December 2009 - Haskell-Cafe

buildExpressionParser prefix recursion
by Warren Harris 02 Dec '09

02 Dec '09

I was wondering why Parsec's buildExpressionParser doesn't allow prefix expressions to be handled recursively, e.g. given a prefix "!" operation, it seems that "!!a" could parse without requiring parentheses ("!(!a)"). Is there an easy way to extend it? (I have a rich expression grammar I'd like to parse which includes several prefix forms, and it would be a shame to have to abandon buildExpressionParser since it is so concise and convenient.) Thanks, Warren

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university courses on type families/GADTs?
by Tom Schrijvers 02 Dec '09

02 Dec '09

Hello Haskell Cafe, I was wondering whether there are any universities that teach about Haskell type families or GADTs? Thanks, Tom -- Tom Schrijvers Department of Computer Science K.U. Leuven Celestijnenlaan 200A B-3001 Heverlee Belgium tel: +32 16 327544 e-mail: tom.schrijvers(a)cs.kuleuven.be url: http://www.cs.kuleuven.be/~toms/ Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm

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Patches and forks for GHC 6.12
by Robin Green 02 Dec '09

02 Dec '09

Some packages will need modifications to build or work with GHC 6.12 (in some cases, just modifications to the .cabal file). I've created this wiki page to track work people have done on that which hasn't yet been included into official packages or repositories: http://haskell.org/haskellwiki/Patches_and_forks_for_GHC_6.12 Please add any ones you know of (or just email me and I'll add them for you). -- Robin

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Trying to sort out multiparameter type classes and their instances
by Jeremy Fitzhardinge 01 Dec '09

01 Dec '09

I'm playing around with some types to represent a game board (like Go, Chess, Scrabble, etc). I'm using a type class to represent the basic Board interface, so I can change the implementation freely: class Board b pos piece where -- Update board with piece played at pos play :: b pos piece -> pos -> piece -> b pos piece -- Query pos to get piece (Nothing if off board) at :: b pos piece -> pos -> Maybe piece -- Empty board empty :: b pos piece and a Position on the board is represented thus: class Position p where up :: p -> p down :: p -> p left :: p -> p right :: p -> p With a concrete implementation using a tuple: instance (Enum c,Enum r) => Position (c,r) where up = second pred down = second succ left = first pred right = first succ My initial Board is a function: position -> Maybe piece, but I'm having a hard time writing the instance for it. My first attempt is: instance Board (pos -> Maybe piece) pos piece where empty = \_ -> Nothing at = ($) play b pos piece = move where move pos' | pos' == pos = Just piece | otherwise = b pos' but ghci complains: board.hs:34:15: Kind mis-match Expected kind `* -> * -> *', but `pos -> Maybe piece' has kind `*' In the instance declaration for `Board (pos -> Maybe piece) pos piece' Playing around with parentheses on the instance line got various similar messages, but I couldn't get anything to work. What am I missing here? One thing that strikes me is that "Board (pos -> Maybe piece) pos piece" has a lot of redundancy, and I'm wondering if I'm defining the Board type class wrong in the first place. Given that the "b" type parameter necessarily defines the position and pieces, I tried using dependent types: class Board b | b -> pos, b -> piece where ... but this complains "Not in scope: type variable `pos'/`piece'", so I tried sprinkling some existential types around as well: class (forall pos piece. b pos piece) => Board b | b -> pos, b -> piece where ... (with and without the dependent types) but this just complains "malformed class assertion". As you can probably tell, I'm just thrashing around trying things without really understanding what's going on here, so I'm hoping someone can give me some useful pointers. J

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Re: [Haskell-cafe] university courses on type families/GADTs?
by Manuel M T Chakravarty 01 Dec '09

01 Dec '09

Tom Schrijvers wrote, > I was wondering whether there are any universities that teach about Haskell type families or GADTs? I do in my course "Language-based Software Safety" (both TFs and GADTs). It's an advanced, research-oriented course for 4th year undergraduate and for postgraduate students. (It wasn't offered last year and this year, but will be again offered next year.) Manuel

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Re: [Haskell] ANN: NoSlow - Microbenchmarks for array libraries
by Henning Thielemann 01 Dec '09

01 Dec '09

On Fri, 27 Nov 2009, Roman Leshchinskiy wrote: > You can get more information (including the ugly tables) from my blog > > http://unlines.wordpress.com/2009/11/27/noslow Btw. storablevector supports 'zip' using http://hackage.haskell.org/package/storable-tuple but you may also use 'zipWith' with an atomar result type for testing. I found that the dependency on Cairo and GTK originates from criterion. I have to build criterion with 'cabal install -f-chart criterion'. Unfortunately I cannot do 'cabal fetch criterion' in order to look into the cabal file, before the dependencies are resolved ...

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module export question
by Sean McLaughlin 01 Dec '09

01 Dec '09

Say I have the following module: ---------------------------------------- module A ( T(T) , t , val ) where data T = T { t :: Int } val :: T val = T 7 ---------------------------------------- When I use A with the following imports, I don't expect this to work, but it does: import qualified A import A(T(..)) main = putStrLn $ show $ t A.val The problem is that I explicitly didn't export 't' as an element of T (by not writing T(..)). Am I just misunderstanding how exports work? I couldn't figure out what the correct behavior should be by looking at the 98 report. Sean

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lazy logging (was: Are there standard idioms for lazy, pure error handling?)
by Evan Laforge 01 Dec '09

01 Dec '09

This is only peripherally related, but I also have a lot of list functions that can possibly be an error, but usually processing can continue. So they tend to return [Either Error Result]. I have another function thus: -- A foldr version is not lazy enough and overflows the stack. partition_either [] = ([], []) partition_either (x:xs) = let (ls, rs) = partition_either xs in case x of Left l -> (l:ls, rs) Right r -> (ls, r:rs) I was a little surprised I couldn't find this in the standard library... or maybe it is? Anyway, the trick is logging the errors while incrementally processing the non-errors. Logging them first forces the whole thing. Mixing the logging in with the processing forces the processing to be in IO and is not as nice as having it separate. If the next processing step is also pure, the errors must be propagated. 'map (fmap f)' should do it, as long as the errors are all the same type. It's still not ideal because the logs happen in a bunch at the end, and are still taking up memory meanwhile. The only thing I can think of is the dreaded lazy IO... plain unsafePerformIO wouldn't work because nothing is forcing it. It would be sort of an output version of getContents, but it would have to have some magic to know when a thunk has been evaluated without forcing it. But hey, the debugger does it! And the RTS already happily logs things in the middle of pure computation, so this would just be giving programmers access to it... so it's not *so* crazy :)

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Fwd: Timing and Atom
by Lee Pike 01 Dec '09

01 Dec '09

[Tom -- resending my reply---I forgot to post to the list.] Neil, could you provide a reference (more on the practical side than the theory side) for the Stochastic Process Algebras you mention? And is there an embedding in Haskell? :) Lee Begin forwarded message: From: Lee Pike <leepike(a)gmail.com> Date: November 30, 2009 10:14:33 PM PST To: Tom Hawkins <tomahawkins(a)gmail.com> Subject: Re: Timing and Atom Tom, Thanks a lot for the detailed reply! > If you need greater timing resolution than what is provided by the > main loop, then I think the only option is to reference a hardware > counter. Right, of course. Use a hardware counter if you're not using a RTOS. That was the quick answer I wasn't considering. :) > I never considered running Atom generated functions in an asynchronous > loop until you posted your "Atomic Fibonacci Server" example Yeah, I always think it's fun to use synchronous languages asynchronously. Usually people think of trying to implement synchrony from asynchronous languages. > But there is a wide field of applications where this approach would > work just > fine -- not having to worry about meeting a hard loop time would > certainly simplify the design. I think it's a trade-off, right? You don't worry as much about time, but you have to worry about doing synchronization using handshakes or similar protocols. I know folks who make fault-tolerant systems generally prefer synchronized systems since asynchrony gets really hard once you start considering the possibility of random faults (See, for example, <http://embedded.eecs.berkeley.edu/giotto/> or _Real-Time Systems_ by Hermann Kopetz). > Could some form of static analysis be applied to provide the timing > guarantees? Of the asynchronous system (e.g., the "Fib. server")? In something like that, it should function under any possible schedule because of the handshakes (I should model-check it to prove that). Did I misunderstand your point? Thanks again for answering my question! Lee

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KiCS (Curry to Haskell interpreter) problem
by Pasqualino "Titto" Assini 01 Dec '09

01 Dec '09

Hi, I am playing around with KiCS and I have a strange problem, when I evaluate a goal the variable bindings are not displayed, I see only the value of the expression. The same expression evaluated in pakcs (another curry interpreter) displays the bindings correctly. Is this a known bug? I would have contacted the author but his email in not in the haskell cabal file. Incidentally, is anyone else using KiCS ? Any comments/remarks ? Best, titto

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