- Haskell-Cafe - Haskell.org

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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by Neslihan ÇETİN 03 Jan '03

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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Lazy streams and unsafeInterleaveIO
by Jyrinx 02 Jan '03

02 Jan '03

As an experiment for a bigger project, I cooked up a simple program: It asks for integers interactively, and after each input, it spits out the running total. The wrinkle is that the function for calculating the total should be a non-monadic stream function (that is, type [Integer] -> [Integer] so that runningTotals [1,2,3,4,5] == [1,3,6,10,15]). The task is then to write a function to return a stream of integers, grabbing them from IO-land lazily (a la getContents). My first attempts had it not displaying a running total until all input (terminated by an input of 0) had finished, at which point it spit out all the totals (i.e. it wasn't an interactive program anymore). I poked around in the docs and on the Web for a while, and found out about unsafeInterleaveIO, which solved the problem neatly (after I modified runningTotals to be less eager, as it was reading ahead by an extra integer each time). I ended up with the attached code (for GHC 5.04.2). My question is this: Is there a more elegant (i.e. non-"unsafe") way to do this? I vaguely recall from the Hudak book (which I unfortunately don't have convenient at the moment) that he used a channel for something like this (the interactive graphics stuff), but IIRC his system would be overkill for my application (including the bigger project). It doesn't seem like it should need any black magic, and concurrency (which channels need, right?) doesn't appear worth the hassle. Really, my desire comes down to a simple, safe, single-threaded way to write a function to generate a lazy stream. Is there such? Luke Maurer jyrinx_list(a)mindspring.com -- running-total -- Haskell program that takes integers as input, outputting a running total -- after each input -- Demonstrates use of lazy streams module Main where import IO import System.IO.Unsafe import Monad runningTotals :: [Integer] -> [Integer] runningTotals [] = [] runningTotals (x:xs) = rt' 0 (x:xs) where rt' tot (x:xs) = (tot+x) `seq` (tot+x):(rt' (tot+x) xs) rt' _ [] = [] -- Note that runningTotals does what appears to be a stateful calculation when -- numbers are read one at a time; however, lazy streams allow this to be a -- pure function. Haskell is cool. inputNumbers :: IO [Integer] inputNumbers = do x <- putStr "? " >> readLn if x == 0 then return [] else do xs <- (unsafeInterleaveIO inputNumbers) return (x:xs) main = do numbers <- inputNumbers mapM_ (putStrLn . (flip shows) "") (runningTotals numbers)

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by promotion＠callez.com 31 Dec '02

31 Dec '02

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RE: AW: slide: useful function?
by Mark Carroll 27 Dec '02

27 Dec '02

On Mon, 2 Dec 2002, David Bergman wrote: (snip) > Till then, we "Haskellers" will probably continue expressing our > patterns either directly in Haskell or using highly formal language, > with terms such as "catamorphisms". > > The virtue, and weakness, of traditional design patterns is their > vagueness and informal character, making them (1) comprehensible to the > 90% of the developer community not familiar with category theory but (2) (snip) If there are any good ways in which non-mathematicians can get to grips with these terms from category theory, they would be well worth promoting. For example, despite having a good computer science degree (in which I was at least introduced to FP, proof, etc. and even learned to draw the dual graph of hypercubes) I'm really not equipped to understand catamorphisms in terms of algebras and homomorphisms, and don't currently have time to take the math degree I fear I'd need in order to do so. Last time I was looking at category theory books I think I came to the conclusion that Lawvere and Schanuel cover things kindly but Pierce seemed to get the syllabus right, so the "right" book wasn't quite out there. My understanding of monads is already a matter of record. Does anyone know of a friendly text that might help new Haskellers to understand functors, etc. and what they mean for program design? I'm not averse to the formal language per se if it can be easily acquired; right now, I worry that I'm using Haskell suboptimally because, not only do I not know the terminology well, but I fear that I'm not even cognisant of the concepts that these terms represent. In a nutshell: if these category theory concepts indeed have an important impact in Haskell land, how to introduce them to working Haskell programmers well enough that they can use them in engineering software that's at least half as good as it could be? (I'm making the assumption here that it would be good for Haskell to be much more widely used - it shouldn't solely be for researchers.) -- Mark

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RE: Record of STRefs better than STRef to a Record?
by Simon Marlow 27 Dec '02

27 Dec '02

> If I use an STRef to a record, will a new record be created > each time I want > to update a single field? Or can I expect GHC to optimize it > and have the field of the record updated in place? You'll get a new record for each update. This might not be so bad though, depending on the number of fields in your record. > Right now I'm using a record of STRefs, like: > data E s = E{ > refi :: STRef s Int, > refc :: STRef s Char > } > > but it can get a little messy, since thread s propagates to > every datatype that uses the record type in it's definition. Here's another trick if you use this route: data E s = E{ refi :: !STRef s Int, refc :: !STRef s Char } and compile with -funbox-strict-fields. This will eliminate the boxing of the STRefs. Cheers, Simon

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ABD G��menlik/Green Card 2003 y�l� kay�tlar�m�z ba�lam��t�r...
by G��MENL�K/ABD 27 Dec '02

27 Dec '02

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