- Haskell-Cafe - Haskell.org

On syntactic sugar
by Lauri Oksanen 12 May '08

12 May '08

Hi, I'm writing my first real Haskell program and I came up with the following code snippet. --- let x' = x \+ dist \* dir nx' = normal geometry wi = (-1) \* dir in do (p, wo) <- brdfCosSampling reflector nx' wi let color' = p \** color q = min 1 (scalarContribution p) in do sampler <- biasedCoinSampler q (radianceSampler surfaces x' wo (q \* color')) (terminalRadianceSampler surfaces x' nx' ((1-q) \* color')) sampler --- This works just fine but I don't like the way I had to indent the code because of alternating lets and dos. I would like to indent the code more like an imperative code i.e. like this --- let { x' = x \+ dist \* dir ; nx' = normal geometry ; wi = (-1) \* dir ; } in do { (p, wo) <- brdfCosSampling reflector nx' wi ; let { color' = p \** color ; q = min 1 (scalarContribution p) ; } in do { sampler <- biasedCoinSampler q (radianceSampler surfaces x' wo (q \* color')) (terminalRadianceSampler surfaces x' nx' ((1-q) \* color')) ; sampler ; }} --- but without braces and semicolons. Following works also, but is ugly (and probably less efficient?). --- do x' <- return $ x \+ dist \* dir nx' <- return $ normal geometry wi <- return $ (-1) \* dir (p, wo) <- brdfCosSampling reflector nx' wi color' <- return $ p \** color q <- return $ min 1 (scalarContribution p) sampler <- biasedCoinSampler q (radianceSampler surfaces x' wo (q \* color')) (terminalRadianceSampler surfaces x' nx' ((1-q) \* color')) sampler --- Is there some nice trick to do this?

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Re: [Haskell-cafe] List concat
by Derek Elkins 12 May '08

12 May '08

On Mon, 2008-05-12 at 02:36 -0400, ajb(a)spamcop.net wrote: > G'day all. > > Quoting Andrew Coppin <andrewcoppin(a)btinternet.com>: > > > The function (++) :: [x] -> [x] -> [x] has O(n) complexity. > > That's not entirely true. > > When you call (++), it does O(1) work. If you evaluate k cons cells. > it takes O(min(k,n)) work. I think we can safely simplify that to O(k) (at least for sequential programs).

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help(!!)
by Galchin, Vasili 12 May '08

12 May '08

Hello, As I said in a Friday posting, I changed the version # of the cabal file that I am using, unix.cabal. Now my build environment is broken and I am dead in the water: vigalchin@ubuntu:~/FTP/Haskell/unix-2.2.0.0$ runhaskell Setup.hs configure Configuring unix-2.3.0.0... vigalchin@ubuntu:~/FTP/Haskell/unix-2.2.0.0$ runhaskell Setup.hs build Setup.hs: unix.cabal has been changed, please re-configure. vigalchin@ubuntu:~/FTP/Haskell/unix-2.2.0.0$ It seems like "runhaskell Setup.hs configure" isn't doing much after the unix.cabal change. How can I force the configure to really run? Do something with some of the config files?? Thanks, Vasili

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Re: [Haskell-cafe] saner shootout programs
by PR Stanley 12 May '08

12 May '08

>I don't know Haskell very well, but Paul: "I'm not racist but . . ." :-) >even I can tell, looking at, for >example, the N-body benchmark, that the Haskell code is probably not >type-safe, and the tricks used in it would not be usable in a larger >program (see below). > >The task is essentially a pure computation: take a list of bodies >having mass, position and velocity; apply Newton laws at discrete >intervals for a large number of times; return new positions and >velocities. > >I could write a C++ procedure that performs this task and have some >piece of mind regarding its type correctness, exception safety and >functional purity: side effects would be local to the procedure, >arguments passed as const or by value, the result returned by value, >no type casts or new/delete operators used. > >On the other hand, the Haskell code makes assumptions about the size >of double-precision floats (obviously not type-safe). Further, the >simulation is not a pure function. > >It is often argued that one only needs these dirty tricks in the most >time-consuming functions. However, if using imperative programming in >these "inner loop" procedures places them in IO monad, the "outer >loops" (the rest of the program - procedures that call it) will have >to go there as well. This makes me doubt the Haskell approach to >functional programming. > >If anyone has a version of the N-body benchmark, where the simulation >is a type-safe pure function, I would very much like to see and time >it. >_______________________________________________ >Haskell-Cafe mailing list >Haskell-Cafe(a)haskell.org >http://www.haskell.org/mailman/listinfo/haskell-cafe

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List concat
by Andrew Coppin 12 May '08

12 May '08

The function (++) :: [x] -> [x] -> [x] has O(n) complexity. If somebody were to invent some type that looks like [x] but actually uses some sort of tree rather than a linked list, you should be able to get O(1) concatenation. Has anybody ever implemented such a thing?

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Interesting critique of OCaml
by Don Stewart 12 May '08

12 May '08

An interesting critique of OCaml. http://enfranchisedmind.com/blog/2008/05/07/why-ocaml-sucks/ One phrase that stood out, regarding GHC's support for deforestation transformations like build/foldr and stream fusion: "Haskell is doing data structure level optimizations with the ease that most other compiler do peephole instruction optimization. This is a non-trivial result." Which I think really captures the joy of being able to write algebraic and data structure transformations, via rewrite rules, without having to extend the compiler -- all thanks to purity, laziness, and static typing. -- Don

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dropping hyphens and \n in words
by Ivan Amarquaye 11 May '08

11 May '08

Generally a hyphen is written at the end of the sentance when moving on to the next line and i managed to achieve this in haskell by using the "\n"- newline which places an index word in the next line i.e. if the words appear indexed like this...([1]),[mangoes] and a hyphen is applied, it becomes ([1],[mang-oes]) and it is valid in my function as i made it accept hyphens as part of a single word. Now my problem is this...I'm assuming that the hyphen normally comes at the end of a sentence like this: "there are so many guys ravis-hing our women" and this can be demonstrated in haskell by "\n" which places the words or characters following it in a new line like this: input: makeIndex"there are so many guys ravis\nhing our women" and output is: (([1],[there]),([1],[ravis]),([2],[hing])) where 1 means the first line and 2 the next. Now i want to write a function that would take away the hyphen and \n from all the words supposed to end on the first line and continue on the next and make all appear on the first line like this: all words in this form: "chip-\nheater" should become "chipheater". hope i can get some guidance on doing this. _________________________________________________________________ Connect to the next generation of MSN Messenger http://imagine-msn.com/messenger/launch80/default.aspx?locale=en-us&source=…

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ANN: xmonad-utils 0.1
by Gwern Branwen 11 May '08

11 May '08

Just dropping in a quick note: I've uploaded to Hackage the 0.1 release of xmonad-utils here: <http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmonad-utils>. There is of course a Darcs repo available as well: <http://gorgias.mine.nu/repos/xmonad-utils>. What is xmonad-utils? It's a couple of small Xlib programs which might be useful for Xmonaders. In particular: * hxsel can replace xclip, and was the foundation for the XMonad.Util.XSelection module in XMonadContrib/; it gets a copy of the copy-paste buffer to manipulate * hhp is a replacement for unclutter, the famous utility which hides your mouse when not in use and so cleans up your screen (but which is also famously unmaintained and buggy). I've personally found hhp to be very helpful, so I packaged it all up. Miscellaneous notes: * There is also a 'heval' program, which tries to provide an X prompt to a GHC eval through the GHC API, but that may or may not work for you (it doesn't compile for me, but I'm fairly sure that's because my 'directory' package is b0rked in an unrelated manner). * There is a requirement on cabal >= 1.4; this was needed because older Cabals didn't properly include in the sdist tarball one of the .hsc files. I don't believe it's actually needed to build. -- gwern nuclear Monarch NSADS Scud foundation World or Kilo SADMS M-x

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Cryptographic hash uniquness (was [Haskell-cafe] Simple network client)
by Peter Verswyvelen 11 May '08

11 May '08

>winds up having a write cache, which is mutable in practice. The >interesting thing is that the block's location is the cryptographic >hash of its contents, which leads to all sorts of neat properties (as >well as requiring immutability). That's interesting. When I developed a version control system for a customer, I also used a cryptographic hash as the database key of file+content in question, but I was afraid I might have clashes (two files with different content generating the same hash)... My intuition told me that the odds of two cryptographic hashes (on meaningful content) colliding was much less than the earth being destroyed by an asteroid... But this is just intuition... What does computer science tell us about this? Thank you, Peter

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Error: Improperly terminated character constant
by Ivan Amarquaye 11 May '08

11 May '08

I'm writing a function dRop to accept words ending in 'es' and drop the last two characters i.e. 'es'.eg. mangoes -> mongo but i keep on getting this error: "Improperly terminated character constant" after running this code which i have left below. Can i get any form of help from anyone in here...? ----------------------------------------- dRop :: String -> String dRop word = if drop (length word - 2) word == 'es' then take (length word - 2) word else word -------------------------------------------------- regards Amarquaye.Ivan _________________________________________________________________ Connect to the next generation of MSN Messenger http://imagine-msn.com/messenger/launch80/default.aspx?locale=en-us&source=…

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