[Haskell-cafe] Subject: ANNOUNCE: grid-3.0.1 (tile maps for board games or maths)
I'm happy to announce a new major release of the grid package: http://hackage.haskell.org/package/grid https://github.com/mhwombat/grid/wiki (wiki) WHAT'S NEW: Functions for reporting the boundary and centre of bounded grid have been added, along with some miscellaneous new utility functions. IMPORTANT: The order of parameters for some functions has changed to make it easier to use them with map and fold operations. ABOUT GRID: Grid provides tools for working with regular arrangements of tiles, such as might be used in a board game or self-organising map (SOM). Grid currently supports triangular, square, and hexagonal tiles, with various 2D and toroidal layouts. If you need a tile shape or layout that isn't currently provided, please let me know. See Math.Geometry.Grid for an example of how to use the package. Suggestions for improvement are welcome. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Subject: ANNOUNCE: grid-3.0.1 (tile maps for board games or maths)
On 18/02/13 13:41, Amy de Buitléir wrote: I'm happy to announce a new major release of the grid package: http://hackage.haskell.org/package/grid https://github.com/mhwombat/grid/wiki (wiki) After taking a peek at the documentation: have you considered removing the size function from Grid? It is the only function that actually uses the type parameter 's'. If you really need it, I would suggest putting it in a separate class, class HasSize a s | a - s where size :: a - s It might also be useful to add a rectangular grid type where diagonally adjacent cells are also neighbors. Another interesting idea is to have modifier types that change which cells are neighbors, for example you could have class Colinear g x | g x where -- | Are three points separated by moves in the same direction? isColinear :: g - x - x - x - Bool -- neighbors are separated by diagonal moves newtype Diagonal g = Diagonal g instance (Grid g, Colinear g x) = Grid (Diagonal g) x where neighbors g x = [z | y - neigbhors x, z - neigbhors y , not (isColinear x y z)] newtype Rook g = ... newtype Knight g = ... -- etc. Twan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] generalized, tail-recursive left fold that can finish tne computation prematurely
Dear Haskellers, while playing with folds and trying to implement `!!` by folding, I came to the conclusion that: - `foldr` is unsuitable because it counts the elements from the end, while `!!` needs counting from the start (and it's not tail recursive). - `foldl` is also unsuitable, because it always traverses the whole list. I came up with the following tail-recursive generalization of `foldl` that allows exiting the computation prematurely: foldlE :: (a - c) - (a - b - Either c a) - Either c a - [b] - c foldlE f g = fld where fld (Left c) _ = c fld (Right a) []= f a fld (Right a) (x:xs)= fld (g a x) xs `foldl` can be defined from it as foldl'' :: (a - b - a) - a - [b] - a foldl'' f z = foldlE id ((Right .) . f) (Right z) and `!!` as: -- Checks for a negative index omitted for brevity. index :: Int - [a] - a index i = foldlE (error $ No such index) f (Right i) where f 0 x = Left x f n _ = Right (n - 1) Is something like that already available somewhere? Best regards, Petr Pudlak ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] generalized, tail-recursive left fold that can finish tne computation prematurely
Dear Haskellers, while playing with folds and trying to implement `!!` by folding, I came to the conclusion that: - `foldr` is unsuitable because it counts the elements from the end, while `!!` needs counting from the start (and it's not tail recursive). - `foldl` is also unsuitable, because it always traverses the whole list. I came up with the following tail-recursive generalization of `foldl` that allows exiting the computation prematurely: foldlE :: (a - c) - (a - b - Either c a) - Either c a - [b] - c foldlE f g = fld where fld (Left c) _ = c fld (Right a) []= f a fld (Right a) (x:xs)= fld (g a x) xs `foldl` can be defined from it as foldl'' :: (a - b - a) - a - [b] - a foldl'' f z = foldlE id ((Right .) . f) (Right z) and `!!` as: -- Checks for a negative index omitted for brevity. index :: Int - [a] - a index i = foldlE (error $ No such index) f (Right i) where f 0 x = Left x f n _ = Right (n - 1) Is something like that already available somewhere? Best regards, Petr Pudlak ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] generalized, tail-recursive left fold that can finish tne computation prematurely
Hi. while playing with folds and trying to implement `!!` by folding, I came to the conclusion that: - `foldr` is unsuitable because it counts the elements from the end, while `!!` needs counting from the start (and it's not tail recursive). What is the problem with the following definition using foldr? index :: Int - [a] - a index n xs = foldr (\ x r n - if n == 0 then x else r (n - 1)) (const (error $ No such index)) xs n Cheers, Andres ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] generalized, tail-recursive left fold that can finish tne computation prematurely
* Petr Pudlák petr@gmail.com [2013-02-18 17:10:26+0100] Dear Haskellers, while playing with folds and trying to implement `!!` by folding, I came to the conclusion that: - `foldr` is unsuitable because it counts the elements from the end, while `!!` needs counting from the start (and it's not tail recursive). - `foldl` is also unsuitable, because it always traverses the whole list. Every structurally-recursive function is definable through foldr, because foldr *is* the structural recursion (aka catamorphism) operation for lists. Here the trick is to make the accumulator a function. This way you can pass a value from left to right. Something like foldr (\x rest n - ...) id list 0 I'll leave filling in the dots as an exercise. Roman ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] generalized, tail-recursive left fold that can finish tne computation prematurely
* Roman Cheplyaka r...@ro-che.info [2013-02-18 18:28:47+0200] * Petr Pudlák petr@gmail.com [2013-02-18 17:10:26+0100] Dear Haskellers, while playing with folds and trying to implement `!!` by folding, I came to the conclusion that: - `foldr` is unsuitable because it counts the elements from the end, while `!!` needs counting from the start (and it's not tail recursive). - `foldl` is also unsuitable, because it always traverses the whole list. Every structurally-recursive function is definable through foldr, because foldr *is* the structural recursion (aka catamorphism) operation for lists. Here the trick is to make the accumulator a function. This way you can pass a value from left to right. Something like foldr (\x rest n - ...) id list 0 I'll leave filling in the dots as an exercise. Er, my template is not quite right — I had 'length' in mind while writing it. Anyway, Andres showed the correct definition. Roman ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] generalized, tail-recursive left fold that can finish tne computation prematurely
Thanks Roman and Andres for the tip. I knew the trick with accumulating a function, but I had never imagined it could work so efficiently. I thought the problem with using foldr would be that the function would be neither tail recursive nor efficient and so I hadn't even tried. Apparently that was wrong. After your suggestion I checked its performance and how it compiles to core and to my surprise GHC optimizes the whole thing into a most-efficient tail recursive function! Best regards, Petr 2013/2/18 Roman Cheplyaka r...@ro-che.info * Petr Pudlįk petr@gmail.com [2013-02-18 17:10:26+0100] Dear Haskellers, while playing with folds and trying to implement `!!` by folding, I came to the conclusion that: - `foldr` is unsuitable because it counts the elements from the end, while `!!` needs counting from the start (and it's not tail recursive). - `foldl` is also unsuitable, because it always traverses the whole list. Every structurally-recursive function is definable through foldr, because foldr *is* the structural recursion (aka catamorphism) operation for lists. Here the trick is to make the accumulator a function. This way you can pass a value from left to right. Something like foldr (\x rest n - ...) id list 0 I'll leave filling in the dots as an exercise. Roman ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] layers: A prototypical 2d platform game
Hi all, also if there's not that much to see and only a few minutes of gameplay, but after spending quite a few hours writing it, getting a feeling for Haskell and it's usage, perhaps it's in some way useful for someone, even if just for a few minutes of distraction. https://github.com/dan-t/layers Greetings, Daniel ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] generalized, tail-recursive left fold that can finish tne computation prematurely
On 18/02/13 16:10, Petr Pudlák wrote: - `foldr` is unsuitable because it counts the elements from the end, while `!!` needs counting from the start (and it's not tail recursive). It is common misconception that foldr processes the list from the right. foldr brackets from the right, but this has nothing to do with processing direction; all [a] are processed left to right, since this is the only way to structurally deconstruct them. This is the reason why it is possible to write foldr (:) [] [1..] If foldr processed the list from the right, it would on infinite lists - and it does. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Maintaining lambdabot
On Sun, Feb 17, 2013 at 8:53 PM, Cale Gibbard cgibb...@gmail.com wrote: On 17 February 2013 18:03, Jan Stolarek jan.stola...@p.lodz.pl wrote: ... This changes would be quite invasive and code wouldn't be compatible with the lambdabot repo on haskell.org. So before I start making any of them I would like to hear from the community if such changes in the source code of lambdabot would be considered helpful and acceptable. Janek I say go for it! I'll be quite happy to start running your new code in #haskell as soon as we get GHC 7.6 installed on Jason's Linode account. :) My name is Jason and I endorse this message :) Jason ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] ~ operator ?
Hi all, I was creating bigger uncurries which I am simply extending from an existing uncurry I found some where, e.g. uncurry4 :: (a - b - c - d - e) - ((a, b, c, d) - e) uncurry4 f ~(a,b,c,d) = f a b c d when I realized, what's the ~ for ? I've only been able to find a partial explanation that it involves preserving laziness, or something, maybe ? I was hoping someone could enlighten me. Thanks Brian ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ~ operator ?
The difference the ~ makes in this case is that `uncurry4 (\_ _ _ _ - ()) undefined` evaluates to `()` instead of bottom. The ~ is called an irrefutable pattern, and it helps make code that pattern matches on constructors more lazy. This seems like a good explanation of the subject: http://en.wikibooks.org/wiki/Haskell/Laziness#Lazy_pattern_matching On Mon, Feb 18, 2013 at 2:02 PM, bri...@aracnet.com wrote: Hi all, I was creating bigger uncurries which I am simply extending from an existing uncurry I found some where, e.g. uncurry4 :: (a - b - c - d - e) - ((a, b, c, d) - e) uncurry4 f ~(a,b,c,d) = f a b c d when I realized, what's the ~ for ? I've only been able to find a partial explanation that it involves preserving laziness, or something, maybe ? I was hoping someone could enlighten me. Thanks Brian ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Aeson + MongoDB, how to effortlessly store and retrieve json?
Good evening guys, suppose I write a very simple parser using Aeson with these types and ToJSON / FromJSON instances: https://github.com/cakesolutions/the-pragmatic-haskeller/blob/master/01-json/Pragmatic/Types.hs https://github.com/cakesolutions/the-pragmatic-haskeller/blob/master/01-json/Pragmatic/JSON/Parser.hs My question is simple: is it possible to automagically store the haskell data structure produced from the aeson encoding using http://hackage.haskell.org/packages/archive/mongoDB/1.3.2/doc/html/Database-MongoDB-Query.html#g:7 Any tip/suggestion is welcome, Thanks, Alfredo ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ~ operator ?
On Mon, 18 Feb 2013 19:13:13 + Mateusz Kowalczyk fuuze...@fuuzetsu.co.uk wrote: On 18/02/13 19:02, bri...@aracnet.com wrote: Hi all, I was creating bigger uncurries which I am simply extending from an existing uncurry I found some where, e.g. uncurry4 :: (a - b - c - d - e) - ((a, b, c, d) - e) uncurry4 f ~(a,b,c,d) = f a b c d when I realized, what's the ~ for ? I've only been able to find a partial explanation that it involves preserving laziness, or something, maybe ? I was hoping someone could enlighten me. Thanks Brian ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe [1] explains it in section 4.4 — ‘Lazy patterns’. [1] - http://www.haskell.org/tutorial/patterns.html how strange - I was sure I looked through the tutorial... Thanks Mateusz and Patrick. Brian ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Aeson + MongoDB, how to effortlessly store and retrieve json?
Not sure if this is helpful, but have a look at aesonbson: https://github.com/nh2/aesonbson/blob/master/Data/AesonBson.hs It can convert aeson to bson and the other way around, so you can easily convert 'Object's to 'Document's. Is that what you are looking for? On 18/02/13 21:37, Alfredo Di Napoli wrote: My question is simple: is it possible to automagically store the haskell data structure produced from the aeson encoding using http://hackage.haskell.org/packages/archive/mongoDB/1.3.2/doc/html/Database-MongoDB-Query.html#g:7 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] FunPtr to C function with #arguments determined atruntime?
Considering that the GHC FFI is already built on libffi (I'm reasonably sure) it seems unnecessary for the Hackage library to depend on an external version. Is it not already getting linked in? On Sun, Feb 17, 2013 at 6:53 PM, Ryan Newton rrnew...@gmail.com wrote: The scenario is pretty simple. I generate C code at runtime. I compile it to a .so. I know how many arguments it expects (but only at runtime), and I get a FunPtr back from 'dlsym'. How do I call it? I feel that I might be confused about the problem, but since I don't see anyone direct answers -- in order to call a FunPtr, you can use foreign import ccall dynamic, to create a regular function. As described in the library documentation for Foreign.Ptr, which I bet you've seen, so you know this. You can cast the FunPtr to whatever type you like, so you can call the function with an argument list different from its initial declaration. My problem is that I can't create a type representing what I want at the Haskell type-check time, and I need such a type for either casting or a foreign import. For example, let's say the function takes a number of Int arguments between 1 and 1000. If I find out at runtime that I need a function with 613 Int arguments, I would need to create the type (Int - Int ... - IO ()) to cast to. I suppose there may be some way to create such a dependent type with Typeable/Data.Dynamic, since it's monomorphic. Or in theory you could dynamically generate new Haskell code to create the type (System.Eval.Haskell)... libffi, which Krzysztof mentioned, is a good solution: http://www.haskell.org/haskellwiki/Library/libffi Because it allows you to pass a list of arguments callFFI :: FunPtr a - RetType b - [Arg] - IO b But it does introduce an extra dependency on a C library (read, deployment liability). It cabal install'd the first time on my linux box, but my mac said The pkg-config package libffi is required but it could not be found. (even though libffi.dylib is definitely installed globally). Anyway, in this case it wasn't *too *painful to just generate a bunch of extra boilerplate C functions for (1) creating a data structure to hold the arguments, (2) loading them in one at a time, and (3) deallocating the structure when the call is done. Yuck. But no extra dependencies. Cheers, -Ryan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] generalized, tail-recursive left fold that can finish tne computation prematurely
As others have pointed out, _in principle_, foldr is not at all deficient. We can, for example, express foldl via foldr. Moreover, we can express head, tail, take, drop and even zipWith through foldr. That is, the entire list processing library can be written in terms of foldr: http://okmij.org/ftp/Algorithms.html#zip-folds That said, to express foldl via foldr, we need a higher-order fold. There are various problems with higher-order folds, related to the cost of building closures. The problems are especially severe in strict languages or strict contexts. Indeed, foldl_via_foldr f z l = foldr (\e a z - a (f z e)) id l z first constructs the closure and then applies it to z. The closure has the same structure as the list -- it is isomorphic to the list. However, the closure representation of a list takes typically quite more space than the list. So, in strict languages, expressing foldl via foldr is a really bad idea. It won't work for big lists. BTW, this is why foldM is _left_ fold. The arguments against higher-order folds as a `big hammer' were made back in 1998 by Gibbons and Jones http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.42.1735 So, the left-fold with the early termination has a good justification. In fact, this is how Iteratees were first presented, at the DEFUN08 tutorial (part of the ICFP2008 conference). The idea of left fold with early termination is much older though. For example, Takusen (a database access framework) has been using it since 2003 or so. For a bit of history, see http://okmij.org/ftp/Streams.html#fold-stream ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Aeson + MongoDB, how to effortlessly store and retrieve json?
Hi Niklas, From a quick look it seems to be suitable for my task. I'll have a look ASAP and I'll keep you posted. Many thanks! Alfredo Di Napoli On 19/feb/2013, at 00:32, Niklas Hambüchen m...@nh2.me wrote: Not sure if this is helpful, but have a look at aesonbson: https://github.com/nh2/aesonbson/blob/master/Data/AesonBson.hs It can convert aeson to bson and the other way around, so you can easily convert 'Object's to 'Document's. Is that what you are looking for? On 18/02/13 21:37, Alfredo Di Napoli wrote: My question is simple: is it possible to automagically store the haskell data structure produced from the aeson encoding using http://hackage.haskell.org/packages/archive/mongoDB/1.3.2/doc/html/Database-MongoDB-Query.html#g:7 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] generalized, tail-recursive left fold that can finish tne computation prematurely
* o...@okmij.org o...@okmij.org [2013-02-19 06:27:10-] As others have pointed out, _in principle_, foldr is not at all deficient. We can, for example, express foldl via foldr. Moreover, we can express head, tail, take, drop and even zipWith through foldr. That is, the entire list processing library can be written in terms of foldr: http://okmij.org/ftp/Algorithms.html#zip-folds That said, to express foldl via foldr, we need a higher-order fold. There are various problems with higher-order folds, related to the cost of building closures. The problems are especially severe in strict languages or strict contexts. Indeed, foldl_via_foldr f z l = foldr (\e a z - a (f z e)) id l z first constructs the closure and then applies it to z. The closure has the same structure as the list -- it is isomorphic to the list. However, the closure representation of a list takes typically quite more space than the list. So, in strict languages, expressing foldl via foldr is a really bad idea. It won't work for big lists. If we unroll foldr once (assuming l is not empty), we'll get \z - foldr (\e a z - a (f z e)) id (tail l) (f z (head l)) which is a (shallow) closure. In order to observe what you describe (a closure isomorphic to the list) we'd need a language which does reductions inside closures. Am I wrong? Roman ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
