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Natural Numbers: Best implementation?
by Mark Spezzano 13 Mar '09

13 Mar '09

Hi, I was wondering what the best way to implement Natural number would be. Is there a package which already does this? Here are some options: 1. Don’t bother. Just use Integer. 2. Use the type data Natural = Zero | Succ !Natural 3. Use the following definition taken from the Gentle Introduction to Haskell 98 newtype Natural = MakeNatural Integer toNatural ::Integer-> Integer toNatural x | x < 0 = error “Can’t create negative naturals!” | otherwise = MakeNatural x fromNatural :: Natural -> Integer fromNatural (MakeNatural i) = i and then... instance Num Natural where fromInteger = toNAtural x + y = toNatural (fromNatural x + fromNatural y) x – y = etc.. x * y = etc... Which method is best? So far, I’ve been picking option #1 – just leaving things as they are and using Integer to keep things simple. I’ve got that feeling that [2] would be fast and [3] would be slow. Comment appreciated on the merits of each. Cheers, Mark Spezzano No virus found in this outgoing message. Checked by AVG. Version: 7.5.557 / Virus Database: 270.11.12/1998 - Release Date: 12/03/2009 6:23 PM

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Combining haddock documentation from multiple packages
by Heikki Aitakangas 12 Mar '09

12 Mar '09

Is it possible to generate a "unified" documentation tree similar to http://www.haskell.org/ghc/docs/latest/html/libraries/index.html from packages installed via cabal from hackage? And if so, how does one go about doing it? -- Heikki Aitakangas

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Using a monad to decompose a function into functions
by Marcin Kosiba 12 Mar '09

12 Mar '09

Hi, I'm doing a bit of research into mobility models and I'm currently exploring implementation language choices for the simulator (yes, sadly it needs to be a custom one). I've been using Haskell here and there for some small tasks, and thought I should consider it as an implementation language for the simulator. While I already have an working implementation in Haskell, there is one thing that I would like to express in a more elegant way, but just can't figure out. The simulation algorithm requires expressing the node's mobility so that it is "stateless". The mobility model algorithm's type should be something like: mobility_model :: WorldState -> NodeState -> OtherInput -> (Action, NodeState) where Action can alter WorldState and the second NodeState is an altered input NodeState. I perform a form of speculative execution on mobility_model so sometimes I need to backtrack to a previous world and node state. This is all fairly simple stuff, and was just an introduction. What I do now is store an enum in NodeState and implement mobility_model as one big case statement. Yes, this is very imperative of me, I know. What I'd like to do is to express mobility_model, so that the code would look like: mobility_model world node input = do do_calculus emit_action if something then emit_action else emit_action do_calculus emit_action mobility_model world node input but I'd like to be able to alter world and node state before continuing from emit_action. I've tried to get this working by using the idea from http://www.haskell.org/pipermail/haskell/2005-April/015684.html but couldn't get the state-altering behavior I was looking for. I've also taken a look at http://monadicheadaches.blogspot.com/2008/01/python-25s-iterators-in-haskel…, the unified concurrency model and Control.Coroutine, but couldn't get the behavior I was going for. Thanks! Marcin Kosiba

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DSLs with {in,}equalities
by Andrew Hunter 12 Mar '09

12 Mar '09

Several times now I've had to define an EDSL for working with (vaguely) numeric expressions. For stuff like 2*X+Y, this is easy, looking pretty much like: > data Expr = Const Integer | Plus Expr Expr | Times Expr Expr > > instance Num Expr where > fromInterger = Const > (+) = Plus > (*) = Times &c. This lets me get a perfectly nice AST, which is what I want. When I want to be able to express and work with inequalities and equalities, this breaks. Suppose I want to write 2*X + Y < 3. I either have to: a) Hide Prelude.(<) and define a simple < that builds the AST term I want. b) Come up with a new symbol for it that doesn't look totally awful. Neither of these work decently well. Hiding Eq and Ord operators, which is what I effectively have to do for a), is pretty much a nonstarter--we'll have to use them too much for that to be practical. On the other hand, b) works...but is about as ugly as it gets. We have lots and lots of symbols that are already taken for important purposes that are syntactically "near" <,<=,==, and the like: << and >> and >>= for monads, >>> for arrows, etc. There...are not good choices that I know of for the symbols that don't defeat the purpose of making a nice clean EDSL for expressions; I might as well use 3*X + Y `lessthan` 3, which is just not cool. Does anyone know of a good solution, here? Are there good substitutions for all the six operators that are important (<,>,>=,<=,==,/=), that are close enough to be pretty-looking but not used for other important modules? Better yet, though a little harder, is there a nice type trick I'm not thinking of? This works for Num methods but not for Ord methods because: (+) :: (Num a) => a -> a -> a (<) :: (Ord a) => a -> a -> Bool i.e. the return type of comparisons is totally fixed. I don't suppose there's a good way to...well, I don't know what the *right* answer is, but maybe define a new typeclass with a more flexible type for < that lets both standard types return Bool and my expressions return Expr? Any good solution would be appreciated. Thanks, AHH

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Hand calculation of Bird's definition of zip using foldr
by R J 12 Mar '09

12 Mar '09

Can someone provide a complete hand calculation of zip [1,2,3] [4,5,6] using the following definition of zip, based on foldr: zip :: [a] -> [b] -> [(a, b)] zip = foldr f e where e ys = [] f x g [ ] = [] f x g (y : ys) = (x , y) : g ys foldr :: (a -> b -> b) -> b -> ([a] -> b) foldr _ e [] = e foldr f e (x : xs) = f x (foldr f e xs) This implementation of zip produces the expected result [(1, 4), (2, 5), (3, 6)], but I'm unable to do the hand calculation and don't understand why it works. Part of my problem is that "e" is defined as a function that takes one argument, I don't see how that fits in with the usual scheme for foldr, which, as I understand it, is: foldr f e [x1, x2, ...] = f x1 (f x2 (f x3 ...(f xn e)))... Thanks, as always, to all in this great community. _________________________________________________________________ Windows Live™: Life without walls. http://windowslive.com/explore?ocid=TXT_TAGLM_WL_allup_1a_explore_032009

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Re: [Haskell-cafe] ThreadScope: Request for features for the performance tuning of parallel and concurrent Haskell programs
by Don Stewart 12 Mar '09

12 Mar '09

karel.gardas: > Don Stewart wrote: > > marlowsd: > >> Ben Lippmeier wrote: > >> > >>> On 12/03/2009, at 12:24 AM, Satnam Singh wrote: > >>>> Before making the release I thought it would be an idea to ask people > >>>> what other features people would find useful or performance tuning. > >>>> So if you have any suggestions please do let us know! > >>>> > >>> Is it available in a branch somewhere to try out? > >> There are three parts to it: > >> > >> - some patches to GHC to generate the log files. The patches are > >> not yet in, but I hope to get them in in the next couple of weeks. > > > > Just as a meta-point, it makes a *lot* of sense for the runtime to > > support proper logging -- it's practically a microkernel after all, so > > logging makes as much sense for GHC's rts as it does for regular > > kernels. > > > > Now we just need a /proc for the rts, so I can peek at GC stats live. > > Seeing this thread and discussion direction I cannot resist to > temptation to ask: is anybody going to write rts DTrace[1] provider? I think this is clear next step in the investigations. -- Don

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Re: [Haskell-cafe] Dynamically typing TH.Exp at runtime
by Martin Hofmann 12 Mar '09

12 Mar '09

This is what hint does, isn't it? However, this still leaves it to me to infer what type [|a|] in [| reverse (a:b:[]) = (b:a:[]) |] is. Thanks a lot anyway. Martin On Thu, 2009-03-12 at 12:59 -0200, Daniel Gorín wrote: > would using ghc-as-a-library to type (strings with) haskell > expressions at runtime help? > > On Mar 12, 2009, at 12:37 PM, Martin Hofmann wrote: > > > I am doing meta-programming at runtime. So my program gets a full > > Haskell declaration in expression quotation ([d|...|]) modifies it and > > returns the modified expression. Therefore, I need type information of > > this expression, and any subexpression, at _runtime_ ! For example: > > > > [d| reverse x1 = y1 |] > > > > - rewrites_to -> > > > > [d| reverse x2:xs = y2:ys |] > > > > - rewrites_to -> > > > > [d| reverse x:xs = reverse xs ++ [x] |] > > > > reverse :: [a] -> [a] implies > > x2,y2 :: a > > x1,y1,xs,ys :: [a] > > > > TH.reify is not applicable, because I need the information at runtime > > and I am in IO. I suppose Data.Dynamic does not work either, because > > [| > > xs|] :: ExpQ and not [a]. > > > > So it looks like I need my own type checker and inference and tag each > > subexpression with its type. If so, I can even omit TH and use my own > > data type for the abstract syntax tree. This annoys me a bit, because > > for me it seems that all I need is already there. > > > > Did anybody have similar problems, because I shouldn't be the only one > > doing dynamic typing in a static language? Is there a Haskell > > implementation of the paper "Typing Dynamic Typing" by Baars and > > Swierstra (http://people.cs.uu.nl/arthurb/dynamic.html ), so I can try > > out if this helps me? > > > > Any help, comments, and how-to-suggestions are highly welcome. > > > > Thanks a lot, > > > > Martin > > > > > > > > _______________________________________________ > > Haskell-Cafe mailing list > > Haskell-Cafe(a)haskell.org > > http://www.haskell.org/mailman/listinfo/haskell-cafe

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Data.Binary patches?
by Denis Bueno 12 Mar '09

12 Mar '09

I've got a small patch for Data.Binary. Should I post it here, or is there some more appropriate forum? http://code.haskell.org/binary/ doesn't specify. Thanks, Denis

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"The Curse of Our Times" in different languages
by Jamie 12 Mar '09

12 Mar '09

See Black-Scholes model/option formula being implemented in different languages including Haskell. http://www.espenhaug.com/black_scholes.html Jamie

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ThreadScope: Request for features for the performance tuning of parallel and concurrent Haskell programs
by Satnam Singh 12 Mar '09

12 Mar '09

Donnie Jones, Simon Marlow and I have been working on infrastructure for logging run-time events and a graphical viewer program called ThreadScope. Hopefully these features will make it into the next release of GHC. We hope the event-log viewer ThreadScope will be useful for the performance tuning of parallel and concurrent Haskell programs. You can see a few screen shots at the program's website http://raintown.org/threadscope Before making the release I thought it would be an idea to ask people what other features people would find useful or performance tuning. So if you have any suggestions please do let us know! Cheers, Satnam Singh ________________________________ Satnam Singh Microsoft 7 JJ Thomson Avenue Cambridge CB3 0FB United Kingdom Email: satnams(a)microsoft.com<mailto:satnams@microsoft.com> UK tel: +44 1223 479905 Fax: +44 1223 479 999 UK mobile: +44 7979 648412 USA cell: 206 330 1580 USA tel: 206 219 9024 URL: http://research.microsoft.com/~satnams Live Messenger: satnam(a)raintown.org<mailto:satnam@raintown.org>

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