January 2009 - Haskell-Cafe

Multiple State Monads
by Phil 17 Jan '09

17 Jan '09

Hi, I¹ve been reading the Monads aren¹t evil posts with interest I¹m a 2nd week Haskell newbie and I¹m doing my best to use them where (I hope) it is appropriate. Typically I¹m writing my code out without using Monads (normally using list recursion), and then when I get them working, I delve into the Monad world.... This has been going well so far with a bit of help from you guys, but I¹ve hit a snag. In the code below I¹m using a state Monad (getEvolution), but unlike simpler cases I¹m passing around two items of state, and one of these states is also ultimately a result although I don¹t care about the result until I reach an end state. My implementation is a bit ugly to say the least and clearly I¹m forcing round pegs into square holes here reading a bit online I get the impression that I can solve the two-state issue using Monad Transformers, by wrapping a StateT around a regular State object (or even two StateT Monads around an Identity Monad??). I think I understand the theory here, but any attempt to implement it leads to a horrible mess that typically doesn¹t compile. The other problem of having a state that is also a result, I¹m sure what to do about this. Was wondering if anyone could give me a push in the right direction how can I rework my state monad so that it looks less wildly. Many thanks, Phil. mcSimulate :: Double -> Double -> Word64 -> [Double] mcSimulate startStock endTime seedForSeed = expiryStock : mcSimulate startStock endTime newSeedForSeed where expiryStock = evalState ( do replicateM_ (truncate(endTime/timeStep)-1) getEvolution; getEvolution ) $ (startStock,ranq1Init seedForSeed) newSeedForSeed = seedForSeed + 246524 discount :: Double -> Double -> Double -> Double discount stock r t = stock * exp (-r)*t payOff :: Double -> Double -> Double payOff strike stock | (stock - strike) > 0 = stock - strike | otherwise = 0 -- Monad Implementation -- Yuk! evolveUnderlying :: (Double, Word64) -> ( Double, (Double, Word64) ) evolveUnderlying (stock, state) = ( newStock, ( newStock, newState ) ) where newState = ranq1Increment state newStock = stock * exp ( ( ir - (0.5*(vol*vol)) )*timeStep + ( vol*sqrt(timeStep)*normalFromRngState(state) ) ) getEvolution :: State (Double, Word64) Double getEvolution = State evolveUnderlying

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Re: Compiling regex-posix-0.93.2 on windows
by Chris Kuklewicz 17 Jan '09

17 Jan '09

I am not sure what is going wrong. I have not been using Haskell on windows. I am also copying this reply to haskell-cafe and libaries mailing lists. Does anyone know? Parnell Flynn wrote: > I am having a terrible time compiling the 0.93.2 version of the > regex-posix library on windows XP. > > I have Cygwin and MinGW installed and I have a version of the regex.h > header in C:/cygwin/usr/include what shared libraries do I need? > > Here is a copy of the error it finds the include but is unable to link. > > This links to the standard c library version of regular expressions. > > The corresponding c header file is regex.h and there is a chance you > > will need to edit the end of the regex-posix.cabal file to find the > > include directory and/or library. > > Preprocessing library regex-posix-0.93.2... > > dist/build/Text/Regex/Posix/Wrap_hsc_make.o(.text+0x85):Wrap_hsc_make.c: > undefined reference to `_impure_ptr' > > dist/build/Text/Regex/Posix/Wrap_hsc_make.o(.text+0xc9):Wrap_hsc_make.c: > undefined reference to `_impure_ptr' > > dist/build/Text/Regex/Posix/Wrap_hsc_make.o(.text+0xf1):Wrap_hsc_make.c: > undefined reference to `_impure_ptr' > > dist/build/Text/Regex/Posix/Wrap_hsc_make.o(.text+0x125):Wrap_hsc_make.c: > undefined reference to `_impure_ptr' > > dist/build/Text/Regex/Posix/Wrap_hsc_make.o(.text+0x159):Wrap_hsc_make.c: > undefined reference to `_impure_ptr' > > dist/build/Text/Regex/Posix/Wrap_hsc_make.o(.text+0x18d):Wrap_hsc_make.c: > more undefined references to `_impure_ptr' follow > > collect2: ld returned 1 exit status > > linking dist\build\Text\Regex\Posix\Wrap_hsc_make.o failed > > command was: y:\ghc\ghc-6.8.2\bin\ghc.exe -optl-LC:/cygwin/lib > dist\build\Text\Regex\Posix\Wrap_hsc_make.o -o > dist\build\Text\Regex\Posix\Wrap_hsc_make.exe > > bash-3.2$ > > Thanks, > > Parnell > > Parnell Flynn • 230 South Lasalle Street • Suite 400 • Chicago, IL 60604 > • 312.244.5317 (o) • parnell___.flynn(a)ronin-capital.com_ > <mailto:parnell.flynn@ronin-capital.com> >

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New blog on Haskell and Haskell in Visualization
by Jeff Heard 17 Jan '09

17 Jan '09

Just started up a blog on my own random lumberings through Haskell and the visualizations I've produced in Haskell. Have plenty of content on backlog, so I should be updating regularly. That's about all! http://vis.renci.org/jeff/ -- Jeff

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Network CGI : outputFPS ByteString problem
by Pieter Laeremans 17 Jan '09

17 Jan '09

Hello, When I try to render a byteString template using outputFPS like this : outputBsTemplate :: StringTemplate ByteString -> CGI CGIResult outputBsTemplate template = let bs = renderFPS template in outputFPS bs I get this error : Couldn't match expected type `Data.ByteString.Lazy.Internal.ByteString' against inferred type `ByteString' Is this a library problem? Is the CGI bound with a too specific ByteString ? Or am I doing something terrible wrong ? thanks in advance, Pieter -- Pieter Laeremans <pieter(a)laeremans.org> "The future is here. It's just not evenly distributed yet." W. Gibson

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Re: [Haskell-cafe] some ideas for Haskell', from Python
by Artyom Shalkhakov 17 Jan '09

17 Jan '09

Hello, 2009/1/16 Immanuel Litzroth <immanuel203(a)gmail.com>: > I don't understand your comment. > 1) If XMonad already uses it the problem is solved, without giving Haskell > import new semantics? Right, but there are some restrictions. > 2) These guys refer to a method to do plugin work in Haskell > http://www.cse.unsw.edu.au/~dons/hs-plugins/ > So the problem of dynamically loading plugins should already be solved? Well, mostly yes, but I think that it should be added to the language. Cheers, Artyom Shalkhakov.

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Simplification of this function?
by Andrew Wagner 17 Jan '09

17 Jan '09

I've been playing around with this, but haven't been able to come up with anything. myFunc f (a,b) (c,d) = (f a c, f b d) It feels as if there should be a nice simple version of this using some combination of {un,}curry, "on", &&&, ***, or something else. Any thoughts?

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Mathematics for Uninterested
by Andrzej Jaworski 16 Jan '09

16 Jan '09

Hi, After recent rant "to Hell with Monoids and Mathematics" I felt like Galileo awaiting execution. I splashed toilet several times but the rant was still there, so I decided to find a scapegoat for this malfunctioning. Of cours your mathematic teachers are to blame. They are the product of negative selection, guys useless for research, mathematics was hard for them so like ancient Egyptian priests used it as an edge over unenlightened crowd, right? I leave this to your conscience;-) Whatever the reason is, they have obviously failed to convince you that MATHEMATICS_IS_ABOUT_SIMPLIFYING_THINGS. MATHEMATICS = MATHEMATICAL_KNOWLEDGE + FORMAL_METHODS_of_THINKING You need only the second part for Haskell. - Then why Monoids and the rest? Isn't that knowledge? - It is knowledge without flesh as it is not tighten to any specific domain. Hence its simplicity and universality. Category Theory generalizes our mathematical experience along the line that we know more that we can prove. - Why generalizations derived along mathematical experience are superior to yours the hacker? - Because the time span between your one mistake or success and another mistake or success is too long for your brain to infer any structural relationship. Mathematicians using general concepts and powerful notation go way beyond human experience. - Why stick with mathematical names? - To have ample reference on them and to make people in different domains speak the same language. Still not convinced that abstraction is your friend? Let me give you a hint, the only prerequisite is to have 10 fingers:-) (0) Try to do some arithmetic without the concept of infinite set. Don't tell me that you can program calculator to do this. You would need axioms of arithmetic which you would never discover without your brain imagining infinite concatenation. (1) The same way you need Cartesian product for tuples as their ultimate holder. (2) The same way you need relational algebra to collect all manipulations of tables (and much more to map them in XML). (3) The same way you need Monoid to contain all Appendable and more. One of you asked if mathematical precision is good enough for programming. My friend, you used words that outside your body experience mean nothing. Take Topology which cannot tell the difference between the fork in your beefburger from that in your compost. You would not use it to calculate N-body problem, would you? But exactly such imprecise (qualitative) methods underpin recent dramatic improvements of precision in Mechanics. Totally imprecise improves precision! Without the high of mathematics our cognition is very misleading. Another of you shrugged over unimportant for him book on abstract algebra. Fine, but the book contains piece of flesh that brought to light methods for your Haskell. If you enjoy Haskell and want stronger feel of the methods this flesh will be your Holy Sacrament. Amen. Students at engineering have functional analysis in their curriculum which takes over 10 times longer to learn than your prerequisites for Haskell. But one of you made an excuse that engineers actually don't know this stuff. If that is so I will leave you with this thought experiment: suppose then that Dr.Wolfram goes crazy (e.g. falling sales) and decides to terminate this civilization by introducing a virus to his Mathematica upgrade... Be happy!

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ANN: dataenc 0.12
by Magnus Therning 16 Jan '09

16 Jan '09

I've just uploaded version 0.12 of dataenc to HackageDB[1]: Data encoding library currently providing Uuencode, Base64, Base64Url, Base32, Base32Hex, Base16, Base85, and yEncoding. Versions 0.9, 0.10.1, 0.10.2, 0.11, 0.11.1, 0.12 Dependencies base License BSD3 Copyright Magnus Therning, 2007 Author Magnus Therning Maintainer magnus(a)therning.org Category Codec Home page http://www.haskell.org/haskellwiki/Library/Data_encoding Upload date Fri Jan 16 20:40:16 UTC 2009 Uploaded by MagnusTherning The changes since 0.11.1 are: * Re-license under BSD3. * Implementation of yEncoding. Enjoy! /M [1]: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dataenc -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus Haskell is an even 'redder' pill than Lisp or Scheme. -- PaulPotts

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Re: Comments from OCaml Hacker Brian Hurt
by John Lato 16 Jan '09

16 Jan '09

> On Fri, 16 Jan 2009, Duncan Coutts wrote: > >> If you or anyone else has further concrete suggestions / improvements >> then post them here now! :-) Show various examples of how monoids apply to programming concepts/problems, e.g. monoids to combine configuration parameters/flags, monoids in writers, etc. Show how using the abstract monoid class is an advantage over a less abstract class/type with concrete examples. > Jonathan Cast wrote: > > > To accumulate a running count, maybe? A fairly common pattern for counting in imperative languages is > int i = 0; > while (<get a value>) i+= <count of something in value> > Using the writer monad, this turns into > > execWriter $ mapM_ (write . countFunction) $ getValues > This should be in the documentation for Writer. It shows how a writer can usefully and simply perform a specific function, and that function is different from what most users would consider using a Writer for. Hence, it's a specific example of both the advantages that generality provides* and a programming use for a monoid. John Lato *) just being more general isn't always an advantage. As abstractions progress, iIt becomes increasingly difficult to apply abstract concepts to the task at hand. This essay, http://discuss.joelonsoftware.com/default.asp?joel.3.219431.12, has a pretty good description of taking this too far.

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Re: [Haskell-cafe] Haskell not ready for Foo [was: Re: Hypothetical Haskell job in New York]
by John A. De Goes 16 Jan '09

16 Jan '09

My statements refer not to the FFI, but as I said, to "FFI code". FFI- based libraries seldom compile without excessive amounts of work, they're often poorly documented, and in general they seem to be maintained much less than pure Haskell libraries. The FFI is necessary, of course, but in general I view it as a bootstrapping process leading to pure Haskell libraries -- a crutch you have to live with until you can afford to pay the price of walking. Regards, John On Jan 8, 2009, at 3:15 PM, John Goerzen wrote: > On Thu, Jan 08, 2009 at 11:14:18AM -0700, John A. De Goes wrote: >> But really, what's the point? FFI code is fragile, often uncompilable >> and unsupported, and doesn't observe the idioms of Haskell nor take >> advantage of its powerful language features. Rather than coding >> through > > That is an extraordinarily cruel, and inaccurate, sweep of FFI. > > I've worked with C bindings to several high-level languages, and I > must say that I like FFI the best of any I've used. It's easy to use > correctly, stable, and solid. If anything, it suffers from > under-documentation. > > The whole point of FFI is to bring other languages into the Haskell > fold. So you can, say, talk to a database using its C library and > wind up putting the strongly-typed HaskellDB atop it. Or you can > write an MD5 algorithm in C and make it look like a regular Haskell > function. > >> You can indeed fit a square peg in a round hole, if you pound hard >> enough. That doesn't mean it's a good thing to do. > > And with that, I fully agree. > > -- Joh

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