Re: [Haskell-cafe] Newbie Instance Of Floating Int Error
One thing to try: type Point = (Float, Float) The problem is that x1, y1, x2, and y2 are Ints, and so (x2-x1)^2 + (y2-y1)^2 is also an Int, but then you try to take the square root, which is an operation only available on floating point values (more specifically, types in the class Floating, which also includes things like Complex Double). Another option is to leave the Point type as-is, but do a conversion just before applying the sqrt, say, by applying the fromIntegral function, which serves to convert from types in the class Integral, like Int and Integer to any numeric type you need. distBetween (x1,y1) (x2,y2) = sqrt . fromIntegral $ (x2-x1)^2 + (y2-y1)^2 hope this helps, - Cale On 27/04/06, Aditya Siram [EMAIL PROTECTED] wrote: Hi all, I just started working with Haskell and am having some difficulties with its type system. Here is function that is supposed to calculate the distance between two coordinates: distBetween (x1,y1) (x2,y2) = sqrt((x2-x1)^2 + (y2-y1)^2) I am trying to explictly give it a type signature. Here is what I have tried and the errors generated by Hugs: type Point = (Int,Int) distBetween :: Point - Point - Float ERROR - Type error in explicitly typed binding *** Term : distBetween *** Type : Point - Point - Int *** Does not match : Point - Point - Float distBetween :: Point - Point - Int Instance of Floating Int required for definition of distBetween Any help is appreciated... Deech ___ 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] Newbie Instance Of Floating Int Error
It looks like somewhere else in your program (or a type signature somewhere) is trying to force the result of sqrt to be an Int which won't work since square roots are irrational (represented by the computer as a Float or Double). You might try (1) making sure the place where distBetween is used isn't trying to use only Ints or Integers and (2) taking off explicit type signatures and seeing if that works (the compiler can usually get things working for you as long as you don't give it misinformation in type signatures). Also, a more useful type of Point if you are taking distances would be type Point = (Float,Float) instead (or you can leave that out because the function type for distBetween will be inferred to be the right thing, i.e. (Float,Float) is already a valid type without needing to be named). Hope that helps, Jared. -- http://www.updike.org/~jared/ reverse )-: On 4/27/06, Aditya Siram [EMAIL PROTECTED] wrote: Hi all, I just started working with Haskell and am having some difficulties with its type system. Here is function that is supposed to calculate the distance between two coordinates: distBetween (x1,y1) (x2,y2) = sqrt((x2-x1)^2 + (y2-y1)^2) I am trying to explictly give it a type signature. Here is what I have tried and the errors generated by Hugs: type Point = (Int,Int) distBetween :: Point - Point - Float ERROR - Type error in explicitly typed binding *** Term : distBetween *** Type : Point - Point - Int *** Does not match : Point - Point - Float distBetween :: Point - Point - Int Instance of Floating Int required for definition of distBetween Any help is appreciated... Deech ___ 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] database access recommendation
Hello Brock, Thursday, April 27, 2006, 7:45:16 PM, you wrote: I looked at http://www.haskell.org/haskellwiki/Libraries_and_tools, and suffice it to say that this page don't reflects current state of the art. during many years it was not updated and when it was moved to the wiki half-year ago it's contents remains the stone-age better sources of information are HCAR (Haskell Communities and Activities Report, http://www.haskell.org/communities/11-2005/html/report.html) and HWN (Haskell Weekly News, http://haskell.org/haskellwiki/HWN) as one person said, There are three active database libraries: HDBC, HSQL and Takusen.. i've included here last announces on two of them (just searched my mail archives for SQL): I'm pleased to (at long last) announce the release of HDBC version 0.99.2, along with 0.99.2 versions of all database backends. If things go well, after a few weeks of testing, this version will become HDBC 1.0.0. HDBC is a multi-database interface system for Haskell -- more on it below. Since 0.99.0, several major things have been updated or enhanced: * Complete rework of memory management system. * Standardized memory management system for all database backends. * Support for querying metadata (type, size, etc.) about columns both in result sets and in database tables * Support for querying the level of transaction support in the underlying DB * ODBC driver has support for working with databases such as MySQL that don't support transactions * Testsuite enhancements for all the above I believe that these modifications will make HDBC suitable as a full-fledged backend for HaskellDB. HDBC is available from: http://quux.org/devel/hdbc gopher://quux.org/1/devel/hdbc Version 0.99.2 has also been uploaded to Debian unstable. Features of HDBC * Ability to use replacable parameters to let one query be executed multiple times (eliminates the need for an escape function) * Ability to access returned rows by column number * Ability to read data from the SQL server on-demand rather than reading the entire result set up front * HUnit testsuite for each backend driver * Well-defined standard API and easy backend driver implementation * Lazy reading of the entire result set (think hGetContents, but for the results of SELECT) (see sFetchAllRows) * Support for translation between Haskell and SQL types * Support for querying database server properties * Add-on package (hdbc-missingh) to integrate with MissingH, providing a database backend for AnyDBM. * Support for querying metadata such as column names. In addition, there is extensive documentation for HDBC. Backend drivers exist for Sqlite v3, PostgreSQL, and ODBC. MySQL is supported via the ODBC driver and the ODBC testsuite passes against MySQL (saving the transaction tests, which aren't supported by MySQL). In addition, there is a odbc-missingh package that can turn and HDBC database into an AnyDBM backend for MissingH. (Sqlite3 could be very handy there) Darcs repositories are available for all of this at http://darcs.complete.org/ I am pleased to announce the HSQL library version 1.6. The HSQL is a simple library, which provides interface to multiple databases. MySQL, PostgreSQL, ODBC, SQLite and MSI (new) are currently supported. What is new? ~~ 1. HSQL is fully Cabalized. 2. Windows users don't need to have cygwin in order to build HSQL. The old build system was using ./configure script which is replaced with hooks in ./Setup.lhs script. 3. HSQL is split into multiple Cabal packages. There is one common package 'hsql' and multiple database specific packages: hsql-odbc, hsql-postgresql, hsql-mysql, hsql-sqlite, hsql-sqlite3, hsql-msi 4. There are two new database frontends: SQLite3 and MSI. The SQLite3 frontend is for SQLite version 3.0 and above. The old SQLite frontend still exists. MSI is fontend to Microsoft Installer database. Each .msi package is a relational database which you can query and modify using HSQL. [Haskell] ANNOUNCE: HSQL 1.7 released What is new? ~~ 1. Driver for Oracle 2. getFieldValueMB is deprecated. Now there is the instance declaration: instance SqlBind a = SqlBind (Maybe a) It allows to use getFieldValue instead of getFieldValueMB. Thanks to Frederik Eaton for this idea. 3. Database.HSQL.SQLite is renamed to Database.HSQL.SQLite2. The trouble was that with Hugs for each module with FFI is generated an extra .dll/.so library. In this case the library is SQLite.dll which on Windows overlaps with the name of the real sqlite.dll library. Home page ~ http://htoolkit.sourceforge.net Download from ~ https://sourceforge.net/project/showfiles.php?group_id=65248package_id=93958 -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list
Re: [Haskell-cafe] short yet featureful grep
On 27/04/06, John Meacham [EMAIL PROTECTED] wrote: my favorite example is the featureful yet short grep, supporting quite a few non-trivial options as well as a detailed '--help' message. :) Does getOptions really take an arbitrary n-tuple? Why? That seems a very odd way of doing things. The only advantage I can think of over lists is that tuples don't have to be typically homogeneous, but I would have thought that you could get your ?? and == operators to spit out the correct types. -- -David House, [EMAIL PROTECTED], http://xmouse.ithium.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] database access recommendation
bulat.ziganshin: Hello Brock, Thursday, April 27, 2006, 7:45:16 PM, you wrote: I looked at http://www.haskell.org/haskellwiki/Libraries_and_tools, and suffice it to say that this page don't reflects current state of the art. during many years it was not updated and when it was moved to the wiki half-year ago it's contents remains the stone-age better sources of information are HCAR (Haskell Communities and Activities Report, http://www.haskell.org/communities/11-2005/html/report.html) and HWN (Haskell Weekly News, http://haskell.org/haskellwiki/HWN) I'm not sure this is fair. About a month ago I went through the entire HWN archives, the haskell@ archives back to 1990 (Check the Old_news page!) and the last HCAR, adding over 100 entries to the libraries page. So, I argue it is the most complete list we have. And if anything's missing, you know how to edit the wiki. -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re[2]: [Haskell-cafe] GetOpt
Hello Anton, Friday, April 28, 2006, 12:54:44 AM, you wrote: I really trying to avoid imperative approach. I do have a terribly big experience in imperative programming (by the way, you might know one application that I made about 3 years ago. It is Uni-K Sensei for no. but may be you are heard about ARJZ? :) windows). Now, I am breaking my previous habits just to think wider and more effective. yes, yes. but you should know that haskell still are perfect imperative language and when pure functional approach will be not enough for your program, you can combine both ways Well, I do not care too much about high-speed. My main goal is to write a prototype of the language that I am creating. It is a kind of Domain-Specific language. I decided to start from a simple thing. A converter of pgn files with chess notation to javascript to visualize it. Just to have some practice. if you don't care about speed and write text-conversion program, pure functional approach may be enough. just use results of GetOpt as argument to all the routines that depends on any program options my own option-processing routines, it's just about 50 lines long (great demonstration of Haskell power!). all processed options are record in one large record that is passed around all the program. if you get accustomed to global variables, it's using in Haskell is possible but that is not the best way. you can also use implicit parameters (at least in hugs and ghc), but this again makes data dependencies somewhat non-understandable Thank you very much. I will see this approach as well. I am still pretty concern of using records instead of lists. what you mean? btw, i suggest you to use WinHugs for debugging program and ghc for final compilation. this makes faster development time together with faster final executable. moreover, making your program compatible with both environments is almost ensure that it will be compatible with coming Haskell standard, Haskell-prime Thanks again. I do not use Windows any more. I use Mac or different Unices. I do use ghc everywhere I work with Haskell. For debugging I use ghci. Well, and everything within GNU Emacs. you can use hugs, it works with many environments. according to my tests, it loads programs 10 times faster than ghci -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] database access recommendation
Hello Brock, Thursday, April 27, 2006, 7:45:16 PM, you wrote: I'm teaching myself Haskell, and was wondering if anyone could recommend a library for accessing databases, PostgreSQL in particular. as Ketil Malde wrote to me in private mail, there is at least one more lib (aside from HDBC/HSQL/Takusen) - HaskellDB. i'm sorry that i have citated phrase that can be no (more) true -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] short yet featureful grep
On Fri, Apr 28, 2006 at 08:47:40AM +0100, David House wrote: On 27/04/06, John Meacham [EMAIL PROTECTED] wrote: my favorite example is the featureful yet short grep, supporting quite a few non-trivial options as well as a detailed '--help' message. :) Does getOptions really take an arbitrary n-tuple? Why? That seems a very odd way of doing things. The only advantage I can think of over lists is that tuples don't have to be typically homogeneous, but I would have thought that you could get your ?? and == operators to spit out the correct types. Because the entire point of the module is to not only be type-safe, but to infer your intent from the type. if everything conformed to the same type, you wouldn't be able to ensure you lined up your arguments and your options properly and worse, you wouldn't be able to use the results directly as youd have to wrap them in some container. each of the return values is of a different type. So, the entire point of the module was to not use a list. unlike the other options out there. :) that is what makes it neat and enables very concise, very typesafe, code. tuples arn't particularly special, you can use HLists with the appropriate instances for instance. John -- John Meacham - ⑆repetae.net⑆john⑈ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re[2]: [Haskell-cafe] database access recommendation
Hello Donald, Friday, April 28, 2006, 12:29:38 PM, you wrote: I looked at http://www.haskell.org/haskellwiki/Libraries_and_tools, and suffice it to say that this page don't reflects current state of the art. during many years it was not updated and when it was moved to the wiki half-year ago it's contents remains the stone-age About a month ago I went through the entire HWN archives, the haskell@ archives back to 1990 (Check the Old_news page!) and the last HCAR, adding over 100 entries to the libraries page. So, I argue it is the most complete list we have. GREAT! but you don't announced this work on the haskell list, so noone can know about it -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] database access recommendation
Hi Bulat, Thanks for all the information. I'm giving HDBC a try as it seems to be the most actively maintained and because I was able to install it. Regards, Brock ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Saving a data structure to a file
Hi, I have a program which needs to cache intermediate data structures to a file (finite, non-cyclic, relatively simple, around ~100Kb when shown). Is there any easy way to acheive this? The current approach I am using is to add deriving Read, Show to all the data structures, and then writeFile output.txt (show x) read = readFile output.txt The problem is that this is too slow, and itsn't lazy. Is there any way to do better than this? I can imagine something like showBinary/readBinary which did this using binary files directly, but is there anything that does this? I'd like it to work with GHC and Hugs at least, and the less extra code I have to write the better :) Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Newbie Instance Of Floating Int Error
On 4/28/06, Aditya Siram [EMAIL PROTECTED] wrote: type Point = (Int,Int) distBetween :: Point - Point - Float ERROR - Type error in explicitly typed binding *** Term : distBetween *** Type : Point - Point - Int *** Does not match : Point - Point - Float distBetween :: Point - Point - Int Instance of Floating Int required for definition of distBetween It's saying that you explicitly gave the function the type Point - Point - Int but that it actually has Point - Point - Int. If you look at the type of sqrt: Prelude :t sqrt sqrt :: (Floating a) = a - a You'll see that it returns a floating-point number. (Also, a minor style point: you should probably get in the habit of putting a space between the sqrt and its arguments. It'll make more sense as you gain more experience.) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Saving a data structure to a file
Hello Neil, Friday, April 28, 2006, 7:59:22 PM, you wrote: I have a program which needs to cache intermediate data structures to a file (finite, non-cyclic, relatively simple, around ~100Kb when shown). Is there any easy way to acheive this? use NewBinary (darcs get http://www.n-heptane.com/nhlab/repos/NewBinary/) or jhc's Binary module together with DrIFT to generate Binary instances. these modules need minimal tweaks to implement Hugs compatibility code generated by DrIFT will use strict get. if you want to get lazy reading, you can replace get/put_ with lazyGet/lazyPut in appropriate instances in order to make it run faster, you should work with memory buffer and read/write entire buffer from/to file: writing: bh - openBinMem 1 undefined lazyPut bh data ... put more data writeBinMem bh filename reading: bh - readBinMem filename data - lazyGet bh get more data -- Best regards, Bulatmailto:[EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
