January 2009 - Haskell-Cafe

Re: [Haskell-cafe] How to define an operation in terms of itself (but of different type)?
by Olex P 24 Jan '09

24 Jan '09

But you know it doesn't make too much sense because I also have to define addition Scalar + Vector (that means construct vector from scalar and add a vector), Vector + Scalar and so on. And as we are not able to overload operations in C++ like way we have to create several different operations even if their meaning is pretty close. Probably it's possible with ad hoc overloading but I don't know is it good idea. On Sat, Jan 24, 2009 at 1:56 PM, Alberto G. Corona <agocorona(a)gmail.com>wrote: > What about making your Vector a instance of Num? you have + and * well > defined , although * is not commutative in this case > > 2009/1/24 Olex P <hoknamahn(a)gmail.com> > >> Yeah guys. I confused myself. I forgot why I had to implement several "+" >> operators (^+^, ^+, ^+. etc.) for Vector class. Now I've got an idea again. >> Different names make a perfect sense. >> >> Thanks a lot. >> >> >> On Sat, Jan 24, 2009 at 6:34 AM, Luke Palmer <lrpalmer(a)gmail.com> wrote: >> >>> 2009/1/23 Brandon S. Allbery KF8NH <allbery(a)ece.cmu.edu> >>> >>>> On 2009 Jan 23, at 17:58, Olex P wrote: >>>> >>>> class Vector v where >>>> (^+^) :: v -> v -> v >>>> >>>> class Matrix m where >>>> (^+^) :: m -> m -> m >>>> >>>> >>>> You can't reuse the same operator in different classes. Vector "owns" >>>> (^+^), so Matrix can't use it itself. You could say >>>> >>>> > instance Matrix m => Vector m where >>>> > (^+^) = ... >>>> >>> >>> No you can't! Stop thinking you can do that! >>> >>> It would be sane to do: >>> >>> class Vector m => Matrix m where >>> -- matrix ops that don't make sense on vector >>> >>> Thus anything that implements Matrix must first implement Vector. Which >>> is sane because matrices are square vectors with some additional structure, >>> in some sense. >>> >>> Luke >>> >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >

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unboxed, functional ATS language beats C++
by Alberto G. Corona 24 Jan '09

24 Jan '09

Language Shootout: ATS is the new top gunslinger. Beats C++<http://shootout.alioth.debian.org/u64q/benchmark.php?test=all&lang=all> http://www.reddit.com/r/programming/comments/72hmw/language_shootout_ats_is… ...Many people somehow think that ATS is fast because of its support for advanced types such as dependent types and linear types. Actually, types are primarily for enhancing safety rather than speed. The efficiency of ATS is largely rooted in its data representation (flat instead of boxed) and its support for tail-call optimization, which is vital for a functional language. What is interesting abut this ML derived language is the speed. The "theoriem proving" facilities seems like a language extensions for property checkings mainly for his non pure features. The type system seems to aid on the safety of pointer arithmetics and other dangerous operations.

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Installing GHC with libedit in nonstandard location
by Grzegorz Chrupala 24 Jan '09

24 Jan '09

Hi all, I'm trying to install the GHC binary for Linux. The problem is my libedit is in $HOME/lib instead of /usr/lib So GHC installs but when I try to run it is fails with: error while loading shared libraries: libedit.so.0: cannot open shared object file: No such file ordirectory I tried setting LIBRARY_PATH and LD_RUN_PATH to $HOME/lib but it doesn't help Is there some way I can run GHC with libedit in a nonstandard location? Thanks -- Grzegorz -- View this message in context: http://www.nabble.com/Installing-GHC-with-libedit-in-nonstandard-location-t… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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"Enumerating" haskell types
by Maurício 24 Jan '09

24 Jan '09

Hi, I'm reading a book where the guy (Abraham Robinson, "Non-Standard Analysis") write a type system like following for his needs. First, he gets an initial type: 0 This is going to be the type of all individuals. Then, types of relations (sets, functions etc.) are built like this: given existing types, a n-uple of them is a new type. So, these are possible types: (0) ((0),0,0) (0,0,0) ( ((0),0,0) , 0 , 0 , (0,0,0) ) I think it's easy, and similar to above, to do that for Haskell kinds. Would it be possible to do something similar for haskell types? If possible, I think that it could be a great help in understanding Haskell type system, specially new things like GADTs and RankN types. Thanks, Maurício

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re: Why monoids will abide...
by David F. Place 24 Jan '09

24 Jan '09

On Jan 20, 7:27 pm, Don Stewart <d...(a)galois.com> wrote: > http://apfelmus.nfshost.com/monoid-fingertree.html > > Thanks Apfelmus for this inspiring contribution! For additional reading material on a use of Monoids may I suggest my article in the Monad.Reader issue 11 "How to Refold a Map." http://haskell.org/sitewiki/images/6/6a/TMR-Issue11.pdf I also sketched an implementation of my incremental map data structure using Finger Trees. http://haskell.org/haskellwiki/The_Monad.Reader/Discuss_Issue11

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Haskell Weekly News: Issue 102 - January 24, 2009
by Brent Yorgey 24 Jan '09

24 Jan '09

--------------------------------------------------------------------------- Haskell Weekly News http://sequence.complete.org/hwn/20090124 Issue 102 - January 24, 2009 --------------------------------------------------------------------------- Welcome to issue 102 of HWN, a newsletter covering developments in the [1]Haskell community. Announcements Monoids and fingertrees. Heinrich Apfelmus posted a [2]nice tutorial explaining the monoid magic behind 2-3 fingertrees. STM-IO-Hooks-0.0.1. Peter Robinson [3]announced the [4]stm-io-hooks library, which provides an STM monad with commit and retry IO hooks. A retry-action is run (once) in a separate thread if the transaction retries, while commit-actions are executed iff the transaction commits. The code is based on the [5]AdvSTM monad by Chris Kuklewicz, but in addition also ensures some atomicity guarantees for commit-actions. 1000 libraries. Don Stewart [6]announced that Hackage has now [7]reached 1000 packages! HTTP-4000.0.4 released. Sigbjorn Finne [8]announced that a new release of [9]HTTP, version 4000.0.4, is now available. The main change is the addition of registering a Browser event handler for capturing state changes to the request-response processing pipeline. Blog noise [10]Haskell news from the [11]blogosphere. * Luke Palmer: [12]Existential Memoization. * Luke Palmer: [13]The Third Virtue. Luke has a Grand Vision (TM)! * Darcs: [14]darcs weekly news #14. * Magnus Therning: [15]More sensible comments on cabal-debian. * GHC / OpenSPARC Project: [16]Wait and perform. * LHC Team: [17]Typeclasses are working, now we're missing a bunch of instances.... * Andy Gill: [18]Memoization in GHC. A cool tutorial on how to memoize a pure function with GHC, without changing the function's definition. * Magnus Therning: [19]Experience with cabal-debian. * Joachim Breitner: [20]darcswatch uploaded to hackage. * GHC / OpenSPARC Project: [21]The Strap. * Real-World Haskell: [22]Bryan O'Sullivan on the Power of Haskell. * Jeff Heard: [23]Visualizing groundwater nitrate concentration. * Don Stewart (dons): [24]Open source releases and growth. * Lennart Augustsson: [25]Performance update on LLVM. * Arch Haskell News: [26]Piet implementation for Arch. * Mark Jason Dominus: [27]Triples and Closure. Did you know that monads are like closure operators? If not, read on. * GHC / OpenSPARC Project: [28]The Grind. * LHC Team: [29]Functions in Haskell.. * Magnus Therning: [30]Building Debian packages of (cabalised) Haskell packages. * Xmonad: [31]xmonad cheatsheet. * Xmonad: [32]xmonad on twitter. * Jeff Heard: [33]A random note on programming with Gtk2Hs.. * Xmonad: [34]xmonad 0.8.1 : GHC 6.10 maintenance release. * Jonathan Tang: [35]Some observations on Kinds. * Martijn van Steenbergen: [36]Type Synonym Families. Quotes of the Week * Jonathan Cast: [on advertising for Haskell programmers and informing them that the codebase is actually in Perl at the interview] <Andrew Wagner> That's......evil. <Jonathan> I know. I'm evil too, though. So it's cool. * rwbarton: A type class is not a type just like a dog house is not a dog. * SPJ: [This is] clearly not a bug in GHC; but it would be more felicitous if it gave you a warning... * Gracenotes: > let o_o = 0.0 ;o' =(, ); ;o (*)=(*) ;( lol, xD :p )= o' o' $o.o$ (:[]) $o.o$ (:[]) o_o in (:[]) o_o :p * Cale: I plan on forming a symbiotic relationship with sandtrout, which, as they merge with my body, will sustain me as I slowly turn into a sandworm, and make use of my billions of other memories and prescience to rule over everyone and everything for millenia. * quicksilver: or in Java, we'd make it more generic by using a PolynomialDivisionOperatorFactory. * roconnor: I was going to read about laziness, but I decided to do it later, when I need to understand it. About the Haskell Weekly News New editions are posted to [37]the Haskell mailing list as well as to [38]the Haskell Sequence and [39]Planet Haskell. [40]RSS is also available, and headlines appear on [41]haskell.org. To help create new editions of this newsletter, please see the information on [42]how to contribute. Send stories to byorgey at cis dot upenn dot edu. The darcs repository is available at darcs get [43]http://code.haskell.org/~byorgey/code/hwn/ . References 1. http://haskell.org/ 2. http://apfelmus.nfshost.com/monoid-fingertree.html 3. http://article.gmane.org/gmane.comp.lang.haskell.cafe/51249 4. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/stm-io-hooks 5. http://www.haskell.org/haskellwiki/New_monads/MonadAdvSTM 6. http://article.gmane.org/gmane.comp.lang.haskell.cafe/51186 7. http://hackage.haskell.org/cgi-bin/hackage-scripts/stats 8. http://article.gmane.org/gmane.comp.lang.haskell.cafe/51165 9. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HTTP 10. http://planet.haskell.org/ 11. http://haskell.org/haskellwiki/Blog_articles 12. http://lukepalmer.wordpress.com/2009/01/24/existential-memoization/ 13. http://lukepalmer.wordpress.com/2009/01/24/the-third-virtue/ 14. http://blog.darcs.net/2009/01/darcs-weekly-news-14.html 15. http://therning.org/magnus/archives/485 16. http://ghcsparc.blogspot.com/2009/01/wait-and-perform.html 17. http://lhc-compiler.blogspot.com/2009/01/typeclasses-are-working-now-were.h… 18. http://blog.unsafeperformio.com/?p=34 19. http://therning.org/magnus/archives/476 20. https://www.joachim-breitner.de/blog/archives/317-darcswatch-uploaded-to-ha… 21. http://ghcsparc.blogspot.com/2009/01/strap.html 22. http://www.realworldhaskell.org/blog/2009/01/21/bryan-osullivan-on-the-powe… 23. http://vis.renci.org/jeff/?p=63 24. http://donsbot.wordpress.com/2009/01/21/open-source-releases-and-growth/ 25. http://augustss.blogspot.com/2009/01/performance-update-ive-continued.html 26. http://archhaskell.wordpress.com/2009/01/20/piet-now-packaged/ 27. http://blog.plover.com/math/monad-closure.html 28. http://ghcsparc.blogspot.com/2009/01/grind.html 29. http://lhc-compiler.blogspot.com/2009/01/functions-in-haskell.html 30. http://therning.org/magnus/archives/458 31. http://xmonad.wordpress.com/2009/01/19/xmonad-cheatsheet/ 32. http://xmonad.wordpress.com/2009/01/19/xmonad-on-twitter/ 33. http://vis.renci.org/jeff/?p=24 34. http://xmonad.wordpress.com/2009/01/19/xmonad-081-ghc-610-maintainance-rele… 35. http://eve-language.blogspot.com/2009/01/some-observations-on-kinds.html 36. http://martijn.van.steenbergen.nl/journal/2009/01/11/type-synonym-families/ 37. http://www.haskell.org/mailman/listinfo/haskell 38. http://sequence.complete.org/ 39. http://planet.haskell.org/ 40. http://sequence.complete.org/node/feed 41. http://haskell.org/ 42. http://haskell.org/haskellwiki/HWN 43. http://code.haskell.org/~byorgey/code/hwn/

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Core questions
by Matthijs Kooijman 24 Jan '09

24 Jan '09

Hi all, for my Master's thesis, I'm looking into functional hardware descriptions, and in particular to translate haskell code into VHDL that can be programmed into an FPGA. For this, I'm using the GHC API to load the haskell source and give me (simplified) core representation. I then walk over the core tree and translate it to VHDL. On the whole the GHC API has been very useful to me. It took me some time to get used to all the (deeply) nested (algebraic) types (I didn't have any previous haskell experience when I started a month or so ago), but things are working out by now. However, there are two issues bothering me still. The first is that the Core types (in particular CoreExpr) are not instances of Show. They are instances of Outputable, which allows them to be pretty printed. However, this pretty printing is good to view the structure of the expression that the CoreExpr represents, but doesn't show the structure of the CoreExpr itself. For example, tuple construction is printed simply as (a, b), while the actual core expression is a nested application of two types, and a and b to the GHC.Tuple.(,) function (or datacon?). Also, the exact constructors used are not quite clear, which made it harder to work with the Core language for me. Instead of looking at the structure of the CoreExpr and write the appropriate patterns, I had to resort to a lot of trial and error. I tried deriving show for CoreExpr myself, but that required me to derive Show for a dozen other types and resulted in type errors beyond my understanding. Is there any compelling reason that CoreExprs are not instances of Show? My second question concerns typle construction. Since tuple types are not primitive types, but dependent types defined in various places (GHC.Tuple and Base IIRC), code working with tuples is not fundamentally different from code working with other (user defined) dependent types and thus not trivial to recognize. I've found that there are some predicate functions that can tell me if a type is a tuple type, but I've had no such luck for the actual tuple construction. In particular, when an expression creates a tuple of two Ints (a, b), this is represented in Core as the application of the element types and the element values to the (,) function (or data constructor? Not sure about this, in Core it's just a Var and the isDataConName [IIRC] predicate crashes when run on the Var). For now, I've manually matched the Var's name to GHC.Tuple.(,) and removed all type arguments to get at the actual values in the tuple. This is of course completely non-portable, to tuples with more than two elements, or unboxed tuples, etc. Is there some predicate function that can do this for me in a more portable way? Gr. Matthijs

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Re: [Haskell-cafe] How to define an operation in terms of itself (but of different type)?
by Olex P 24 Jan '09

24 Jan '09

Well if telepaths on vacation... class Vector v where (^+^) :: v -> v -> v data Vector3 = V3 !Double !Double !Double instance Vector Vector3 where (V3 x1 y1 z1) ^+^ (V3 x2 y2 z2) = V3 (x1 + x2) (y1 + y2) (z1 + z2) class Matrix m where (^+^) :: m -> m -> m data Matrix3 = M3 !Vector3 !Vector3 !Vector3 instance Matrix Matrix3 where (M3 r11 r12 r13) ^+^ (M3 r21 r22 r23) = M3 (r11 ^+^ r21) (r12 ^+^ r22) (r13 ^+^ r23) Hope this is better :) So yeah... r11 ^+^ r21 just doesn't work.

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Generalizing a filter-making function
by Dominic Espinosa 24 Jan '09

24 Jan '09

Novice question here. Sorry if the post is wordy. In the following code (which doesn't actually compile as-is), I'm trying to generalize these 'make*Filter' functions into a single 'makeFilter' function. However, I can't get the types to work right. Foo is a tuple type on which a large number of accessor functions are defined. All of them have type Foo -> Int, Foo -> String, or Foo -> [a] (so far). I tried defining 'Query' using exsistential types instead, but had difficulty with 'escaped type variables' when I tried to write a generalized 'makeFilter' function. The general point of makeFilter is to take as parameters a Query, a value, a comparison function, and then return a function (Foo -> Bool). This returned function takes as its argument an object of type Foo, applies the function 'q' to it, compares that value to 'val', and finally returns a Bool. Later on in the program, a list of these filter functions is to be used with a list of Foo objects, to determine which Foo objects satisfy all of the filters. Advice would be greatly appreciated. --- code --- data Query a = Query { query_identifier :: String, query_func :: (Foo -> a) } makeIntFilter :: Query Int -> Int -> (Int -> Int -> Bool) -> (Foo -> Bool) makeIntFilter q val cmp = (\k -> val `cmp` (query_func q $ k)) makeStringFilter :: Query String -> String -> (String -> String -> Bool) -> (Foo -> Bool) makeStringFilter q val cmp = (\k -> val `cmp` (query_func q $ k)) -- ??? broken, and the 'cmp' argument is thrown away, which seems wrong makeMemberFilter :: Eq a => Query [a] -> a -> (a -> a -> a) -> (Foo -> Bool) makeMemberFilter q val cmp =(\k -> val `elem` (query_func q $ k))

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Error in HTTP 4.004 + CouchDB?
by Alex Ott 23 Jan '09

23 Jan '09

Hello (i sent this message to the mail of author, specified at hackage, but it returned to me, so i decided to resend it here) I found strange thing during work with latest versions of CouchDB & HTTP module on my Mac OS X 10.4. The problem is, that when i send document to the couchdb server, it name was cut to the 2 characters. After some investigation i found, that this is problem in combination of the CouchDB + HTTP usage. CouchDB module fills out the Request structure, and specify scheme & authority fields. This lead that i see (in network dumps) following data: PUT http://127.0.0.1:5984/test1/Users_ott_tmp_1_tst HTTP/1.1 Content-Type: ... ... Host: 127.0.0.1:5984 ... instead of proper PUT /test1/Users_ott_tmp_1_tst HTTP/1.1 I can eliminate this by explicitly setting of the scheme & authority into empty values, but this lead to improper creation of the Host header in request: PUT /test1/Users_ott_tmp_1_tst HTTP/1.1 Content-Type: ... ... Host: ... I tried to fix this problem myself, but I'm novice in the industrial Haskell programming ;-) P.S. this code from (CouchDB/HTTP.hs) will restore proper behaviour of the code, but break Host header makeURL path query = CouchMonad $ \conn -> do return ( (ccURI conn) { uriPath = '/':path , uriQuery = if null query then "" else '?':(urlEncodeVars query) , uriScheme = "" , uriAuthority = Nothing } ,conn ) -- With best wishes, Alex Ott, MBA http://alexott.blogspot.com/ http://xtalk.msk.su/~ott/ http://alexott-ru.blogspot.com/

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