFoo3 :: acc -> as -> bs > > instance (HMapFoo3 (b :* bs) as res, Foo a b) => HMapFoo3 bs (a :* as) res where > hMapFoo3 acc (x :* xs) = hMapFoo3 (foo x :* acc) xs > > instance HMapFoo3 acc HNil acc where > hMapFoo3 acc _ = acc The only difference between hMapFoo2 and hMapFoo3 is that the order of constraints on the inductive case has been reversed, with the recursive constraint first and the immediately checkable constraint second. Now, if we enable the following line, compilation time rockets to ~6s! > testHMapFoo3 = hMapFoo3 HNil sampleData Slowdowns such as those described here are not a purely hypothetical issue, but have caused real problems in production code. The example given above is fairly simple. The constraints used are minimal and easily checked. In real programs, the constraints are more difficult, increasing constant factors significantly. These constant factors, combined with unexpected algorithmic slowdowns due to the type inference engine, can lead (and have lead) to compilation times of HList-style code becoming deeply unwieldy-to-unusable, and can lead (and have lead) to this occuring only well after this code has been introduced and used on smaller cases without trouble. The constraint solver should certainly be smart enough to reduce the compile times of HMapFoo3 to those of HMapFoo2. In fact, with type families, the there is no difference (see below). Could the compiler be smart enough to do the same for HMapFoo1? I'm not sure. Certainly, it could at least knock down its own constant factors a bit, even if it can't improve the big-O performance there. ---- Appendix: Examples with Type Families As the below code demonstrates, the same issues demonstrated with Functional Dependencies also appear with Type Families, although less horribly, as their code-path seems more optimized in the current constraint solver: > class TFoo x where > type TFooFun x > tfoo :: x -> TFooFun x > tfoo = undefined > > instance TFoo Int where > type TFooFun Int = Double > instance TFoo Double where > type TFooFun Double = Int > instance TFoo String where > type TFooFun String = String > > class THMapFoo1 as where > type THMapFoo1Res as > thMapFoo1 :: as -> THMapFoo1Res as > > instance (TFoo a, THMapFoo1 as) => THMapFoo1 (a :* as) where > type THMapFoo1Res (a :* as) = TFooFun a :* THMapFoo1Res as > thMapFoo1 (x :* xs) = tfoo x :* thMapFoo1 xs > > instance THMapFoo1 HNil where > type THMapFoo1Res HNil = HNil > thMapFoo1 _ = HNil The following, when enabled, takes ~3.5s. This demonstrates that slowdown occurs with type families as well. > testTHMapFoo1 = thMapFoo1 sampleData > class THMapFoo2 acc as where > type THMapFoo2Res acc as > thMapFoo2 :: acc -> as -> THMapFoo2Res acc as > > instance (TFoo a, THMapFoo2 (TFooFun a :* acc) as) => THMapFoo2 acc (a :* as) where > type THMapFoo2Res acc (a :* as) = THMapFoo2Res (TFooFun a :* acc) as > thMapFoo2 acc (x :* xs) = thMapFoo2 (tfoo x :* acc) xs > > instance THMapFoo2 acc HNil where > type THMapFoo2Res acc HNil = acc > thMapFoo2 acc _ = acc The following, when enabled, takes ~0.6s. This demonstrates that the tail recursive transform fixes the slowdown with type families just as with fundeps. > testTHMapFoo2 = thMapFoo2 HNil sampleData > class THMapFoo3 acc as where > type THMapFoo3Res acc as > thMapFoo3 :: acc -> as -> THMapFoo3Res acc as > > instance (THMapFoo3 (TFooFun a :* acc) as, TFoo a) => THMapFoo3 acc (a :* as) where > type THMapFoo3Res acc (a :* as) = THMapFoo3Res (TFooFun a :* acc) as > thMapFoo3 acc (x :* xs) = thMapFoo3 (tfoo x :* acc) xs > > instance THMapFoo3 acc HNil where > type THMapFoo3Res acc HNil = acc > thMapFoo3 acc _ = acc The following, when enabled, also takes ~0.6s. This demonstrates that, unlike the fundep case, the order of type class constraints does not, in this instance, affect the performance of type families. > testTHMapFoo3 = thMapFoo3 HNil sampleData --Gershom

2 1

ANN: unification-fd: simple generic unification
by wren ng thornton 12 Jul '11

12 Jul '11

-------------------------------------------- -- unification-fd 0.5.0 -------------------------------------------- The unification-fd package offers generic functions for first-order structural unification (think Prolog programming or Hindley--Milner type inference). I've had this laying around for a few years, so I figured I might as well publish it. An effort has been made to try to make this package as portable as possible. However, because it uses the ST monad and the mtl-2 package it can't be H98 nor H2010. However, it only uses the following common extensions which should be well supported[1]: Rank2Types MultiParamTypeClasses FunctionalDependencies FlexibleContexts FlexibleInstances UndecidableInstances [1] With the exception of fundeps which are notoriously difficult to implement. However, they are supported by Hugs and GHC 6.6, so I don't feel bad about requiring it. Once the API stabilizes a bit more I plan to release a unification-tf package which uses type families instead, for those who feel type families are easier to implement or use. -------------------------------------------- -- Description -------------------------------------------- The unification API is generic in the type of the structures being unified and in the implementation of unification variables, following the two-level types pearl of Sheard (2001). This style mixes well with Swierstra (2008), though an implementation of the latter is not included in this package. That is, all you have to do is define the functor whose fixed-point is the recursive type you're interested in: -- The non-recursive structure of terms data S a = ... -- The recursive term type type PureTerm = Fix S And then provide an instance for Unifiable, where zipMatch performs one level of equality testing for terms and returns the one-level spine filled with pairs of subterms to be recursively checked (or Nothing if this level doesn't match). class (Traversable t) => Unifiable t where zipMatch :: t a -> t b -> Maybe (t (a,b)) The choice of which variable implementation to use is defined by similarly simple classes Variable and BindingMonad. We store the variable bindings in a monad, for obvious reasons. In case it's not obvious, see Dijkstra et al. (2008) for benchmarks demonstrating the cost of naively applying bindings eagerly. There are currently two implementations of variables provided: one based on STRefs, and another based on a state monad carrying an IntMap. The former has the benefit of O(1) access time, but the latter is plenty fast and has the benefit of supporting backtracking. Backtracking itself is provided by the logict package and is described in Kiselyov et al. (2005). In addition to this modularity, unification-fd implements a number of optimizations over the algorithm presented in Sheard (2001)--- which is also the algorithm presented in Cardelli (1987). * Their implementation uses path compression, which we retain. Though we modify the compression algorithm in order to make sharing observable. * In addition, we perform aggressive opportunistic observable sharing, a potentially novel method of introducing even more sharing than is provided by the monadic bindings. Basically, we make it so that we can use the observable sharing provided by the previous optimization as much as possible (without introducing any new variables). * And we remove the notoriously expensive occurs-check, replacing it with visited-sets (which detect cyclic terms more lazily and without the asymptotic overhead of the occurs-check). A variant of unification which retains the occurs-check is also provided, in case you really need to fail fast for some reason. * Finally, a highly experimental branch of the API performs *weighted* path compression, which is asymptotically optimal. Unfortunately, the current implementation is quite a bit uglier than the unweighted version, and I haven't had a chance to perform benchmarks to see how the constant factors compare. Hence moving it to an experimental branch. I haven't had a chance to fully debug these optimizations, though they pass some of the obvious tests. If you find any bugs, do be sure to let me know. Also, if you happen to have a test suite or benchmark suite for unification on hand, I'd love to get a copy. -------------------------------------------- -- References -------------------------------------------- Luca Cardelli (1987) /Basic polymorphic typechecking/. Science of Computer Programming, 8(2):147--172. Atze Dijkstra, Arie Middelkoop, S. Doaitse Swierstra (2008) /Efficient Functional Unification and Substitution/, Technical Report UU-CS-2008-027, Utrecht University. <http://www.cs.uu.nl/research/techreps/repo/CS-2008/2008-027.pdf> Oleg Kiselyov, Chung-chieh Shan, Daniel P. Friedman, and Amr Sabry (2005) /Backtracking, Interleaving, and/ /Terminating Monad Transformers/, ICFP. <http://www.cs.rutgers.edu/~ccshan/logicprog/LogicT-icfp2005.pdf> Tim Sheard (2001) /Generic Unification via Two-Level Types/ /and Paramterized Modules/, Functional Pearl, ICFP. <http://web.cecs.pdx.edu/~sheard/papers/generic.ps> Tim Sheard & Emir Pasalic (2004) /Two-Level Types and/ /Parameterized Modules/. JFP 14(5): 547--587. This is an expanded version of Sheard (2001) with new examples. <http://web.cecs.pdx.edu/~sheard/papers/JfpPearl.ps> Wouter Swierstra (2008) /Data types a la carte/, Functional Pearl. JFP 18: 423--436. <http://www.cs.ru.nl/~wouters/Publications/DataTypesALaCarte.pdf> -------------------------------------------- -- Links -------------------------------------------- Homepage: http://code.haskell.org/~wren/ Hackage: http://hackage.haskell.org/package/unification-fd Darcs: http://community.haskell.org/~wren/unification-fd Haddock (Darcs version): http://community.haskell.org/~wren/unification-fd/dist/doc/html/unification… -- Live well, ~wren

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Re: [Haskell-cafe] [Haskell] Call for GUI examples - Functional Reactive Programming
by Paul Liu 12 Jul '11

12 Jul '11

Hi James, I think you meant to reply to the list. For one, I wouldn't consider GUI programming based on HTML as declarative, since more often than not, the format says nothing about its behavior, and when it does, it's through javascript, which is hardly declarative. Regards, Paul Liu On Tue, Jul 12, 2011 at 5:17 AM, James Deng <cnjamesdeng(a)gmail.com> wrote: > These days, more and more GUI programming are done in declarative, such as > HTML(jQuery), Mozilla XUL, android UI XML. > > Haskell is an excellent declarative language, so should have the advantages > of constructing GUI in declarative, intuitive and powerful way. But so far, > there is not such a GUI library intuitive like Html, powerful like XUL. > > I expect something like html to layout GUI, and a haskell GUI engine would > render the user interfaces files. > Where is the gap lying? > > On 07/12/2011 05:51 AM, Paul Liu wrote: > > You guys might want to checkout the recent work on Euterpea at Yale > CS. In particular: > > 1. Paul Hudak is writing a new book. > http://plucky.cs.yale.edu/cs431/reading.htm > 2. It uses FRP and arrows for sound synthesis. > 3. It combines FRP signals with monadic (which recently gets > re-written in arrows) GUI composition. > 4. New novel techniques is being developed to handle I/O within arrows > framework. > > The code can be obtained through darcs, details at > http://plucky.cs.yale.edu/cs431/software_resources.htm > > Notably, the way it handles GUI is that the composition of widgets are > static, but the signals flowing between them are dynamic. This closely > follows Conal Elliott's Phooey approach, and greatly reduces the > complexity of GUI programming. > > Disclaimer: I was an ex-student who worked on this project. > > Regards, > Paul Liu > > On Thu, Jul 7, 2011 at 11:08 PM, Heinrich Apfelmus > <apfelmus(a)quantentunnel.de> wrote: > > Dear Haskellers, > > Can GUI programming be liberated from the IO monad? > > > Regards, > -- > James Deng > http://cnjdeng.appspot.com -- Regards, Paul Liu

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Inconsistent trailing comma in export list and record syntax
by L Corbijn 12 Jul '11

12 Jul '11

Hello, I'm wondering why the trailing comma is allowed in export syntax, but not in record syntax, here an example module Foo ( export1, -- is allowed ) where data Type = Type { record1 :: Foo, -- is not allowed } To me this seems quite inconsistent and sometimes quite frustrating, imagine the case that you want to temporarily remove the last record: data Type = Type { record1 :: Foo, -- record2 :: Bar } this would fail due to an extra comma that has to be commented out. You could of course say that I'm using a bad style, but it remains that it seems to be inconsistent to allow a trailing comma in one place and not in the other. So is there an reason for this? Lars Corbijn

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External system connections
by Richard Wallace 12 Jul '11

12 Jul '11

Hello all, I'm trying to understand how to properly structure a connection to an external system. I'm writing an application that processes requests (it's an IRC bot - ya, I've looked at lambabot but it's a bit beyond my current understanding and I'm really trying to learn this stuff and find the best way to do that is to write it myself) and sends requests to an external system, via SOAP. The SOAP requests should be sent on separate threads. The SOAP server requires an initial request for authentication be made to obtain a token used on subsequent requests. There are three ways I can structure this. One is to do the authentication with the SOAP server when the application starts up and just use that token for all subsequent requests. This will fall over if the token times out and I'd have to restart the application. Another way is to do authenticate with the server and get a new token for each request. This is obviously really inefficient. What I'd like to do is something a bit smarter. When a request to the SOAP server is to be made, if we have a token then we try and use it. If it fails, we reauthenticate and get a new token. When establishing a new token, other threads trying to send SOAP requests should block until a new token is available. There are other conditions that should be handled - such as an exponential back-off when the SOAP server can't be reached - but I feel like reestablishing the authentication token is really the key concern here. Unfortunately, I have no idea right now how to write this in Haskell. Any pointers would be awesome. Thanks! Rich

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Fwd: The Typeable class is changing
by Alberto G. Corona 11 Jul '11

11 Jul '11

What to do when the data has been defined in other package and provides no Typeable instance? 2011/7/11 Yitzchak Gale <gale(a)sefer.org> > Simon Marlow has announced[1] on the Haskell Libraries > list that the Typeable class is changing. > > The standard way to create a Typeable instance is > just to derive it. If you do that, you will not be affected > by this change. > > But it seems that many packages create Typeable > instances by explicitly using mkTyCon. If your package > does this, it will eventually break, after a deprecation > period. > > Please respond to this thread if you own a package > that will be affected by this change. > > Can someone who has quick access to the entire contents > of Hackage please do a grep and find out exactly which > packages on Hackage will be affected? Thanks. > > -Yitz > > [1] http://www.haskell.org/pipermail/libraries/2011-July/016546.html > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Re: [Haskell-cafe] Inconsistent trailing comma in export list and record syntax
by Johan Tibell 11 Jul '11

11 Jul '11

On Mon, Jul 11, 2011 at 3:54 PM, Henning Thielemann <thunderbird(a)henning-thielemann.de> wrote: > Johan Tibell wrote: >> I've found this quite annoying, especially when using CPP to >> conditionally include something in a list, as it might force you to >> reorder the list to make the commas appear correctly when the >> conditional section is enabled/disabled. > > In this case I use the colon like a terminator. > > http://www.haskell.org/haskellwiki/List_notation Sorry, I wasn't being clear. I meant for import/export lists. Johan

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Monad.Reader #19 call for copy -- special issue on parallelism and concurrency
by Brent Yorgey 11 Jul '11

11 Jul '11

Call for Copy: The Monad.Reader - Issue 19 SPECIAL ISSUE on parallelism and concurrency -------------------------------------------- Whether you're an established academic or have only just started learning Haskell, if you have something to say, please consider writing an article for The Monad.Reader! Issue 19 will be a special issue focusing on parallelism and concurrency, construed broadly. The submission deadline for Issue 19 will be: **Tuesday, August 16** The Monad.Reader ~~~~~~~~~~~~~~~~ The Monad.Reader is a electronic magazine about all things Haskell. It is less formal than journal, but somehow more enduring than a wiki- page. There have been a wide variety of articles: exciting code fragments, intriguing puzzles, book reviews, tutorials, and even half-baked research ideas. Submission Details ~~~~~~~~~~~~~~~~~~ Get in touch with me if you intend to submit something -- the sooner you let me know what you're up to, the better. Please submit articles for the next issue to me by e-mail (byorgey at cis.upenn.edu) Articles should be written according to the guidelines available from http://themonadreader.wordpress.com/contributing/ Please submit your article in PDF, together with any source files you used. The sources will be released together with the magazine under a BSD license. If you would like to submit an article, but have trouble with LaTeX please let me know and we'll work something out.

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Re: [Haskell-cafe] IterIO: How to write use my inumReverse
by Maciej Wos 11 Jul '11

11 Jul '11

Sorry, my previous message got truncated. I was trying to say that many iteratees like iterReverse can be defined nicely using combinators from Control.Applicative. You end up with much cleaner code. Also, iterLines doesn't work as the name would suggest. It only consumes one line from the input stream and returns it inside a singleton list. Instead, iterLines can be defined like this: iterLines :: (Monad m) => Iter L.ByteString m [L.ByteString] iterLines = do el <- tryBI lineI case el of -- no more "full" lines left, return the remaining data Left (e :: SomeException) -> (:[]) <$> dataI -- read one line; add it to the list and read more Right line -> (line:) <$> iterLines enumPure "line1\nline2\nline3" |$ iterLines [Chunk "line1" Empty,Chunk "line2" Empty,Chunk "line3" Empty] However, the above returns the resulting list only after consuming the whole stream, which is something to avoid. In case of iterReverse it is better to read and accumulate characters until \n is found and then return the reversed string. One way to do this is to read one character at a time: reverseLineSlow :: Iter L.ByteString IO L.ByteString reverseLineSlow = iter "" where iter acc = do c <- headI case c of 10 -> return $ L.reverse acc _ -> iter (acc `mappend` L.singleton c) But this will be really slow. Instead, the data should be read one chunk at a time: reverseLine :: Iter L.ByteString IO L.ByteString reverseLine = iter "" where iter acc = do -- read the data from the stream one chunk at a time Chunk c eof <- chunkI -- check if there is any \n, i.e. if we read a whole line let (a,b) = L.break (==10) c if b == "" -- haven't found any \n yet; append all data to the accumulator then iter (acc `mappend` a) -- have found \n else do -- put the data after \n back (while removing \n itself) ungetI $ L.tail b -- return reversed accumulator plus the data up to \n return $ L.reverse $ acc `mappend` a Hope this helps! -- Maciej On Tue, Jul 12, 2011 at 12:47 AM, Maciej Wos <maciej.wos(a)gmail.com> wrote: > Don't forget Applicative instance! > iterReverse = L.reverse <$> lineI > > On Monday, 4 July 2011 at 22:54, dm-list-haskell-cafe(a)scs.stanford.edu > wrote: > > At Mon, 4 Jul 2011 20:36:33 +1000, > John Ky wrote: > > Hi Haskell Cafe, > > enum |$ inumLines .| inumReverse .| inumUnlines .| iter > ... > > iterLines :: (Monad m) => Iter L.ByteString m [L.ByteString] > iterLines = do > line <- lineI > return [[line] > > iterUnlines :: (Monad m) => Iter [L.ByteString] m L.ByteString > iterUnlines = (L.concat . (++ [C.pack "\n"])) `liftM` dataI > > iterReverse :: (Monad m) => Iter [L.ByteString] m [L.ByteString] > iterReverse = do > lines <- dataI > return (map L.reverse lines) > > inumLines = mkInum iterLines > inumUnlines = mkInum iterUnlines > inumReverse = mkInum iterReverse > > It all works fine. > > My question is: Is it possible to rewrite inumReverse to be this: > > iterReverse :: (Monad m) => Iter L.ByteString m L.ByteString > iterReverse = do > line <- dataI > return (L.reverse line) > > inumReverse = mkInum iterReverse > > And still be able to use it in the line: > > enum |$ inumLines .| {-- inumReverse goes in here somehow --} .| > inumUnlines .| iter > > The reason I ask is that the Haskell function reverse has the type [a] -> > [a], > not [[a]] -> [[a]]. > > I thought perhaps the alternative inumReverse is cleaner than the original > as > it behaves more similarly to Haskell's own reverse function. > > I'm not sure what you are trying to achieve. If you want an iter that > works on L.ByteStrings, then you can say: > > iterReverse :: (Monad m) => Iter L.ByteString m L.ByteString > iterReverse = do > line <- lineI > return (L.reverse line) > > In that case you don't need inumLines and inumUnlines. If, however, > you want the type to be [L.ByteString], and you would rather do this > one line at a time, instead of calling map, then you could do > something like the following: > > iterReverse :: (Monad m) => Iter [L.ByteString] m [L.ByteString] > iterReverse = do > line <- headI > return [L.reverse line] > > But the code you have above should also work, so it all depends on > what you are trying to achieve. > > David > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > >

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Good XML-Enumerator Examples
by Michael Xavier 11 Jul '11

11 Jul '11

Hi Cafe. I'm writing a library which will be parsing XML data from a web API. I'm fairly well versed at this point in parsing JSON (aeson being my library of choice). For XML, I like the API of xml-enumerator over hxt because it appears to be nice and simple .The problem is that the example in the documentation isn't really sufficient for me to get a grasp of how to traverse XML and parse elements with multiple child nodes into a record. My first question: Am I barking up the wrong tree using xml-enumerator for anything beyond fairly flat, basic XML? If not, can anyone direct me to some good code samples using xml-enumerator to parse more complicated data? If I am indeed barking up the wrong tree, what would you all suggest? Thanks, -- Michael Xavier http://www.michaelxavier.net LinkedIn <http://www.linkedin.com/pub/michael-xavier/13/b02/a26>

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Re: [Haskell-cafe] External system connections
by Richard Wallace 11 Jul '11

11 Jul '11

On Sun, Jul 10, 2011 at 2:54 PM, Brandon Allbery <allbery.b(a)gmail.com> wrote: > On Sun, Jul 10, 2011 at 17:34, Richard Wallace > <rwallace(a)thewallacepack.net> wrote: >> Rather than a single separate thread that makes requests I was hoping to >> make several soap requests concurrently, rather than have them be made >> serially. I would only want to block other threads making soap request if >> one of them returns a response indicating the token they were using is no >> longer valid. > > Is there one shared token, or one per requesting thread, or some other > arrangement? There should be one shared token. > In either case, separation of concerns makes me think > the token handling belongs on the other end: a thread writes a > request down a Chan, a queue dispatcher thread reads it if there are > available workers and the token is available (with callers blocking > otherwise), dispatcher sends request to available worker. If the > worker finds the token is invalid, it calls back in to the dispatcher > with a token renewal request, which causes anything else that needs > that token to block as above, and itself blocks until a valid token is > returned. Ahhh... I see. That sounds like a much better plan. It also keeps the fact that the token is "mutable" confined to a single thread, the dispatcher thread. One thing I'd have to be careful to handle is iIf multiple worker threads find that the token is now invalid. In that case they would all send the token renewal request to the dispatcher and they would each try and do the re-auth. To avoid that, part of the token renewal request could be the used token. When the dispatcher does the token renewal, it checks if the used token is the same as the current token. If so, it does the token renewal. If not, it dispatches to the SOAP worker thread with the new token. > Another advantage of this is that adding more workers is > done in an obvious place (the queuer thread). > > That said, I still don't know enough details to say if that token > management implementation actually makes sense. I'd still go with the > worker/queuer thread setup, which is I think what you were pointed to > (the pool hackage). > Ok, I still don't see how the Pool package helps. I had assumed the pooled resource would be the token, so the size of the pool would be 1. Now that I rethink it, it seems like you are suggesting the resource is the worker threads. Is that right? When I read what you were talking about above with the dispatcher thread I had thought it would use forkIO to start the worker threads. The worker threads would read from a Chan that the dispatcher wrote to. In that way, the dispatcher doesn't have to worry about worker threads being available. Am I misunderstanding something? Thanks, Rich > -- > brandon s allbery allbery.b(a)gmail.com > wandering unix systems administrator (available) (412) 475-9364 vm/sms >

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NLP libraries and tools?
by Dmitri O.Kondratiev 11 Jul '11

11 Jul '11

Hi, Please advise on NLP libraries similar to Natural Language Toolkit ( www.nltk.org) First of all I need: - tools to construct 'bag of words' ( http://en.wikipedia.org/wiki/Bag_of_words_model) which is a list of words in the article. - tools to prune common words, such as prepositions and conjunctions, as well as extremely rare words, such as the ones with typos. - stemming tools - Naive Bayes classifier - SVM classifier - k-means clustering Thanks!

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unbuffered raw keyboard input on Windows
by José Romildo Malaquias 10 Jul '11

10 Jul '11

Hello. I want to write a Haskell console application (a game) in ghc where the user will interact using the keyboard. I need to read the keys as soon as they are typed and without echoing to the console. Although Haskell provides this capability in the standard libraries (using hSetEcho and hSetBuffering from System.IO), it does not work on Windows due to a bug (http://hackage.haskell.org/trac/ghc/ticket/2189) I have tried installing packages like vty, hscurses, and ncurses, but they did not install on Windows. Which workarounds are available for that? Romildo

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Re: [Haskell-cafe] error with nanocurses-1.5.2
by José Romildo Malaquias 10 Jul '11

10 Jul '11

On Sat, Jul 09, 2011 at 01:59:12PM -0300, José Romildo Malaquias wrote: > Hello. > > When trying to install nanocurses-1.5.2 on my Fedora Linux system, I am > getting the following error: > > $ cabal install nanocurses > Resolving dependencies... > Configuring nanocurses-1.5.2... > Preprocessing library nanocurses-1.5.2... > In file included from Curses.hsc:42:0: > cbits/utils.h:7:20: fatal error: config.h: No such file or directory > compilation terminated. > compiling dist/build/UI/Nanocurses/Curses_hsc_make.c failed (exit code 1) > command was: /usr/bin/gcc -c dist/build/UI/Nanocurses/Curses_hsc_make.c > -o dist/build/UI/Nanocurses/Curses_hsc_make.o -fno-stack-protector > -fno-stack-protector -D__GLASGOW_HASKELL__=700 -Dlinux_BUILD_OS > -Dlinux_HOST_OS -Dx86_64_BUILD_ARCH -Dx86_64_HOST_ARCH -Icbits > -I/usr/lib64/ghc-7.0.2/unix-2.4.2.0/include > -I/usr/lib64/ghc-7.0.2/bytestring-0.9.1.10/include > -I/usr/lib64/ghc-7.0.2/base-4.3.1.0/include > -I/usr/lib64/ghc-7.0.2/include -I/usr/lib64/ghc-7.0.2/include/ > cabal: Error: some packages failed to install: > nanocurses-1.5.2 failed during the building phase. The exception was: > ExitFailure 1 > > It would be nice to have this buf fixed. The attached patch fixes the compilation issues. Basically it changes the build type from simple to configure in the cabal file, and it renames the function getline in cbits/utils.{h,c} to mygetline. If appropriate, please release a new version with the fixes. Romildo

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JSON Querry examples
by Dmitri O.Kondratiev 10 Jul '11

10 Jul '11

Hi, I am trying to search known-in-advance fields in JSON object with unknown structure. It looks like Data.JSON2.Query is most appropriate for this task. Yet, I can not figure out how to use filters this package provides to filter JSON arrays and objects. Are there any end-to-end examples of making queries of unknown JSON objects? Thanks!

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class and instance
by Patrick Browne 10 Jul '11

10 Jul '11

Hi, I am trying to understand the following code. I have written my current (mis-)understanding and questions below. I do not wish to improve the code, it is from a research paper[1] that I am trying to understand. Pat [1] ftp://ftp.geoinfo.tuwien.ac.at/medak/phdmedak.pdf -- A specification. The class Points takes two type variables. -- The functions take variables of type p, a; return a value of type a. class Points p a where getX :: p a -> a getY :: p a -> a -- A parameterized representation -- One constructor takes two elements of type a and produces a Point. data Point a = Pt a a -- An implementation of the class Points on the data type (Point a) -- In Pt b c constructor the variables b and c areboth of type a. -- The type a is left undefined until evaluation instance Points Point a where getX (Pt b c) = b getY (Pt b c) = c -- This runs with any type e.g. Integers -- getX(Pt 1 2) -- :t getX(Pt 1 2) -- getX(Pt 1 2) :: forall t. (Num t) => t My main question is in understanding the relationship between the arguments of the functions getX and getY in the class and in the instance. It seems to me that the constructor Pt 1 2 produces one element of type Point which has two components. How does this square with the class definition of getX which has two arguments? Is there a difference between: getX :: p a -> a and getX :: p -> a -> a This message has been scanned for content and viruses by the DIT Information Services E-Mail Scanning Service, and is believed to be clean. http://www.dit.ie

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Haddock and the type inference
by Yves Parès 10 Jul '11

10 Jul '11

Hello Café, As far as I know, there is currently no way to tell haddock to run the type inferer to document functions without explicit type signature. Is that done on purpose (e.g. to force the developpers to write at least the type signatures of the public API, which is a fair reason) or is it a limitation? Is it planned to add that support to haddock?

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Re: [Haskell-cafe] NLP libraries and tools?
by wren ng thornton 10 Jul '11

10 Jul '11

(Psst, the nlp list is <nlp(a)projects.haskell.org> :) On 7/9/11 3:10 AM, Jack Henahan wrote: > On Jul 7, 2011, at 10:53 PM, wren ng thornton wrote: >> I can't help but be a (meta)theorist. But then, I'm of the firm opinion >> that theory must be grounded in actual practice, else it belongs more to >> the realm of theology than science. > > Oof, you're liable to wound my (pure) mathematician's pride with remarks like that, wren. :P How's that now? Pure mathematics is perfectly grounded in the practice of mathematics :) I've no qualms with pure maths. Afterall, mathematics isn't trying to model anything (except itself). The problems I have are when the theory branch of a field loses touch with what the field is trying to do in the first place, and consequently ends up arguing over details which can be neither proven nor disproven. It is this which makes them non-scientific and not particularly helpful for practicing scientists. Linguistics is one of the fields where this has happened, but it's by no means the only one (AI, declarative databases, postmodernism,...) There's nothing wrong with not being science. I'm a big fan of the humanities, mathematics, and philosophy. It's only a problem when non-science is pretending to be science: it derails the scientists and it does a disservice to the non-science itself. Non-science is fine; pseudo-science is the problem. For the same reason, I despise math envy and all the pseudo-math that gets bandied about in social sciences wishing they were economics (or economics wishing it were statistics, or statistics wishing it were mathematics). -- Live well, ~wren

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