Re: [Haskell-cafe] Numerical methods in Haskell
I finally got some time to answer Simon's posting: Simon P-J: | Between google searching and looking through the activity | report, I take it that no one has really developed serious | libraries for matrix manipulations, diff eqs, etc. | | Are there any practical reasons for this or is it just a | matter of the haskell community being small and there not | being many people interested in something so specialized? The latter I think, but it's just the sort of thing that a functional language should be good at. Two other difficulties (a) It's hard to compete with existing libraries. The obvious thing is not to compete; instead, just call them. But somehow that doesn't seem to be as motivating. Perhaps some bindings exist though? Hard to compete, yes. But on the other hand, the rewards can be high. Numerical library code (especially matrix libraries) tends to be highly optimized for the hardware architecture at hand. As a consequence a small change in, say, the cache architecture, might require a thorough rewrite of the library code to regain high utilisation of the hardware. This is since languages like Fortran and C force you to code so close to the hardware. A high-level language, with good optimizing compilation, would make also library code more portable across hardware architectures. N.b. these optimizations are non-trivial to say the least. (b) A concern about efficiency, because numerical computation is typically an area where people really care about how many instructions you take. It's a legitimate concern, but I don't think that it'll turn out to be justified. With unboxed arrays, and/or calling external libraries for the inner loops -- and the potential for aggressive fusion and/or parallelism, there is plenty of upward potential. I also want to work on nested data parallelism (a la NESL, and NEPAL) which fits right in here. The number of instructions is only one side of the coin. For high-performance computing, memory issues are at least as important: both the amount of memory used (e.g., will the computation fit into memory at all), and how the memory hierarchy is utilized (caches, TLB:s, virtual memory, ...). This is a really sweet spot of functional languages, and laziness adds to it. On the other hand, the increased abstraction of functional languages gives an optimizing compiler larger freedom to reorder computations and choose memory layouts of data structures like matrices. This is potentially very useful, since optimizing for memory hierarchy utilization typically involves both data layout and order of memory accesses. However, to achieve a good result, the compiler must be able to predict a great deal of the computing and the memory usage. For instance, dynamic memory handling of numeric data structures will surely kill any serious attempt to predict the cache behavior. To achieve good optimizing compilation, we need either very good program analyses, or a library of recursion patterns or templates for which the compiler knows how to allocate memory statically and order the computations well, or possibly both. Some encouraging examples: Sven-Bodo Scholz has achieved very good performance for the restricted functional matrix language SAC, using optimizations for cache. My former student Peter Drakenberg invented a restricted functional matrix language, with analyses to infer matrix sizes statically, and sharing analysis, to find opportunities to allocate memory statically and update in-place. He also got some good experimental figures. This leads me to believe that compilers in more general languages could do something similar, by recognizing certain patterns or through advanced program analyses. However, both these languages are strict, and I am not sure at all how to do this in a lazy language. In any case, this is nontrivial compiler work and considerable research efforts would be needed. Unfortunately, I don't see how to fund such research, since the high-performance computing community largely seems to have given up on functional languages since the demise of the data-flow languages. I'd love to see a little community of matrix manipulators spinning up. Yes. There might me a niche for high-level numerical coding, somehwere where MATLAB is today. MATLAB isn't exactly blazingly fast, still very widespread. On the other hand, MATLAB is already in that niche. The question to answer is what advantages a functional language like Haskell could offer in this niche. We need to come up with these answers, and then convince enough people outside our own community. Björn Lisper ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] library sort
Strangely, Hoogle isn't easy to find at haskell.org. I'm not sure where the best place to add a link would be: perhaps near the top of the libraries-and-tools page? It's all wikified now, so would someone like to add it somewhere appropriate? Simon | -Original Message- | From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of | Radu Grigore | Sent: 17 February 2006 14:11 | To: Haskell Cafe | Subject: Re: [Haskell-cafe] library sort | | On 2/16/06, Jared Updike [EMAIL PROTECTED] wrote: | If you need an easier way to search the Haskell APIs, use Hoogle: | | Hoogle is very nice. Thanks to everyone who answered my question about | finding a sort library function. | | -- | regards, | radu | http://rgrig.blogspot.com/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Constructor classes implementation
Am Freitag, 17. Februar 2006 03:34 schrieb Sean Seefried: Hey all, If you're interested in an implementation of constructor classes (type classes which can take constructors as arguments; already implemented in Haskell) please see: http://www.cse.unsw.edu.au/~sseefried/code.html This should help understanding the paper by Mark P. Jones called A system of constructor classes: overloading and implicit higher-order polymorphism much easier. The implementation not only infers the type but also prints out a trace of the derivation tree for the syntax directed rules. Cheers, Sean p.s. If you find any bugs, please let me know. Re bugs: 1. printGamma [] would print an unmotivated }, as witnessed by typeInf [] term14. 2. the case unify (ConT c) (AppT t1 t2) is missing. 3. too many shadowed bindings, this is always dangerous, I believe 4. I'm not sure, the datatypes are appropriate; as far as I know, expressions have a type and not a kind, which is what the use of the same Var type for Type and Exp entails. I have only just glimpsed at Jones' paper, so I don't yet see, what this type inference algorithm (quite nice, btw) has to do with constructor classes. If I still don't after reading it, I'll come back to ask. Cheers, Daniel -- In My Egotistical Opinion, most people's C programs should be indented six feet downward and covered with dirt. -- Blair P. Houghton ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Numerical methods in Haskell
On Mon, Feb 20, 2006 at 11:47:49AM +0100, Bjorn Lisper wrote: (a) It's hard to compete with existing libraries. The obvious thing is not to compete; instead, just call them. But somehow that doesn't seem to be as motivating. Perhaps some bindings exist though? Hard to compete, yes. But on the other hand, the rewards can be high. Numerical library code (especially matrix libraries) tends to be highly optimized for the hardware architecture at hand. As a consequence a small change in, say, the cache architecture, might require a thorough rewrite of the library code to regain high utilisation of the hardware. This is since languages like Fortran and C force you to code so close to the hardware. A high-level language, with good optimizing compilation, would make also library code more portable across hardware architectures. N.b. these optimizations are non-trivial to say the least. The only particularly relevant numerical libraries today (atlas and fftw) already do far better optimization than any compiler is going to acheive, and in the case of fftw can respond to changes in memory configuration at runtime. In both cases they're written in ANSI C (although the C code for fftw is written by an OCaml program... or at least some dialect of ML). In order to take advantage of cache behavior properly, it's necesary to allow adjustments in the actual algorithm used, which isn't something that a clever compiler is likely to accomplish. Which is to say that while I'd love native Haskell code to run incredibly fast, noone can compete with atlas or fftw when writing in any other language either, unless they're willing to do either runtime or compile-time (or coding-time) timings on the actual machine on which the code will run. There really is no reason to try to compete with these libraries. Unless, I suppose, your interest is in writing libraries... fwiw, atlas takes close to a day to compile on my machine, since it spends so long performing timings of various algorithms with various parameters. I don't want my haskell compiler to take that long to compile *anything*. -- David Roundy http://www.darcs.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Numerical methods in Haskell
David Roundy: On Mon, Feb 20, 2006 at 11:47:49AM +0100, Bjorn Lisper wrote: (a) It's hard to compete with existing libraries. The obvious thing is not to compete; instead, just call them. But somehow that doesn't seem to be as motivating. Perhaps some bindings exist though? Hard to compete, yes. But on the other hand, the rewards can be high. Numerical library code (especially matrix libraries) tends to be highly optimized for the hardware architecture at hand. As a consequence a small change in, say, the cache architecture, might require a thorough rewrite of the library code to regain high utilisation of the hardware. This is since languages like Fortran and C force you to code so close to the hardware. A high-level language, with good optimizing compilation, would make also library code more portable across hardware architectures. N.b. these optimizations are non-trivial to say the least. The only particularly relevant numerical libraries today (atlas and fftw) already do far better optimization than any compiler is going to acheive, and in the case of fftw can respond to changes in memory configuration at runtime. In both cases they're written in ANSI C (although the C code for fftw is written by an OCaml program... or at least some dialect of ML). In order to take advantage of cache behavior properly, it's necesary to allow adjustments in the actual algorithm used, which isn't something that a clever compiler is likely to accomplish. That's a valid point. You may want to change, e.g., the size of blocks in a block-oriented matrix algorithm to match the cache. This will, in general, require the use of algebraic laws like associativity and commutativity, which are not valid for floating-point arithmetics and thus can change the numerical behaviour, so a compiler shouldn't fiddle around with them unless under strict control of the programmer. Interestingly, the language invented by my aforementioned former PhD student contains a nondeterministic matrix decomposition primitive, which allows the partitioning of a matrix into a fixed number of blocks, but where block sizes can vary. This is exactly to let the programmer give an optimizing compiler this degree of freedom when deemed safe. Alas, he never got around to any serious experiments with this feature. Björn Lisper ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell as scripting language?
On Wed, 15 Feb 2006, Marc Weber wrote: Is there a way to use haskell as scripting language in a) your own project? b) other projects such as vim (beeing written in C)? For German readers, I put an example of a scripting task on that Wiki: http://www.wikiservice.at/dse/wiki.cgi?SpracheHaskell/Skripte ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] module for probability distributions
On Thu, 16 Feb 2006, Matthias Fischmann wrote: I wrote a module for sampling arbitrary probability distribution, so far including normal (gaussian) and uniform. http://www.wiwi.hu-berlin.de/~fis/code/ For those who need something like this: feel free to take it, it's BSD. For those who feel like growing their karma: - There are a few questions in the code (marked with 'XXX'). - There are probably far better way to do this. I am eager to learn. - There is probably a better implementation out there already. Please point me to it. In HaskellDSP there are some routines for generating random numbers with specific distributions: http://haskelldsp.sourceforge.net/doc/index.html e.g. http://haskelldsp.sourceforge.net/doc/Numeric.Random.Distribution.Gamma.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] getChar + System.Cmd.system + threads causes hangups
Hello Using system or any variant of it from System.Process seems broken in multithreaded environments. This example will fail with and without -threaded. When run the program will print hello: start and then freeze. After pressing enter (the first getChar) System.Cmd.system will complete, but without that it will freeze for all eternity. What is the best way to fix this? I could use System.Posix, but that would lose windows portablity which is needed. import Control.Concurrent import System.Cmd main = do forkIO (threadDelay 10 hello) getChar getChar hello = do putStrLn hello: start system echo hello world! putStrLn hello: done - Einar Karttunen ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] HaXml: ampersand in attribute value
Lennart Augustsson wrote: But speaking of HaXml bugs, I'm pretty sure HaXml doesn't handle % correctly. It seem to treat % specially everywhere, but I think it is only special inside DTDs. I have many XML files produced by other tools that the HaXml parser fails to process because of this. Indeed. This is an area that I found required a fair amount of work on the version of HaXML I was playing with, some time ago. The change log at the end of: http://www.ninebynine.org/Software/HaskellUtils/HaXml-1.12/src/Text/XML/HaXml/Lex.hs has some clues to what I had to do. Notably: [[ -- Revision 1.12 2004/06/04 21:59:13 graham -- Wortk-in-progress: creating intermediate filter to handle parameter -- entity replacement. Separated common features from parse module. -- Created new module based on simplified use of parsing utilities -- to dtect and substitute PEs. The result is a modifed token sequence -- passed to the main XML parser. ]] The parameter entity filter is defined by: http://www.ninebynine.org/Software/HaskellUtils/HaXml-1.12/src/Text/XML/HaXml/SubstitutePE.hs The parameter and entity entity handling aspect of the code was not pretty, due mainly to the somewhat quirky nature of XML syntax, especially concerning parameter and general entities. #g -- Graham Klyne For email: http://www.ninebynine.org/#Contact ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] HaXml: ampersand in attribute value
Malcolm Wallace wrote: Lennart Augustsson wrote: But speaking of HaXml bugs, I'm pretty sure HaXml doesn't handle % correctly. It seem to treat % specially everywhere, but I think it is only special inside DTDs. I have many XML files produced by other tools that the HaXml parser fails to process because of this. I believe I fixed at least one bug to do with % characters around version 1.14. But that is the development branch in darcs, not formally released yet. Nevertheless, if you know of such bugs, do report them; even better if you can send a small test case. Malcolm, Did you come across the HaXml test harness I created based on a subset of W3C conformance tests? http://www.ninebynine.org/Software/HaskellUtils/HaXml-1.12/test/ This covers all the parameter entity problems I fixed some time ago. #g -- Graham Klyne For email: http://www.ninebynine.org/#Contact ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] getChar + System.Cmd.system + threads causes hangups
Here is a version that works fine: myRawSystem cmd args = do (inP, outP, errP, pid) - runInteractiveProcess cmd args Nothing Nothing hClose inP os - pGetContents outP es - pGetContents errP ec - waitForProcess pid case ec of ExitSuccess - return () ExitFailure e - do hPutStrLn stderr (Running process ++unwords (cmd:args)++ FAILED (++show e++)) hPutStrLn stderr os hPutStrLn stderr es hPutStrLn stderr (Raising error...) fail Running external command failed pGetContents h = do mv - newEmptyMVar let put [] = putMVar mv [] put xs = last xs `seq` putMVar mv xs forkIO (hGetContents h = put) takeMVar mv ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] HaXml: ampersand in attribute value
Graham Klyne [EMAIL PROTECTED] wrote: Did you come across the HaXml test harness I created based on a subset of W3C conformance tests? http://www.ninebynine.org/Software/HaskellUtils/HaXml-1.12/test/ This covers all the parameter entity problems I fixed some time ago. Indeed, and an excellent resource. I have been wondering how to merge it back into my version of HaXml ever since. Regards, Malcolm ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] building ghc on win with gcc of cygwin, without cygwin1.dll (-mno-cygwin flag) ?
| Googling around I've found that there exists the -mno-cygwin flag which | you can use to not include this lib.. So would it might be possible to | get a ghc build not using cygwin.dll with just cygwin ? I don't know. It sounds plausible. But we only have enough resource to maintain one route for building GHC on Windows. We'd be delighted if other folk were engaged enough to push other routes through and make them work. Another route we don't have, but which in principle is definitely feasible is to compile GHC using Cygwin for Cygwin. The advantage would be that the resulting GHC would have a working Posix library. The disadvantage is that programs it compiles would require Cygwin to run. Anyway, it's up for grabs. Please grab away! We need more Windows-savvy expertise on GHC. Simon | -Original Message- | From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Marc | Weber | Sent: 12 February 2006 04:45 | To: haskell-cafe@haskell.org | Subject: [Haskell-cafe] building ghc on win with gcc of cygwin,without cygwin1.dll (-mno-cygwin flag) | ? | | Hi.. I've found the nice documentation about building ghc on | haskell.org/ghc - documentation. | There is one chapter (#1) about C compilers and environments to use: | Either MSYS or cywin and | gcc of MinGW because gcc of cygwin will link to cygwin1.dll by default | which may change and therefore can brake your apps. | Googling around I've found that there exists the -mno-cygwin flag which | you can use to not include this lib.. So would it might be possible to | get a ghc build not using cygwin.dll with just cygwin ? | I haven't read anything about that in those docs. | | Or does this flag simply include cygwin.dll within your apps (statically | linked) ? | | #1 :http://www.haskell.org/ghc/docs/latest/html/building/platforms.html#ghc -cygwin | | Marc | ___ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] haskell programming guidelines
Hi, haskell admits many programming styles and I find it important that several developers of a prject agree on a certain style to ease code review. I've set up guidelines (still as plain text) for our (hets) project in http://www.informatik.uni-bremen.de/agbkb/forschung/formal_methods/CoFI/hets/src-distribution/versions/HetCATS/docs/Programming-Guidelines.txt These were inspired by C programming guidelines, http://haskell.org/hawiki/ThingsToAvoid and the problems I came across myself. It like to get comments or proposals for our or other haskell grogramming guidelines. Thanks Christian ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] haskell programming guidelines
On Mon, 20 Feb 2006, Christian Maeder wrote: I've set up guidelines (still as plain text) for our (hets) project in http://www.informatik.uni-bremen.de/agbkb/forschung/formal_methods/CoFI/hets/src-distribution/versions/HetCATS/docs/Programming-Guidelines.txt It seems we share the preference for 'case', 'let', 'map', 'filter' and 'fold'. :-) I prefer a definite choice between all_lower_case_with_underscore and camelCase identifier style. 'you should probably' -- should probably what? Is the function size restriction still sensible for Haskell? I think Haskell functions should be at most a few lines, but not one or two screenfuls of text. formJust - fromJust These were inspired by C programming guidelines, http://haskell.org/hawiki/ThingsToAvoid and the problems I came across myself. It like to get comments or proposals for our or other haskell grogramming guidelines. http://www.haskell.org/hawiki/UsingQualifiedNames http://www.haskell.org/hawiki/PairsOfIdentifiers ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] haskell programming guidelines
On Feb 20, 2006, at 12:48 PM, Christian Maeder wrote: Hi, haskell admits many programming styles and I find it important that several developers of a prject agree on a certain style to ease code review. I've set up guidelines (still as plain text) for our (hets) project in http://www.informatik.uni-bremen.de/agbkb/forschung/formal_methods/ CoFI/hets/src-distribution/versions/HetCATS/docs/Programming- Guidelines.txt These were inspired by C programming guidelines, http://haskell.org/ hawiki/ThingsToAvoid and the problems I came across myself. It like to get comments or proposals for our or other haskell grogramming guidelines. I personally disagree with your preference for custom datatypes with a value representing failure to lifting types with Maybe. I tend to like using the Maybe monad for composing large partial functions from smaller ones, but your suggestion makes that impossible. Also, if you bake in your failure case into your datatype, you can't use the type system to differentiate explicitly partial functions (which use Maybe X), from ones that are not expected to be partial (which just use X). Final point, using Maybe gives you an easy route to go to Either String X or some other richer monad to represent failure. Rob Dockins Speak softly and drive a Sherman tank. Laugh hard; it's a long way to the bank. -- TMBG ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] haskell programming guidelines
On Feb 20, 2006, at 2:26 PM, Henning Thielemann wrote: On Mon, 20 Feb 2006, Robert Dockins wrote: I personally disagree with your preference for custom datatypes with a value representing failure to lifting types with Maybe. I understood that part of the guidelines as a pleading for Maybe. Humm. Well clearly I read it the opposite way. I suppose that means that whatever technique is being recommended should be put forth with more clarity ;-) Rob Dockins Speak softly and drive a Sherman tank. Laugh hard; it's a long way to the bank. -- TMBG ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Unexpected results with simple IO
I'm also using GHC 6.4.1 and rxvt v2.7.10. The problem does occur in compiled code, but everything is OK in ghci! hFlush stdout did solve the problem, as expected. I've just started using rxvt. If you have tips on how to make ghci work well with rxvt, please share them with me (for instance, how to set the top arrow to repeat the last line, instead of moving the cursor one live above. I don't understand very well how those applications handle keyboard). Best, Maurício Emil Axelsson wrote: What version of GHC are you using? Your code works for me in rxvt in Cygwin, with GHC 6.4.1. But I remember having that same problem earlier (in some earlier GHC version, so it may be fixed by now). The solution was to run hFlush after each putStr, like so: import System.IO (hFlush, stdout) do putStr ... hHlush stdout ... If I remember correctly, the problem only occurred in GHCi and Hugs -- not when compiling the code. / Emil Maurício skrev: You're right... I was running the example in rxvt, in cygwin. Now I tried in Windows command shell and it works. Thanks, Maurício Cale Gibbard wrote: That doesn't happen for me at all, it works just fine. Maybe it's something wrong with your terminal? You could possibly try playing with the buffering settings on stdout, using hSetBuffering in System.IO. - Cale On 17/02/06, Maurício [EMAIL PROTECTED] wrote: Dear Haskell users, I have a problem using IO. The small test program below asks the user to guess from a list of random numbers between 1 and 10. Everything works well excepts for one problem: all the messages (Guess a number..., Right... and Wrong...) are printed after the program finishes, i.e., I have to use it blind. I'm afraid I misunderstand something important about lazyness or monads... What am I doing wrong? Thanks, Maurício module Main where import Random main = do r_gen - getStdGen --random generator let r_list = (randomRs (1,10) r_gen) --random list guess_loop (r_list) guess_loop (r:r_others) = do putStrLn Guess a number between 1 and 10: n - readLn if n==r then do putStrLn Right! :) return () else do putStrLn Wrong... :( guess_loop r_others ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] haskell programming guidelines
maeder: Hi, haskell admits many programming styles and I find it important that several developers of a prject agree on a certain style to ease code review. I've set up guidelines (still as plain text) for our (hets) project in Perhas you'd like to put up a Style page on thew new Haskell wiki, perhaps under the Idioms category? You could take some hints from the old style page, http://www.haskell.org/hawiki/HaskellStyle -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lazy read
On Thu, Feb 16, 2006 at 11:28:08PM +, Malcolm Wallace wrote: Essentially, rather than having an indication of success/failure in the type system, using the Maybe or Either types, you are asking to return the typed value itself, with no wrapper, but perhaps some hidden bottoms buried inside. One could certainly define such a parsing set-up, but there are no standard ones I know of. The attraction of lazy parsing is obvious, but there is a cost in terms of safety. Yeah, I have thought about the best way to do this before, something like all combinators by default being irrefutable meaning they always succeed perhaps placing bottoms in the structure, with a specific 'try' like in parsec that will let you try a path and backtrack locally. I was thinking something like a continuation based error monad where the error continuation was 'bottom' unless overridden locally, but I never could get the types to work out quite right. perhaps rank-n types or following the process outlined in the parallel parsing combinators paper will afford a better result. John -- John Meacham - ⑆repetae.net⑆john⑈ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] haskell programming guidelines
There is a more straightforward way to get localized error messages rather than using 'maybe' and hand-writing an appropriate error, and that is to rely on irrefutable bindings. f x = ... y ... where Just y = Map.lookup x theMap now if the lookup fails you automatically get an error message pointing to the exact line number of the failure. or if the failure message of the routine is more important than the source location you can do f x = ... y ... where Identity y = Map.lookup x theMap it is anoying you have to make a choice between these two possibilities, but this can be mitigated with CPP magic or the SRCLOC_ANNOTATE pragma. John -- John Meacham - ⑆repetae.net⑆john⑈ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Code completion? (IDE)?
vim7 has introduced omni-completion... So I'm interested wether there are any projects which support any kind of completion.? I have been working on some code completion support for EclipseFP. It is right now in a really infant stage, but it at least is something. Just take a look at the latest integration build that you are able to find at http://eclipsefp.sourceforge.net/download Could you describe the general approach taken? I'd like to see something like that added to Emacs's haskell-mode as well, and I expect a fair bit of the work can be done on the Haskell side and thus shared among editors. Stefan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Associated Type Synonyms question
Stefan Wehr writes: Martin Sulzmann [EMAIL PROTECTED] wrote:: Stefan Wehr writes: [...] Manuel (Chakravarty) and I agree that it should be possible to constrain associated type synonyms in the context of class definitions. Your example shows that this feature is actually needed. I will integrate it into phrac within the next few days. By possible you mean this extension won't break any of the existing ATS inference results? Yes, although we didn't go through all the proofs. You have to be very careful otherwise you'll loose decidability. Do you have something concrete in mind or is this a more general advice? I'm afraid, I think there's a real issue. Here's the AT version of Example 15 from Understanding FDs via CHRs class D a class F a where type T a instance F [a] where type T [a] = [[a]] instance (D (T a), F a) = D [a] ^^^ type function appears in type class Type inference (i.e. constraint solving) for D [a] will not terminate. Roughly, D [[a]] --_instance D (T [a]), F [a]) --_type function D [[a]], F [a] and so on Will this also happen if type functions appear in superclasses? Let's see. Consider class C a class F a where type T a instance F [a] where type T [a] = [[[a]]] class C (T a) = D a ^ type function appears in superclass context instance D [a] = C [[a]] -- satisfies Ross Paterson's Termination Conditions Consider D [a] --_superclassC (T [a]), D [a] --_type function C [[[a]]], D [a] --_instance D [[a]], D [a] and so on My point: - The type functions are obviously terminating, e.g. type T [a] = [[a]] clearly terminates. - It's the devious interaction between instances/superclasss and type function which causes the type class program not to terminate. Is there a possible fix? Here's a guess. For each type definition in the AT case type T t1 = t2 (or improvement rule in the FD case rule T1 t1 a == a=t2 BTW, any sufficient restriction which applies to the FD case can be lifted to the AT case and vice versa!) we demand that the number of constructors in t2 is strictly smaller than the in t1 (plus some of the other usual definitions). Then, type T [a] = [[a]] although terminating, is not legal anymore. Then, there might be some hope to recover termination (I've briefly sketched a proof and termination may indeed hold but I'm not 100% convinced yet). Martin ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Unexpected results with simple IO
Unfortunately, I don't know how to make the arrow keys work in rxvt. I'm not the right person to ask about such things... I don't think it's possible (unless GHC is built for Cygwin, or something). Does anybody else know? I use an alias alias ghciW='cmd /c start ghci' That way I can start GHCi in a new DOS window from rxvt. I usually want two windows anyway -- one for the interactive session and one to compile other modules. / Emil Maurício skrev: I'm also using GHC 6.4.1 and rxvt v2.7.10. The problem does occur in compiled code, but everything is OK in ghci! hFlush stdout did solve the problem, as expected. I've just started using rxvt. If you have tips on how to make ghci work well with rxvt, please share them with me (for instance, how to set the top arrow to repeat the last line, instead of moving the cursor one live above. I don't understand very well how those applications handle keyboard). Best, Maurício Emil Axelsson wrote: What version of GHC are you using? Your code works for me in rxvt in Cygwin, with GHC 6.4.1. But I remember having that same problem earlier (in some earlier GHC version, so it may be fixed by now). The solution was to run hFlush after each putStr, like so: import System.IO (hFlush, stdout) do putStr ... hHlush stdout ... If I remember correctly, the problem only occurred in GHCi and Hugs -- not when compiling the code. / Emil Maurício skrev: You're right... I was running the example in rxvt, in cygwin. Now I tried in Windows command shell and it works. Thanks, Maurício Cale Gibbard wrote: That doesn't happen for me at all, it works just fine. Maybe it's something wrong with your terminal? You could possibly try playing with the buffering settings on stdout, using hSetBuffering in System.IO. - Cale On 17/02/06, Maurício [EMAIL PROTECTED] wrote: Dear Haskell users, I have a problem using IO. The small test program below asks the user to guess from a list of random numbers between 1 and 10. Everything works well excepts for one problem: all the messages (Guess a number..., Right... and Wrong...) are printed after the program finishes, i.e., I have to use it blind. I'm afraid I misunderstand something important about lazyness or monads... What am I doing wrong? Thanks, Maurício module Main where import Random main = do r_gen - getStdGen --random generator let r_list = (randomRs (1,10) r_gen) --random list guess_loop (r_list) guess_loop (r:r_others) = do putStrLn Guess a number between 1 and 10: n - readLn if n==r then do putStrLn Right! :) return () else do putStrLn Wrong... :( guess_loop r_others ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
