February 2005 - Haskell-Cafe

Re: New to haskell: unresolved overloading question
by Christian Maeder 22 Feb '05

22 Feb '05

Blair Fraser wrote: > I'm new to haskell and I'm working through the class section of the > gentle tutorial. I decided to implement an ApproxEq class (just for > fun), but have run into problems. The code below works fine for units > and bools, but gives an unresolved overloading error for Floats and > Doubles. What's going on here? What does the error message mean and > what did I do wrong? (I'm in Hugs.) > > -- ApproxEq: My first class. > class (Eq a) => ApproxEq a where > (~=) :: a -> a -> Bool > x ~= y = x == y -- By default, check equality > > -- These two override default with same, just checking if it works > instance ApproxEq () where () ~= () = True > instance ApproxEq Bool where x ~= y = x == y > > -- These two override default with different > -- consider floating points equal if they differ by one or less > instance ApproxEq Float where x ~= y = (abs (x-y) <= 1) > instance ApproxEq Double where x ~= y = (abs (x-y) <= 1) More elegant seems to be: instance (Ord a, Num a) => ApproxEq a where x ~= y = (abs (x-y) < 1) However, this requires extensions to "Allow unsafe overlapping instances": hugs -98 +O ghci -fglasgow-exts -fallow-overlapping-instances -fallow-undecidable-instances -fallow-incoherent-instances > -- This one dosn't work either, but it just depends on the other two > instance ApproxEq a => ApproxEq [a] where > [] ~= [] = True > (x:xs) ~= (y:ys) = x ~= y && xs ~= ys > _ ~= _ = False > > Thanks, > Blair

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Re: New to haskell: unresolved overloading question
by Christian Maeder 22 Feb '05

22 Feb '05

Tomasz Zielonka wrote: > Why not forget about ApproxEq for () and Bool and simply define a > function? > > (~=) :: (Ord a, Num a) => a -> a -> Bool > x ~= y = abs (x-y) < 1 Indeed, this works best. The instance for lists is also not too important. Christian

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Literate Haskell
by Dmitri Pissarenko 22 Feb '05

22 Feb '05

Hello! I'm curious what experienced Haskellers think about using literate Haskell in daily work. It seems to me like a good idea, since during coding it often helps to write down one's thoughts (often, I find a solution to a complicated problem in this way). What are your experiences with using literate Haskell? TIA Dmitri Pissarenko -- Dmitri Pissarenko Software Engineer http://dapissarenko.com

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Re: [Haskell-cafe] Problems with building GCJNI 1.2
by Antony Courtney 21 Feb '05

21 Feb '05

Hi, Sorry for not jumping in on this discussion sooner. I am the author of GCJNI. GCJNI was developed back when Hugs was the dominant Haskell implementation and GreenCard was the easiest/best way to do foreign language bindings for Haskell because the FFI proposal was still in the formative stages. These days, the Haskell FFI addendum is stable and widely implemented, ghc has become much more widely available and easy to install and use, and (without wishing to sound harsh), GreenCard's FFI code generator was broken and useless the last time I tried to use it (a few years ago). I actually ported GCJNI to the Haskell FFI a few years ago, getting rid of ALL use of GreenCard. I used ghc's 'hsc2hs' tool to help with the marshalling, and made no attempt to port to hugs. I called this newer system 'hsjni'. It worked very well for my needs (using the Java2D rendering engine as the back-end for a prototype Haskell GUI system). I never released hsjni simply due to lack of time while finishing my thesis. I invested a little effort in trying to test hsjni with the latest JDK and get a release together in December, but it got snagged on a silly little bug in hsc2hs's handling of paths which have embedded spaces. A minor bug, to be sure, but since the default installation location for the JDK under Windows is under "C:\Program Files", the bug meant hsjni couldn't work out-of-the-box for the most common installation of the prerequisites. Simon Marlow has since fixed this bug in CVS head; I'm just waiting for the next release of ghc with this fix available before I try again. If you think you'd be interested in having a shot at building 'hsjni' (or you are interested in maintaining it), let me know, and I'll make the sources available for you. Caveat is that I haven't tested it since JDK 1.3, because of this hsc2hs issue. My appologies for not mentioning any of the above on the GCJNI web page. Hopefully the whole GCJNI release and web pages can be deprecated soon, with hsjni offered in its place. Kind Regards, -Antony -- Antony Courtney email: antony(a)aya.yale.edu New York, NY WWW: http://www.apocalypse.org/~antony

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[darcs #222] performance problem with massive changes in a single file
by David Roundy via RT 21 Feb '05

21 Feb '05

I'm cc'ing haskell-cafe on this darcs bug, since I imagine there might be a student (or professor) hanging around there who might be interested in the implementation of an efficient LCS algorithm in haskell. [markjugg - Sun Feb 20 09:15:57 2005]: > Typically in software, 1000's of line aren't changed at the time, which > is why this hasn't been an big issue before. Indeed. The issue here is that darcs uses the exact Hunt-Szymanski LCS algorithm when it computes the diff, while GNU diff uses an approximate --but faster--algorithm, which works "almost" as well. The approximate algorithm is reasonably well-documented, but more complex to implement than the exact one, just because it's harder to understand. This would be a reasonable project for an interested haskell CS student. The LCS is implemented in pure haskell 98, and is of signature lcs :: Ord a => [a] -> [a] -> [a] It's an interesting problem that's easily testable, and has different scaling behavior in different limits (finite alphabet, infinite alphabet, permutations of unique objects). David (ever optimistic for new contributors to darcs!) http://darcs.net

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ANN: Haskell news/community site
by John Goerzen 21 Feb '05

21 Feb '05

Hi, I'm setting up a Haskell news & community site. Check it out at: http://sequence.complete.org/ It supports aggregation of Haskell blogs and discussion from various sources (including this list). Any registered user can submit content for the front page, and all registered users can vote to approve or reject such submissions. The first thing to do is pick a real name. I hope that eventually this can be a *.haskell.org site. -- John

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Alternative lambda syntax (not entirely serious)
by Fritz Ruehr 21 Feb '05

21 Feb '05

I was introducing a fresh set of students to lambda abstraction in Haskell yesterday and had the following inspiration about a possible alternative syntax. I didn't end up showing the idea to the students -- too confusing -- but I thought I would float it here, as long as nobody takes it too seriously (this is the Cafe, after all :) ) . The idea arose from two things I'd done recently in class: introducing the underscore pattern as "an anonymous variable", and also commenting on the relationship between the final "->" in a type signature and the "=" of the corresponding function definition. So, since a lambda abstraction denotes an anonymous function, the underscore signifies anonymous things, and given that "->" of the current syntax connotes a type than a value, I thought "why not this syntax instead?": (_ x = x) -- the identity function (_ (x,y) = x) -- the fst function (_ f g x = f (g x)) -- composition In other words, at a lexical level we simply write "_" in place of "\" and "=" in place of "->". The use and placement of the "_" nicely indicates that what we are denoting is both anonymous and a function, and the use of the "=" makes it more parallel to the definition of a named function. This looks pretty nice in the right context, e.g., when talking about the kinds of code transformation one does when introducing lambda. For example: foo = map f xs where f x = blah VERSUS foo = map (_ x = blah) xs OK, so why not a serious proposal? Well first off, from a pedagogical perspective, we may not *want* to stress the similarity between the definition of a function and an expression which denotes one: this is a point of some confusion for newcomers, after all. Second, I imagine it could make parsing more difficult (although an *initial* underscore wouldn't otherwise come up in an expression context, right?). Third (and this is probably the death-knell), the use of "_" and "=" would be especially confusing in cases where an abstraction appears immediately on the RHS of a definition, or where one of the parameter patterns was itself an underscore. Or (and especially!) both; for example: const = (_ x _ = x) -- bleah! or worse yet: const x = (_ _ = x) -- yikes! Now, such definitions don't come up much in actual production code, I suppose, but they do tend to come up in classrooms, where the basic ideas are being introduced, and confusion would be especially unwelcome there. Finally, and not least, this proposal would further distance Haskell's syntax from the original lambda notation: this is not only a sad thing in general, but it would also make lambda-based T-shirt & merchandise logos even more obscure than they are now :) . -- Fritz

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Strange HTTP module behavior
by John Goerzen 21 Feb '05

21 Feb '05

Hello, I am trying to send a simple SOAP message to an Apache server. I got the HTTP library from the XML-RPC library and thought this would make an easy client. However, I'm having two problems. First, even though I am sending an accurate Content-Length header with my POST message, the server is timing out and returning a 500 Internal Server Error document. Second, after reading that error document, Network.HTTP is generating an error of its own. Here's an strace illustrating it: recvfrom(3, "ssResponse> </soapenv:Body>\n</soapenv:Envelope>", 1000, 0, NULL, N ULL) = 47 recvfrom(3, "", 1000, 0, NULL, NULL) = 0 shutdown(3, 1 /* send */) = 0 shutdown(3, 0 /* receive */) = 0 close(3) = 0 ioctl(1, SNDCTL_TMR_TIMEBASE or TCGETS, {B38400 opost isig icanon echo ...}) = 0 fcntl(2, F_GETFL) = 0x8002 (flags O_RDWR|O_LARGEFILE|0x800 0) write(2, "\nFail: ", 7) = 7 write(2, "Network.Socket.recv: end of file (end of file)\n", 47) = 47 write(2, "\n", 1) = 1 I *think* that what's happening is that ghc threw an exception for the empty recvfrom response, then the exception handler closed the conection, and then the exception was prnited to stderr. Oh, tcpdump verified that both client and server are sending Connection: close headers. Any ideas? -- John

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Re: Strange HTTP module behavior [PATCH]
by John Goerzen 21 Feb '05

21 Feb '05

On Fri, Feb 18, 2005 at 11:36:57PM +0100, Bjorn Bringert wrote: > John Goerzen wrote: > >It turns out that Network.Socket.recv raises an EOF error when it gets > >back 0 bytes of data. HTTP is expecting it to return an empty list for > >some reason. > > > >The below patch fixed it for me. > > [...] > > Hmm, strange. Is that recv behavior a bug or a feature? I don't know, but it's explicitly there whatever it is. From ghc 6.2.2: let len' = fromIntegral len if len' == 0 then ioError (mkEOFError "Network.Socket.recv") else peekCStringLen (ptr,len') It appears this change was committed to fptools in version 1.26 of Socket.hsc on July 15, 2002. http://cvs.haskell.org/cgi-bin/cvsweb.cgi/fptools/libraries/network/Network… Which arguably is not what one would expect recv to do, and in any case is undocumented at http://www.haskell.org/ghc/docs/latest/html/libraries/network/Network.Socke… Still, 2002 was awhile back, so I'm still surprised nobody else noticed. -- John

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Re: [Haskell-cafe] Literate Haskell
by yeoh＠cs.wisc.edu 20 Feb '05

20 Feb '05

> Date: Fri, 18 Feb 2005 21:21:00 +0100 > From: Dmitri Pissarenko <mailing-lists(a)dapissarenko.com> > > Hello! > > I'm curious what experienced Haskellers think about using literate > Haskell in daily work. > > It seems to me like a good idea, since during coding it often helps to > write down one's thoughts (often, I find a solution to a complicated > problem in this way). I use LHS by default. Here's some stats on the single LHS file I've been hacking away at: Lines of comments over code: 53% Char count of comments/code: 49% This is for a 754-line file. I'm real big on notes/comments, since I have often stared in bewilderment at my own code years later wondering what the heck I was doing. The puzzlement is unusual; normally, it's a good laugh. I use TextPad, and some of the comments are sidelined Haskell snippets, like on a pasteboard. Syntax coloring is active throughout the file, except in comments enclosed by "--/", which is my little kludge on TextPad to get it to work out right. Works nicely so far.

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