October 2009 - Haskell-Cafe

Non-traversable foldables
by Martijn van Steenbergen 09 Oct '09

09 Oct '09

Hallo café, Can anyone think of datatypes that are Foldable but not Traversable? If not, what is the purpose of having a separate Foldable class? Thanks, Martijn.

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Different semantics in "identical" do statement?
by staafmeister 09 Oct '09

09 Oct '09

In my program do x <- do blah <- someFunc return blah return $ Constructor x behaves differently from do blah <- someFunc return $ Constructor blah where the dots are identical. I would think that these programs should behave identically, by the monad laws. The result of my program is that the second gives correct behaviour, while the first loops forever. Greetings, Gerben -- View this message in context: http://www.nabble.com/Different-semantics-in-%22identical%22-do-statement--… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Applicative do?
by Philippa Cowderoy 09 Oct '09

09 Oct '09

I do a lot of work with parsers, and want to do more using Applicatives. That said, I'm finding it a little tedious being forced to use pointless style for a task that's well-suited to having a few names around. The idea of an applicative do notation's been kicked around on #haskell a few times (I can't find any trace of it on the mailing list, but I confess to not having searched too hard), so I thought I'd propose it here. The basic idea is to turn this: do a <- f g b <- h pure $ foo a b into this: (\a b -> pure $ foo a b) <*> (f <*> g *> h) Aside from changing >>= and >> into <*> and *>, the most significant difference from monadic do is that all the generated lambda abstractions go in front of the final "return" statement which is then fmapped across the rest of the code. Bindings are thus only in scope in the "return" statement. I believe sugared let statements can be handled similarly so long as they respect the binding discipline. This leads us to the bikeshed topic: what's the concrete syntax? The obvious way is to replace do with a new keyword - for example, ado for "applicative do". There's a nice alternative though: we can check whether a do statement meets the binding rules for an applicative block and treat it as one if so, or a monadic one if not. While not all Monads are Applicatives, code can readily be changed back using the WrappedMonad newtype - whereas existing code needn't turn on the appropriate extension in the first place. Thoughts, comments? -- flippa(a)flippac.org

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Num instances for 2-dimensional types
by Sönke Hahn 09 Oct '09

09 Oct '09

Hi! I often stumble upon 2- (or 3-) dimensional numerical data types like (Double, Double) or similar self defined ones. I like the idea of creating instances for Num for these types. The meaning of (+), (-) and negate is clear and very intuitive, i think. I don't feel sure about (*), abs, signum and fromInteger. I used to implement fromInteger n = (r, r) where r = fromInteger n , but thinking about it, fromInteger n = (fromInteger n, 0) seems very reasonable, too. Any thoughts on that? How would you do it? btw: These are two examples i looked at in hackage: Data.Complex.Complex (which is special, cause it is mathematically defined, what (*), abs, signum and fromInteger should do (i think)) and Physics.Hipmunk.Common.Vector (http://hackage.haskell.org/packages/archive/Hipmunk/5.0.0/doc/html/Physics- Hipmunk-Common.html#9) Sönke

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ANNOUNCE: htzaar-0.0.1
by Tom Hawkins 09 Oct '09

09 Oct '09

HTZAAR is a Haskell implementation of TZAAR, a great little two-player abstract strategy game designed by Kris Burm. TZAAR won Games Magazine's Game-of-the-Year in 2008. TZAAR has some interesting game dynamics. At each turn a player must decide whether to attack or strengthen his own pieces. The unbalanced number of tzaars, tzarras, and totts pieces leads to several phase transitions in the game. At first tzaars are targeted, but once they're fortified, tzarras become the targets, then totts. It also has a rather distinct transition from mid to end game. Midway through, most effort is on positioning power pieces for attack. But as more pieces come off the board, movement is restricted to the point where groups become isolated from others, which can work for or against you. Anyway, I thought the game would be a fun AI problem. HTZAAR comes with a very simple AI opponent (lame), but I did my best to structure the interface to make it easy to create new AI strategies. I hope a few people try to build and contribute AI strategies to HTZAAR. And if there's anyway I can improve the AI interface, let me know. HTZAAR can also pit AI against AI. The following is a game between two instances of the "lame" AI. For an idea of the complexity of TZAAR, the number in parentheses indicates the number of possible moves at each turn. TZAAR has a large number of choices at each turn because each turn consists of two different moves (n^2). Typical games are between 15 to 25 turns. $ htzaar -s 22 lame lame 1. white (lame) : D7 -> E7 (42) 2. black (lame) : F6 -> F5 G1 -> F2 (6355) 3. white (lame) : D6 -> E6 A5 -> A4 (5867) 4. black (lame) : F3 -> E4 H2 -> G2 (5227) 5. white (lame) : I2 -> I1 C5 -> C6 (4992) 6. black (lame) : F5 -> G4 H1 -> H3 (4345) 7. white (lame) : H5 -> G6 C6 -> C7 (4176) 8. black (lame) : F7 -> E7 D4 -> C3 (4265) 9. white (lame) : G6 -> E5 H4 -> G5 (3464) 10. black (lame) : H6 -> I5 F2 -> E2 (2870) 11. white (lame) : E5 -> G4 D2 -> D1 (2303) 12. black (lame) : B2 -> C2 G7 -> I5 (2742) 13. white (lame) : G5 -> I5 E6 -> I3 (1965) 14. black (lame) : E7 -> B5 G3 -> E3 (2055) 15. white (lame) : E1 -> F1 I4 -> I3 (1303) 16. black (lame) : E2 -> D1 C4 -> D5 (1656) 17. white (lame) : I5 -> F8 C7 -> I3 (913) 18. black (lame) : B5 -> A4 E4 -> B4 (1480) 19. white (lame) : D3 -> C2 B1 -> C2 (608) 20. black (lame) : B4 -> B6 C3 -> C2 (526) 21. white (lame) : D8 -> E8 F1 -> F4 (185) 22. black (lame) : H3 -> G4 B3 -> B6 (314) 23. white (lame) : C1 -> A1 F4 -> G4 (47) 24. black (lame) : B6 -> E8 E8 -> F8 (125) black (lame) Wins! Screenshots and links: http://tomahawkins.org Known bugs: Occasionally HTZAAR throws a Prelude.head: empty list. I'm still trying to track this down.

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Can HXT and Parsec 3 be used in the same library?
by John Millikin 09 Oct '09

09 Oct '09

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 I'm writing a library that needs to parse both plain text (with Parsec) and XML (with HXT). HXT's .cabal file specifies that it only works with parsec (>= 2.1 && < 3), but it still builds when I depend on parsec >= 3. It's only during "cabal install" that the following error is displayed: - -------------------------------------------- Resolving dependencies... cabal-1.7.3: cannot configure hxt-8.3.1. It requires parsec >=2.1 && <3 For the dependency on parsec >=2.1 && <3 there are these packages: parsec-2.1.0.0 and parsec-2.1.0.1. However none of them are available. parsec-2.1.0.0 was excluded because dbus-core-0.5 requires parsec >=3.0.0 parsec-2.1.0.1 was excluded because dbus-core-0.5 requires parsec >=3.0.0 - -------------------------------------------- Is there a good solution to this? I was thinking about changing my library to use Parsec 2, but that just pushes the problem to my users, it doesn't really solve it. -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) iEYEARECAAYFAkrI3BgACgkQIkiy1+fm1TrmJACghhgKDBJNIIa9OVhrZ6zrSii+ HEYAnjqfi0KiIQSNrhGCi6eN1DQoHHs+ =aODO -----END PGP SIGNATURE-----

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type inference question
by minh thu 09 Oct '09

09 Oct '09

Hi, I'd like to know what are the typing rules used in Haskell (98 is ok). Specifically, I'd like to know what makes let i = \x -> x in (i True, i 1) legal, and not let a = 1 in (a + (1 :: Int), a + (1.0 :: Float)) Is it correct that polymorphic functions can be used polymorphically (in multiple places) while non-functions receive a monomorphic type ? Also, I'd like to know why id id True is permitted but not (\f -> f f True) id Is it possible to change the later expression so it type checks ? Are there any type inference algorithms that would accept it ? I'm asking this because I have implemented a simple type inference algorithm following PLAI and I can infer the type for id id True (because I use fresh type variable names for id whenever I apply it before unification) and have an occur check fail for the later expression, just like in haskell. Thanks, Thu

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Any example of concurrent haskell application?
by Daryoush Mehrtash 09 Oct '09

09 Oct '09

I am trying to learn more about concurrent applications in Haskell by studying an existing a real application source code. I would very much appreciate if you can recommend an application that you feel has done a good job in implementing a real time application in Haskell. Daryoush

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GPipe example and screenshot
by Tobias Bexelius 08 Oct '09

08 Oct '09

I've put a simple GPipe example (including a screenshot) on the haskellwiki now, showing off an animated spinning box. You'll find the page here: http://www.haskell.org/haskellwiki/GPipe Later on, I will add more examples, and also some kind of GPipe-tutorial. Hang in there... Cheers Tobias Bexelius _________________________________________________________________ Windows Live: Gör det enklare för dina vänner att se vad du håller på med på Facebook. http://www.microsoft.com/windows/windowslive/see-it-in-action/social-networ…

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New seed. New random number. Right? Wrong.
by michael rice 08 Oct '09

08 Oct '09

I'm entering a new number to make a different seed for my generator each time through the loop, but my resulting numbers are anything but random. Where am I going wrong? Michael ============================= import System.Random import Control.Monad(when) main = do numberString <- getLine when (not $ null numberString) $ do let seed = read numberString let (randNumber, newGen) = randomR (1,6) (mkStdGen seed) :: (Int, StdGen) in putStrLn $ "Number is " ++ show randNumber main ============================= [michael@localhost ~]$ runhaskell zz.hs 12317 Number is 6 12371 Number is 6 37829 Number is 6 8713 Number is 6 71392 Number is 5 29492 Number is 6 41089 Number is 6 28483 Number is 6 78687 Number is 5 40703 Number is 6 86910 Number is 5 15608 Number is 6 56155 Number is 5 31575 Number is 6 72231 Number is 5 51815 Number is 6 20189 Number is 6 71533 Number is 5 17755 Number is 6 91161 Number is 5 [michael@localhost ~]$

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