August 2008 - Haskell-Cafe

Unboxing VT_VARIANT in hscom
by Praki Prakash 19 Aug '08

19 Aug '08

I am a Haskell newbie trying to do COM automation using Haskell. I am using hscom (Krasimir's implementation of COM automation). I have run into a problem and need some help. I have a Variant returned from a COM method invocation. When I print it, it shows up as below. Variant VT_DISPATCH <interface 0x00039f00> I need to invoke methods on the wrapped interface. My attempt to unbox it as below runs into 'rigid type' error. someFunc (Variant VT_DISPATCH val) query = do dispId <- getMethodID "MethodName" val The code above generates this error. Couldn't match expected type `IDispatch a' against inferred type `a1' `a1' is a rigid type variable bound by... I am probably missing something pretty basic. Any help on this is greatly appreciated! Thanks

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Reader monad, implicit parameters, or something else altogether?
by Bjorn Buckwalter 19 Aug '08

19 Aug '08

All, I have a growing amount of astrodynamics code that relies on various physical "constants". The problem with these so called "constants" are that they either aren't really constants or aren't well known. An example is the leap second table (see Data.Time.Clock.TAI). I'd like to be able to fetch current values of these constants at runtime and make them accessible to my astrodynamics functions by some means. To clarify, once initialized the constants will be considered constant for the remainder of the program. I'd store the constants in a data structure along the lines of: > data AstroData a = AstroData > { mu_Earth :: GravitationalParameter a > , leapSeconds :: LeapSecondTable > } I would like to know if there is any consensus on what is the best way to make such a data structure accessible in pure functions. Passing it explicitly would be a mess. It seems that two options are to use either a Reader monad or implicit parameters. Using a Reader monad is straight forward enough though it requires writing/converting code in/to monadic style and adds some clutter to the formulae. It seems implicit parameters could be cleaner but I've seen them referred to as everything from evil to "just what you need" and rendering the Reader monad obsolete... What do you people recommend? Thanks, Bjorn Buckwalter

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ANN: posix-realtime 0.0.1 (system category)
by Galchin, Vasili 19 Aug '08

19 Aug '08

Hello, 1) I have added more to "the posix realtime" support. - I put in parens above because I have currently uploaded outside the Haskell Unix library .. i.e. I am still open to submitting to the Unix library. - I have also put in parens because I have not personally implemented all of the posix realtime support .... others have also implemented Posix realtime support. 2) I am still trying to learn the Haskell FFI .. my bad .. thank you Brandon, Derek, Don(2?), Jason, Bulat, Duncan, Felipe, Bryan, Ketli, Luke, Jonthan, Jeremy, Jules. . .. Sorry for list .. alittle corney .. but nonetheless very grateful! Who did I forget who answered my questions? Is this a pre-order? ;^) 3) There are still issues with RTDataTypes.hsc that I am working with .. in particular Sigevent. 4) This has been a learning exercise for me as I said in terms of the Haskell API and Haskell monad class. Someone one in the 2) list suggested aHaskell Posix AIO (async IO) API implemented in terms of the State monad or Continuation monad ... I didn't ignore this suggestion ... I am still thinking about .. i.e. the AIO "API" is a somewhat moving target. Kind regards, Vasili

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(fwd) OT: Cartoon Guide to Lob's Theorem
by Benjamin L.Russell 19 Aug '08

19 Aug '08

While this is somewhat off-topic, I found this interesting reference to a cartoon guide to Lob's Theorem, so I am forwarding it to this mailing list for some enjoyment in mathematical logic: On Mon, 18 Aug 2008 12:31:11 -0400, in gmane.lisp.scheme.plt Psy-Kosh <psykosh(a)earthlink.net> wrote: >This is a bit off topic, but I suspect some here would appreciate this: >http://www.overcomingbias.com/2008/08/lobs-theorem.html > >Remenicent of the graphical lambda game thing. > >Psy-Kosh >_________________________________________________ > For list-related administrative tasks: > http://list.cs.brown.edu/mailman/listinfo/plt-scheme -- Benjamin L. Russell

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how can I receive the mail?
by shoulders 18 Aug '08

18 Aug '08

-----邮件原件----- 发件人: haskell-cafe-bounces(a)haskell.org [mailto:haskell-cafe-bounces@haskell.org] 代表 haskell-cafe-request@haskell. org 发送时间: 2008年8月19日 4:23 收件人: haskell-cafe(a)haskell.org 主题: Haskell-Cafe Digest, Vol 60, Issue 20 Send Haskell-Cafe mailing list submissions to haskell-cafe(a)haskell.org To subscribe or unsubscribe via the World Wide Web, visit http://www.haskell.org/mailman/listinfo/haskell-cafe or, via email, send a message with subject or body 'help' to haskell-cafe-request(a)haskell.org You can reach the person managing the list at haskell-cafe-owner(a)haskell.org When replying, please edit your Subject line so it is more specific than "Re: Contents of Haskell-Cafe digest..." Today's Topics: 1. Re: permutations and performance (John D. Ramsdell) 2. Houston area FPers.... (Galchin, Vasili) 3. Re: Fwd: Haskell job opportunity: Platform Architect at Peerium, Inc. (Benjamin L.Russell) 4. the cabal category field and "uploads" (Galchin, Vasili) 5. Re: Houston area FPers.... (Galchin, Vasili) 6. Re: the cabal category field and "uploads" (Don Stewart) 7. Re: the cabal category field and "uploads" (Galchin, Vasili) 8. Re: Re: Phantoms (Jason Dagit) 9. "build depends:" in a .cabal file (Galchin, Vasili) 10. Re: "build depends:" in a .cabal file (Galchin, Vasili) 11. Serial COM port access on Windows ( Kol?? Du?an ) 12. Re: FRP question (Ryan Ingram) 13. Re: "build depends:" in a .cabal file (Henning Thielemann) 14. missingH 0.18.6 - problems with cabal configure (Andrew U. Frank) 15. Re: "build depends:" in a .cabal file (Duncan Coutts) 16. Reader monad, implicit parameters, or something else altogether? (Bjorn Buckwalter) 17. Reader monad, implicit parameters, or something else altogether? (Bjorn Buckwalter) 18. Re: Reader monad, implicit parameters, or something else altogether? (Henning Thielemann) 19. Re: Reader monad, implicit parameters, or something else altogether? (Bjorn Buckwalter) 20. Re: Reader monad, implicit parameters, or something else altogether? (Brandon S. Allbery KF8NH) 21. Re: Reader monad, implicit parameters, or something else altogether? (Brandon S. Allbery KF8NH) 22. Re: Reader monad, implicit parameters, or something else altogether? (Henning Thielemann) 23. Re: Reader monad, implicit parameters, or something else altogether? (Henning Thielemann) 24. Fwd: [Haskell-cafe] Reader monad, implicit parameters, or something else altogether? (Bjorn Buckwalter) 25. Re: Fwd: [Haskell-cafe] Reader monad, implicit parameters, or something else altogether? (Henning Thielemann) 26. Re: X Haskell Bindings (David Roundy) 27. Re: Fwd: [Haskell-cafe] Reader monad, implicit parameters, or something else altogether? (Evan Laforge) 28. Re: Reader monad, implicit parameters, or something else altogether? (Brandon S. Allbery KF8NH) ---------------------------------------------------------------------- Message: 1 Date: Sun, 17 Aug 2008 22:40:30 -0400 From: "John D. Ramsdell" <ramsdell0(a)gmail.com> Subject: Re: [Haskell-cafe] permutations and performance To: "Henning Thielemann" <schlepptop(a)henning-thielemann.de> Cc: Haskell Cafe <haskell-cafe(a)haskell.org> Message-ID: <7687290b0808171940y49c67e93n4866a276a5b8a50(a)mail.gmail.com> Content-Type: text/plain; charset=UTF-8 On Sat, Aug 16, 2008 at 5:28 PM, Henning Thielemann <schlepptop(a)henning-thielemann.de> wrote: > There is _one_ permutation with no elements, namely [], thus it must be > | n == 0 = [[]] I see. John ------------------------------ Message: 2 Date: Sun, 17 Aug 2008 21:47:33 -0500 From: "Galchin, Vasili" <vigalchin(a)gmail.com> Subject: [Haskell-cafe] Houston area FPers.... To: mad.one(a)gmail.com Cc: Galchin Vasili <vigalchin(a)gmail.com>, "haskell-cafe(a)haskell.org" <haskell-cafe(a)haskell.org> Message-ID: <5ae4f2ba0808171947v52dae7a2gf9582c1b31f50d20(a)mail.gmail.com> Content-Type: text/plain; charset="iso-8859-1" Hi Austin, I live next to the NASA Johnson Space Center. JSC is minutes away from Uof H Clear Lake. Which campus are you going to? (main campus?). Kind regards, Vasili

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Fwd: [Haskell-cafe] Reader monad, implicit parameters, or something else altogether?
by Bjorn Buckwalter 18 Aug '08

18 Aug '08

On Mon, Aug 18, 2008 at 2:02 PM, Henning Thielemann <lemming(a)henning-thielemann.de> wrote: > > On Mon, 18 Aug 2008, Bjorn Buckwalter wrote: > >> On Mon, Aug 18, 2008 at 11:16 AM, Henning Thielemann >> <lemming(a)henning-thielemann.de> wrote: >>> >>> On Mon, 18 Aug 2008, Bjorn Buckwalter wrote: >>> >>>> I would like to know if there is any consensus on what is the best way >>>> to make such a data structure accessible in pure functions. Passing it >>>> explicitly would be a mess. It seems that two options are to use >>>> either a Reader monad or implicit parameters. Using a Reader monad is >>>> straight forward enough though it requires writing/converting code >>>> in/to monadic style and adds some clutter to the formulae. It seems >>>> implicit parameters could be cleaner but I've seen them referred to as >>>> everything from evil to "just what you need" and rendering the Reader >>>> monad obsolete... >>> >>> I expect that you will get the same range of opinions as you got from >>> your >>> search. As far as I know implicit parameters break referential >>> transparency. >>> >>> http://www.haskell.org/haskellwiki/The_Monad.Reader/Issue2/FunWithLinearImp… >>> So I prefer Reader monad. The burden of converting to monadic style pays >>> off when you need to use the same code with different values for the >>> "constants". (E.g. find out for which value of the Planck constant the >>> universe collapses and for which it oscillates etc. :-) >> >> Love the example but could you elaborate a little on how monadic style >> helps with this? (This is probably a matter of it not being obvious to >> me what approach you would take to solving the problem.) > > Instead of > muEarth :: GravitationalParameter a > muEarth = ??? > > escapeVelocity :: a > escapeVelocity = ... muEarth ... > > you would write > > data AstroData a = AstroData > { muEarth :: GravitationalParameter a > , leapSeconds :: LeapSecondTable > } > > escapeVelocity :: Reader (AstroData a) a > escapeVelocity = > do mu <- asks muEarth > return (... mu ...) > > Even better you would introduce a newtype for Reader (AstroData a). This way > you can add any monadic functionality later (Writer et.al.). Right, and I'd evaluate it using e.g.: runReader escapeVelocity myAstroData But with implicit params I suppose I'd define (untested) e.g.: escapeVelocity :: (?astro :: AstroData a) => a escapeVelocity = ... mu ... where mu = muEarth ?astro To evaluate this I'd use: let ?astro = myAstroData in escapeVelocity Which is comparable to the Reader version (with the advantage/disadvantage of the body of 'escapeVelocity' not being monadic). In retrospect I think I misunderstood what you were saying in you first email. I thought you were arguing that the monadic style would have an advantage over implicit params in the Planck problem. But you probably only meant to reemphasize the advantage (of either solution) over hard-coding constants... Thanks again, your Reader example is virtually identical to what I started off with so at least I know I'm not completely off target for a monadic implementation. (Sorry about the reposts Henning, I keep forgetting to cc the café!)

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Kieran Beltran/Watson/IBM is out of the office.
by Kieran Beltran 18 Aug '08

18 Aug '08

I will be out of the office starting 08/18/2008 and will not return until 08/25/2008. I will be on vacation for a few days. If this is urgent please reach my Admin Kristine Smester ((917) 472-3387), otherwise I will respond to your message when I return.

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X Haskell Bindings
by Antoine Latter 18 Aug '08

18 Aug '08

Haskellers, I'm slowly porting XCB (the X C Bindings) to Haskell, and I would like input from any interested parties. The following is a summary of my plan so far. I'm interested in hearing any suggestions or concerns about what a Haskell library for writing X clients should look like. This is not a release announcement, and I can't make any promises about when this will be delivered. Code is available in darcs: -- darcs get http://community.haskell.org/~aslatter/code/xhb Some of the advantages XCB claims over xlib are: + smaller API + latency hiding when communicating with the X server + direct access to the X protocol + improved threading support + easier to extend What I plan for the X Haskell Bindings (XHB) are as follows: + All requests to the server are non-blocking (under most circumstances) + Requests which produce a reply from the server return a "token" or "receipt" + The caller may then, at a time of their choosing, query the receipt for the response (or error) from the server. This query is blocking. The API will look something like: > -- | Create a window as specified > createWindow :: Connection -> CreateWindow -> IO () > -- | Instruct the server that it should begin displaying the named window > mapWindow :: Connection -> WINDOW -> IO () > -- | List all of the extensions supported by the server > listExtensions :: Connection -> IO (Receipt (ListExtensionsReply)) > -- | Query a receipt for a response > getReply :: Receipt a -> IO (Either XError a) Note that the first two requests do not have replies, whereas the third request expects a reply from the server. Since the request to create a window has so many parameters, these parameters are all wrapped up into a "CreateWindow" data type, which is only ever used by the "createWindow" function. The "mapWindow" request only has one parameter, so it does not need it's own "MapWindow" data type. What I don't have planned out is what to do with the stream of events and errors that come back from the server. If an error is related to an outstanding receipt, it gets dumped there for the caller to examine directly. Other errors go into an error queue. Events go into a similar event queue. How should this queue be exposed in the API? Should the user of the library register an error/event callback? > registerErrorCallback :: Connection -> (XError -> IO ()) -> IO () Or is something like this enough: > pollForError :: Connection -> IO (Maybe (XError)) > waitForError :: Connection -> IO XError Each X extension defines its own set of errors and events (potentially). Should all of these be lumped together into one giant sum-type for errors and one for events? Take care, Antoine

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Serial COM port access on Windows
by Kolář Dušan 18 Aug '08

18 Aug '08

Dear all, Is there any more or less specific library for access to serial COM port from Haskell on Windows? Or am I expected to use a file mapping? Could you point me to some texts/examples? I tried some google (GHC specific) with no expected result. The issue is the following - I have a device communicating via Bluetooth with my PC, PC can create a serial COM port over the Bluetooth connection... I used C and a specific library for C so far, but I'd rather not encapsulate it by some Foreign calls in Haskell. Thanks for hints, Dusan

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Phantoms
by Andrew Coppin 18 Aug '08

18 Aug '08

I just (re)discovered that I can do things like data Foo x = Foo Int Int Now "Foo Int" and "Foo Double" are, as far as the type checker cares, two completely different types, even though in fact they are the same. This is actually Quite Useful, in the particular case I'm working on. Here's the thing though... How do I get it so that "Foo Int" and "Foo Double" produce slightly different strings when printed?

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