[Haskell-cafe] ANNOUNCE: graphtype — A simple tool to illustrate dependencies between Haskell types
While developing applications which deal with complex data it is crucial to know how exactly you manipulate this data. Haskell provides excellent tools for expressing a data scheme you work with: ADTs, `type` and `newtype` declarations, type classes and much more is hidden in rich Haskell's type system. Obviously, when types of data in your domain you work with grow — all declarations grow, and it becomes hard to grasp all dependencies, to change them and to remove them deliberately. graphtype was developed to visualise type declarations in you Haskell source files. It produces .dot-file for subsequent processing with graphviz. Results for example file bundled with graphtype: http://i.piccy.info/i4/00/90/bfa07290012c2d3b455696bdaa86.png To play with it, you can use hackage: http://hackage.haskell.org/package/graphtype or hack some code: http://github.com/explicitcall/graphtype Visualisation of dependencies in complex type class hierarchies is still on the way. It isn't obvious how do to this nicely, as in most cases type class declarations are imported from other libraries, and you don't always have source files for them. Anyway, graphtype is fairly usable. Leave here your questions, suggestions and have fun looking at type dependencies in your code. WBR, Max ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Generics for constructing Rows
Hi, all. I've come into trouble defining function `gmap` which will work on these data types: data Row = Row (E Name) (E Salary) (E Department) type E a = Either (Maybe RowIndex) (Maybe a) type RowIndex = Int `RowIndex`, `Name`, `Salary`, `Department` have kind * pseudocode: gmap :: (E a - E a) - Row - Row readRow :: [String] - Row - Row readRow l = gmap (\(Left (Just ri)) - Right $ l `atMay` c = readMay) `atMay` and `readMay` are defined in module `Safe` from package `safe` atMay :: [a] - Int - Maybe a readMay :: (Read a) = String - Maybe a Basically we have optional Row indices and try to read raw row (list of Strings) into the same Row type if index is present. At this moment I just have separate data type for row which has been read and just a list of row indices, but it is definitely flawed when I need to add fields to Row from time to time, as it is too easy to introduce bugs while positioning in list of row indices, not mentioning all those boilerplate code flowing around. I've tried to define gmap using libraries for generic programming but failed each time while type checking for different reasons. With `Data.Generics` and `gmapT` it fails because gmapT doesn't allow traversion function to have `Read a` constraint (Data and Typeable instances for Row and inner data types of course were derived). EMGM's map demands traversion function to be non-polymorphic, i.e. type-checker fails with the message, complaining it cannot match `E a` against `E Name`, against `E Salary` etc. For each of this I've spent smth about five hours just reading API docs, papers, and trying to beat type-checker. Maybe I'll look into Smash, RepLib and others, but I hope someone was also trying to define such gmap and succeeded. So, generic programming folks, is it even possible to define such function? I don't really care about using GHC extensions, I don't care about code being portable, I just want to remove boilerplate and prevent introducing bugs. Thanks in advance, Max. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Generics for constructing Rows
Sorry for a type and probably confusing you, it must be: readRow l = gmap (\(Left (Just ri)) - Right $ l `atMay` ri = readMay) instead of l `atMay` c ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Where does documentation get installed with cabal?
Colin Paul Adams co...@colina.demon.co.uk writes: I'm trying to find the API documentation for happstack 0.3 (online is for 0.2). So I did: cabal install happstack --reinstall --enable-documentation but I can't find it anywhere within ~/.cabal - where should I look? In most cases it is installed in ~/.cabal/share/doc/happstack*/html. Is there any files at that directory? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Generics for constructing Rows
Sean Leather leat...@cs.uu.nl writes:
I'm not sure the problem you're running into is strictly a generic
programming (GP) one. Typically, GP takes code that is often written
and generalizes it, so that it doesn't have to be written for multiple
datatypes.
That seems to be GP problem, as your solution doesn't scale well when I
wan't to add/remove/change fields in the `Row` record. The perfect way
as I see it, would be just editing `Row` data declaration, nothing else.
Studying few papers about GP in Haskell, I reckon this could be
represented as generic traversal, using my `Row` declaration with
`Either`. I don't see really good way to write a generic producer from
`[String]` to version of `Row` without `Either`. But SYB doesn't
provide a way for passing type-class-parametric functions to gmapT, and
SYB-with-class has large overhead of its usage. I don't have enough
time to find out how this can be written in SYB-with-class, if it really can be
written. The restriction of EMGM was described in my initial message.
For your problem, I think the first issue is figuring out how to write
the non-generic function. I don't know if this is exactly what you
want, but you can write a version of gmap using GADTs and rank-2
types. I've simplified some types, but it should be easily
transferable to your code. For example, change the String, Float,
etc. to your Salary, Department, whatever.
---
{-# LANGUAGE GADTs #-}
{-# LANGUAGE Rank2Types #-}
module Main where
data T a where
String :: T String
Float :: T Float
Integer :: T Integer
data Row = Row (Maybe String) (Maybe Float) (Maybe Integer)
deriving Show
f :: T a - Maybe a - Maybe a
f String (Just a) = Just z
f _ x = x
gmap :: (forall a . T a - Maybe a - Maybe a) - Row - Row
gmap f (Row x y z) = Row (f String x) (f Float y) (f Integer z)
main = do
print $ gmap f $ Row Nothing (Just 5.4) (Just 3) -- == Row Nothing (Just
5.4) (Just 3)
print $ gmap f $ Row (Just a) Nothing Nothing -- == Row (Just z)
Nothing Nothing
If this is what you're looking for, then I think it might be possible to do
this more generally, though I haven't looked
into it.
Many thanks for this code, I'll try to integrate it at this point. It
seems to remove the burden of managing row indices list and implements
some intended restrictions that will make my code less error prone, I
hope.
WBR, Max.
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Re: [Haskell-cafe] Can't derive Binary for StdGen
Grigory Sarnitskiy sargrig...@ya.ru writes: Hello! I'm trying to derive Binary for StdGen with DrIFT: [...] but I got error DrIFT: can't find module System/Random What shall I do? I'd use http://hackage.haskell.org/packages/archive/derive/2.0.1/doc/html/Data-Derive-Binary.html instead. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Planning for a website
Simon Michael si...@joyful.com writes: I can give a +1 vote for the Hack api and related libs. (Jinjing Wang is a one-man army.) Below hack you'll run happstack or another web-serving lib. Above hack you might run some combination of loli, maid, the hack middleware modules, hsp. The advantage is that changing the low-level server in future is a matter of changing one or two lines; and the upper-level utilities seem more usable to me than current happstack's. The problem is that `hack` isn't documented at all and that prevents it from being in wide use. At least, when I started my web app, I preferred happstack, as low-level and documented API is better than high-level API without a little bit of documentation, examples and tutorials. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [Haskell-beginners] Ambigous Types with Haskell Functional Graph Library
Joe joesmo...@gmail.com writes: I tried using ucycle directly from Data.Graph.Inductive.Example by itself. I didn't realize there were two instances of Graph. How would you use the PatriciaTree Graph instance in the ucycle type signature? ucycle :: Graph gr = Int - gr () () You must have that Graph instance in scope. I can't give any more suggestions as I don't see the code in which you use `ucycle`. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Changelogs and available since
Laszlo Nagy rizso...@gmail.com writes: Hi All, I was volunteer to solve this problem: http://hackage.haskell.org/trac/summer-of-code/ticket/1565 In a conversation on the librar...@haskell.org I was suggested to scratch my idea here. I would use hoogle for this. Currently it stores the package name and the symbols of the modules about a package. What do you think about hayoo? I prefer this to hoogle, as hayoo has more complete database across hackage packages, AFAIK I would extend it to store the version of the package as well. Extend the query methods to use all the available package description at search. And make a VersionRange from the matched Versions. I wrote some code for doing this a while ago. Take a look and make some comments, I can extend this library to satisfy your needs http://hackage.haskell.org/package/cabal-query ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: simple hsql question
Alexander Kotelnikov sa...@myxomop.com writes: Ok, let me ask it in another way. Is there a good way to access databases, mysql in particular, from haskell program? Use HDBC or Takusen. You can find them on hackage. HDBC is fairly usable, but you must write SQL queries by yourself or use some simple machinery to construct queries. I've written a bunch of helper functions to construct queries as strings, e.g.: insert :: String - [String] - String - String insert t = (++) . ( INSERT ++) . ( INTO++) . (t ++). ( ( ++) . (++ ) ) . join , select :: [String] - String - String select = (++) . ( SELECT ++) . join , from :: [String] - String - String from = (++) . ( FROM++) . join , In the end your query looks like: query' (select [max(cheque)] . from [history] $ ; In this way you avoid typical errors which emerge when you write simple SQL query strings. Though you don't get true static typing. There are haskelldb and Takusen, which provide more elaborate way of connecting to RDBMS, they demand more investigation though. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] unsafeDestructiveAssign?
Job Vranish jvran...@gmail.com writes: Does anybody know if there is some unsafe IO function that would let me do destructive assignment? Something like: a = 5 main = do veryUnsafeAndYouShouldNeverEveryCallThisFunction_DestructiveAssign a 8 print a 8 Aren't StateT or IORefs the exact thing you are looking for? I'm also looking for a way to make actual copies of data. so I could do something like this: a = Node 5 [Node 2 [], Node 5 [a]] main = do b - makeCopy a veryUnsafeAndYouShouldNeverEveryCallThisFunction_DestructiveAssign b (Node 0 []) -- 'a' is unchanged It would be even more fantastic, if the copy function was lazy. I think the traverse function might actually make a copy, but I would be happier with something more general (doesn't require membership in traversable), and more explicit (was actually designed for making real copies). Same thing, IORefs could help you. Anyway, I can't imagine any case where veryUnsafeAndYouShouldNeverEveryCallThisFunction_DestructiveAssign could be useful with its imperative semantics as you've described. The point is that Haskell is pure language and I use it because of this feature (not only because of this, to be exact). I don't want to use any library code that brokes pure semantics and launches nuclear bombs behind the IO monad. GHC is smart enough these days to do all optimised destructive assignments, copies and all that imperative stuff and there are plenty of other ways to get a performance boost without unsafeHorribleThings. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] unsafeDestructiveAssign?
Job Vranish jvran...@gmail.com writes: I am well aware of the usual ST/IORefs as the usual solutions to data mutability in haskell. I very very much understand purity, and why it is a good thing, and why we should try to stay away from IO and ST as much as possible. I am very much away that even if I had such a function that it will probably break everything. I am not just trying to make things run faster. What I am trying to do is hyper unusual and I really do need an unsafeHorribleThings to do it. Well, is purpose of this a big secret? Maybe if you describe the whole direction of such efforts, somebody could propose better solution that satisfies your needs. I recommend you to describe semantics of unsafeHorribleThings in details, and in what exact cases this could be useful. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell's type system compared to CLOS
Matthias-Christian Ott o...@mirix.org writes: That's true. This is a big advantage when compiling programmes. But as far as I know type inference is not always decidable in Haskell. Am I right? Decidability of type inference depends on features you use (GADTs, type classes etc). Type inference in Haskell doesn't mean you avoid providing type signatures at all costs. It is good programming practice to provide type signatures, as elaborate design of ADTs, type classes you use in your program gives you possibility to encode specification of how program behaves in static, that is type signatures help you to document your code. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] ANNOUNCE: cabal-query 0.1
This package was written to assist you at finding a set of packages, which satisfy your needs. At the moment it doesn't have a standalone executable, but you can do the queries from your Haskell code. It uses Data.Generics.PlateData, so * when Cabal package format changes, we don't have to rewrite anything * all queries are statically typed * as a disadvantage, we may suffer some performance loss when doing very complex queries, anyway most of processing goes while we read package descriptions, not querying them Example of enduser querying code: module Main where import qualified Data.ByteString.Lazy as B import System.Environment import Distribution.Query import Distribution.Compiler import Distribution.License import Distribution.ModuleName hiding (main) import Distribution.Package import Distribution.PackageDescription import Distribution.Version import Distribution.Text import Language.Haskell.Extension main = (head `fmap` getArgs) = B.readFile = mapM_ (putStrLn . show . (x - (display $ package x, display $ license x))) . queryIndex (Not (Id (== GPL)) : Not (Id (== BSD3))) This queries an index file, which is commonly located at ~/.cabal/packages/hackage.haskell.org/00-index.tar in POSIX systems. You can query any field of PackageDescription no matter how deep it is. You don't need to provide any type signature for comparison functions, which are wrapped in Id, as long as you use data constructors for which type can be inferred. In the future versions I want to add some query expressions parser, for invoking cabal-query from command-line. I suppose this will look like: cabal-query -q 'license == BSD3 stability == stable testedWith == GHC repoType == (git | darcs)' path_to_cabal-files_or_index-file ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Request for Changelogs
Did you see these tickets http://hackage.haskell.org/trac/hackage/ticket/299 http://hackage.haskell.org/trac/hackage/ticket/244 ? Probably the real proposal could be fixed in comments for that tickets, so anyone who wants to implement that feature would see all possible solutions in one place, without browsing through haskell-cafe archives. Joachim Breitner m...@joachim-breitner.de writes: If the changelog had a properly specified format and location, cabal upgrade could, if the user wants it, tell him all the downloaded changes. This really helps him to keep up-to-date. It is also a good channel for the authors to talk to his users („see this nice module I added to the package“, „some parts of the API are deprecated now, please move to this part“). Last but not least having documented changes is QA measure that the Haskell platform in the wider sense deserves. Greetings, Joachim ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: SQL Database in Haskell?
As I can say from my experience of usage of hdbc-sqlite3 and happstack-state, the latter covers everything you ever wanted from sqlite3 and more. It you aren't too concerned about performance, you can free yourself from many tedious routines that are imminent when you work with relational database. Elaborated data model design coupled with some generics technique (uniplate with derive, e.g.) gives you a possibility to write down your domain problem directly to haskell. CK Kashyap ck_kash...@yahoo.com writes: I'd be very interested to see a rdbms implementation in Haskell ... perhaps a port of sqlite Regards, Kashyap ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] [GSoC] WWW::Mechanize-like package for Haskell
Hi, I'm interested in working on a library for a stateful web browsing in Haskell during Google Summer of Code. The basic idea is described at http://hackage.haskell.org/trac/summer-of-code/ticket/1107. WWW::Mechanize is a ready to use library written in Perl, though I used python's mechanize when I wrote some simple scripts (http://wwwsearch.sourceforge.net/mechanize/), which provides much cleaner interface. Anyway, it gives simple and convenient way to retreive web-sites, to handle cookies, history and to process retrieved content and forms. There are basics of it Network.Browser module from Haskell's HTTP library (http://hackage.haskell.org/packages/archive/HTTP/3001.0.4/doc/html/Network-Browser.html), but it's ugly (uses unsafePerformIO for error reporting) and lacks a greater part of needed functionality. My aim is to greatly improve Network.Browser module and to make coding small scripts with it in more functional way possible. At this moment it uses BrowserAction state monad. Though, the deadline is approaching, I still seek some ways to improve my proposal. So here are the questions: are there any other data structures that will make programming with this library more convenient, besides simple state monad? Should we contrive more sophisticated system with other other separate data structures? What other improvements you'd like to see? Thanks in advance for any advice. -- Max. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [GSoC] WWW::Mechanize-like package for Haskell
On Mon, Apr 7, 2008 at 4:11 PM, Thomas Schilling [EMAIL PROTECTED] wrote: It doesn't have to be perfect. Make sure you know how to use monad transformers. Also take a look at tag soup and the various HTML/XML parsers. I'm sure there's plenty to work on. My guess would be, that you try to write non-trivial example applications and see what is needed. For example, you could write a script to download/upload a Haskell wiki page logging in if necessary. Take a look of what other WWW::Mechanize packages are used. That kind of stuff. Also, for a GSoC proposal you should try to convince the mentors, why your project is useful for Haskell in general. So maybe you have some more arguments there, too. / Thomas There's many benefits of having such library in Haskell: improved automated testing (as Yitzchak Gale mentioned) due to pure nature of inner algorithms (BrowserAction can be pure and be transformed into IO only on demand), static typing (just hate a bunch of stupid bugs while writing all those scripts in python/perl). We can use powerful HTML/XML parsers available there in Haskell (HXT with its arrowed XML filters). Haskell community will definitely benefit from such library. Firstly, as I see, in indirect way: I know many people that don't want to use or learn more about Haskell, saying it lacks libraries for their everyday work. Network libraries still aren't cool enough, and personally I want to improve them at least to the point when I can say look! here's the network libraries and they aren't worse than yours, even better: pure and checked! :). Secondly, new libraries are useful for the community directly, we have aforementioned lambdabot, e.g. I'd like to write some bots looking at new changes at haskellwiki or something like that, thing I'm doomed to code in shcurl/perl/python now. -- Max ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
