January 2003 - Haskell-Cafe

Re: A newbie question about Arrows.
by Ashley Yakeley 08 Jan '03

08 Jan '03

At 2003-01-06 03:14, Ross Paterson wrote: >class PreArrow ar where > arr :: (a -> b) -> ar a b > (>>>) :: ar a b -> ar b c -> ar a c ... > class (PreArrow ar, Monoidal p u) => GenArrow ar p u where > first :: ar a b -> ar (p a c) (p b c) My own preference is something like this: class (PreArrow ar) => GenArrow' ar where arrApply :: ar p (q -> r) -> ar p q -> ar p r My GenArrow' is actually equivalent to Hughes' Arrow and I assume your GenArrow. In an ideal world, I could leverage my FunctorApply class to have something like this: class (Functor f) => FunctorApply f where -- first arg (fab) "executed" first fApply :: f (a -> b) -> (f a -> f b) -- first arg (fa) "executed" first fPassTo :: f a -> f (a -> b) -> f b fPassTo fa fab = fApply (fmap (\a ab -> ab a) fa) fab class (PreArrow ar,forall p. FunctorApply (ar p)) => GenArrow'' ar Unfortunately, GHC does not yet allow this kind of superclassing. <http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/*checkout*/hbase/Source/HBa se/Category/Functor.hs?rev=HEAD&content-type=text/plain> <http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/*checkout*/hbase/Source/HBa se/Category/Arrow.hs?rev=HEAD&content-type=text/plain> -- Ashley Yakeley, Seattle WA

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OT: Project Idea: Aristo - A Framework for Embedding and Integrating High-Level Languages
by Shlomi Fish 08 Jan '03

08 Jan '03

I just returned from a dinner a few members of the Linux Israeli scene had with Richard Stallman (of GNU fame), as he his now visiting Israel. I got the opportunity to talk with him, and asked him the GNU project insistance of using guile for extensibility. He said the reason for that was that converting any other language to Scheme was a good a choice out of the fact Scheme was so simple, powerful and generic. I said converting Perl 5 code to Scheme would not be too trivial at all, and he said that was the case for converting Perl to almost everything else. :-) (despite this joke, I'm sure converting many languages to Scheme would be incredibly problematic) Then I noted that they should try the GIMP approach instead: and use a procedural database to integrate several languages back-ends. Then, he said that the type system would not be flexible enough for that. After a while that it was a challenge: create a bridge in ANSI C, that will be flexible enough to interoperate code between many virtual machines: perl 5, python, Ruby, Java, Haskell, O'Caml, etc. The codename is dubbed "Aristo", which is the Hebrew pronouncation and spelling of "Aristotle". (it's simpler and without the "totel" at the end). Now what I need is some advice on designing a type system: OK, I can have arrays of ints, and associative arrays from strings to ints, etc. But an array of records whose one field is an array of ints, the other a linked list of strings, the other an associative array from floats to this record itself, is something I'd rather not have to do type interference with. (usually you don't need such strict typing requirements). Here are some of my ideas, so far. Let me know what you think: 1. Aristo can only be written in ANSI C. Any other language will be unacceptable, as all implementations of the back-ends are written in ANSI C. 2. Aristo should have a nice nested namespace organization, with import and export features, package-scope globals etc. The gimp flat list (in which namespace collision is maintained by prefixes) is not scalable. 3. There should be a union type which is a type identifier followed by the type value, which could assume any value. It is possible some types will not be unions and will be plain types without the type value. 4. A standard enum can be used to store the object meta-type, and a standard pointer/long union can be used to store the data. Almost anything will be done using indirection. 5. Types should be declared using a machine-readable type definition, that can be understood by humans. 6. Maybe I can take a look at GIMP PDB for code and ideas, but probably not much. From my impression it's more of an ad-hoc system for the GIMP. 7. A procedure can restrict the types that pass to it, but I'm not sure to what extent. 8. I'll have to translate between conventions of different languages inside each language back-end. 9. It's first implementation will probably not have all the functionality I'd like it to have, but better to release early and release often. Your ideas and comments are most welcome. At the moment Aristo is at the pre-coding stage. (I'm just thinking about its design). But I'd rather have something well-designed and well-scrutinized in mind before I start actual coding. Regards, Shlomi Fish ---------------------------------------------------------------------- Shlomi Fish shlomif(a)vipe.technion.ac.il Home Page: http://t2.technion.ac.il/~shlomif/ He who re-invents the wheel, understands much better how a wheel works.

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Ambiguous defaults
by Ferenc Wagner 08 Jan '03

08 Jan '03

Hello, What's the way to express the following: a compound object is generally made up of two components with identical type. For this common case I'd like to provide some default methods, which take the object apart, operate on the parts and put the results back together. In other cases this default method should be overridden, eg. when the compound object isn't made of two identical components. My first try (below) doesn't work, can you recommend a solution? Thanks: Feri. \begin{code} module Test where class Component b where property :: b -> Int class Compound a where decompose :: Component b => a -> (b,b) additive :: a -> Int additive x = property l + property r where (l,r) = decompose x \end{code} ghci: test.hs:9: Ambiguous type variable(s) `b' in the constraint `Component b' arising from use of `property' at test.hs:9 hugs: ERROR test.hs:9 - Cannot justify constraints in default member binding *** Expression : additive *** Type : Compound a => a -> Int *** Given context : Compound a *** Constraints : Component b

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RE: Space explosion
by Simon Marlow 06 Jan '03

06 Jan '03

> > -- Specs contains the options for the simulation; > rollBounds simply > finds the lowest and highest possible rolls > > -- type RollValue = Int > > -- type RollValueDist = Array RollValue Int > > probDist :: Specs -> [RollValue] -> RollValueDist > > probDist specs rolls = > > accumArray (+) 0 (rollBounds specs) [(r, 1) | r <- rolls] > > It should be doing an in-place array update as it evaluates the > comprehension, but apparently something's being kept in memory longer > than it has an excuse for. It does do a sequence of in-place updates, but this is another instance of the foldl space leak. The additions aren't being performed strictly, so the array is filling up with suspensions. For example: accumArray (+) 0 (1,10) [(3,1), (3,1), (3,1), ...] would produce this sequence of arrays as accumArray processed its input list: { (3,1), ... } { (3,1+1), ... } { (3,1+1+1), ... } we should really have a strict version of accumArray (the lazy version *is* useful sometimes: you can refer recursively to the final array, for example). A quick workaround in your case (GHC only, I'm afraid) is to use UArray instead, available from Data.Array.Unboxed. These arrays contain unboxed values and are therefore strict by definition; and you can use all the usual array operations including accumArray. This workaround only applies if you're using one of the types supported by UArray though (Int, Char, and other base types). Cheers, Simon

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Using SGML/XML in literate code
by Peter Simons 06 Jan '03

06 Jan '03

I understand that I can mark-up code segments in a literate source module using the syntax: \begin{code} ... \end{code} But is it somehow possible to re-define this to be <haskellcode> ... </haskellcode> instead? I am asking because the latter style would enable me to write literate programs in DocBook, HTML, or any other SGML-based mark-up format. And _that_ would be very useful indeed. :-) -peter

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Re: Web Application Development
by Christopher Milton 06 Jan '03

06 Jan '03

--- Dominic Steinitz <dominic.steinitz(a)blueyonder.co.uk> wrote: [snip] > The original code was in PHP and MySQL so rather than re-invent the wheel I [snip] > So my question is: what should I do to re-write this in Haskell? I haven't tried it out, but I found this project on SourceForge: http://sourceforge.net/projects/functional-php/ on to the Haskell stuff... > From the discussion on this mailing list in June and reading through the > libraries section of haskell.org, I think there are the following options: > > HTML > > 1. Use Wash/CGI but maybe somehow move to mod_haskell if the webserver > performance isn't adquate. HaXml could also be quite useful for generating HTML or XML: http://www.cs.york.ac.uk/fp/HaXml/ > SQL > > 2. Use MySQL-HS which requires HaskellDirect or use HSQL. > > I'd be very grateful for any advice and any progress as a result of the > discussion in June. Ashley Yakeley has an interface to MySQL: http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/hbase/Source/Foreign/ and I was thinking of extending the DBI model as found in Perl and the libpq binding as listed on Haskell.org: http://www.astercity.net/~khaliff/haskell/dbi/dbi.tgz Searching Freshmeat and SourceForge for Haskell turns up a few things. Actually, there's quite a bit of alpha and beta Haskell code out on the net, but we don't seem to get very far with much of it... ===== Christopher Milton cmiltonperl(a)yahoo.com __________________________________________________ Do you Yahoo!? Yahoo! Mail Plus - Powerful. Affordable. Sign up now. http://mailplus.yahoo.com

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Editor Tab Expansion
by Jim Greer 03 Jan '03

03 Jan '03

Greets - [ Better late than never. ] Ran across this thread while perusing list trying to figure out whether to join or not. In the interest of sharing info with the great collective unconscious (i.e. Internet), thought I'd include part of my .vimrc so that Vim users (and others considering the same) can see how it handles the embedded tab/whitespace layout problems of various languages: set background=dark syntax on filetype on set textwidth=72 autocmd Filetype haskell set ai autocmd Filetype haskell set expandtab autocmd Filetype haskell set tabstop=8 autocmd Filetype haskell set shiftwidth=8 autocmd Filetype mail set ai autocmd Filetype mail set tabstop=4 autocmd Filetype mail set shiftwidth=4 autocmd Filetype ruby set ai autocmd Filetype ruby set tabstop=2 autocmd Filetype ruby set shiftwidth=2 A complete ('enhanced') install of Vim includes syntax recognition for a great number of file formats, including Haskell (apparently, haven't done much yet). Setting syntax on manages the syntax recognition part, so that you get colorized syntax. Setting filetype on recognizes files (by extension?), so that you can use the autocmd FileType haskell .... commands to tailor settings for particular filetypes. In this case, I'm using expandtab and setting the tab settings to 8, as mentioned previously. Cordially, Jim G p.s. http://www.vim.org/ p.p.s. I'm suprised that the *macs crowd hasn't chimed in with ways to configure their editos^Hr to set tabs differing by day of week, phase of moon, etc. -- James B. Greer <jgreer(a)midsouth.rr.com> [expires: 2004-12-27] 7F49 3D20 03C6 E859 E4F9 0735 E1F9 B259 21FA 1494

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03 Jan '03

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RE: Parsing date and time specifications
by Simon Marlow 03 Jan '03

03 Jan '03

> I think it is widely agreed that the time and date structures in the > standard libraries are brok^H^H^H^Hslightly less than useful. > > I suggest you feel free to rewrite it as you see fit, and then lobby > for its inclusion in the hierarchical libraries. > > (My preju^H^Hference would be to store a date-time internally in a > posix-like manner (seconds,microsecond since the epoch).) But a ClockTime *is* implemented as (seconds,picoseconds) since the epoch. CalendarTime is just a human-readable interpretation of the ClockTime, or a way to synthesise a ClockTime from a known date/time. (BTW Peter: the weekday/yearday in a CalendarTime are ignored by the toClockTime in GHC's Time library, so you can set them to anything). I'm still not sure I understand why the Time library is considered to be broken; sure I can see that our implementation of addToClockTime is wrong (adding one minute shouldn't always be the same as adding 60 seconds, for example), but is the design fundamentally wrong? GHC's TimeExts library (package lang) is allegedly a better implementation of addToClockTime/diffClockTime, if anyone is looking for one. Cheers, Simon

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Sets
by Mark T.B. Carroll 02 Jan '03

02 Jan '03

Am I right in thinking that sets of sets don't really work with GHC's Data.Set because "Set a" things aren't instances of Ord? How should I do sets of sets with it? (e.g. Set.mkSet [Set.unitSet 1] gives "No instance for (Ord (Set a))") -- Mark

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