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Variants of a recursive data structure
by Klaus Ostermann 07 Aug '06

07 Aug '06

Hi all, I have a problem which is probably not a problem at all for Haskell experts, but I am struggling with it nevertheless. I want to model the following situation. I have ASTs for a language in two variatns: A "simple" form and a "labelled" form, e.g. data SimpleExp = Num Int | Add SimpleExp SimpleExp data LabelledExp = LNum Int String | LAdd LabelledExp LabelledExp String I wonder what would be the best way to model this situation without repeating the structure of the AST. I tried it using a fixed point operator for types like this: data Exp e = Num Int | Add e e data Labelled a = L String a newtype Mu f = Mu (f (Mu f)) type SimpleExp = Mu Exp type LabelledExp = Mu Labelled Exp The "SimpleExp" definition works fine, but the LabeledExp definition doesn't because I would need something like "Mu (\a -> Labeled (Exp a))" where "\" is a type-level lambda. However, I don't know how to do this in Haskell. I'd need something like the "." operator on the type-level. I also wonder whether it is possible to curry type constructors. The icing on the cake would be if it would also be possible to have a function unlabel :: LabeledExp -> Exp that does *not* need to know about the full structure of expressions. So, what options do I have to address this problem in Haskell? Klaus

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Compiling To C And Why Not Compiling To Java Or C#?
by Kaveh Shahbazian 06 Aug '06

06 Aug '06

1 & 2 - Maybe this is not a proper question but I think It has a point. Why Not Compiling To Java Or C#? What is the need of implementing a class interoperability between Haskell and other plateforms? Maybe we donot need that. Haskell can be the big infrastructure and code snippets in Java or C# can serve in a monadic wrapper. It looks like that we can scripting Haskell in that languages. For example F# 's target code is ILX (an extension of MSIL for functional languages). But I think interoperablity between languages - like what .NET claims - is totaly pointless. Again think of an infrastructure that has a Haskell core which defines and controls very high level processes. Then there will be a running machine that Haskell is sitting on top of it. This running machine implement haskell in the plateform language. This is what I mean. 3 - And for an OOP style interoperability between languages, I think most usefull is to have a robust standard for Object-Broking. Something like IEC 61850 in new power systems in industry. A tool from siemens can understand what a hitachi high voltage key commands and all of these devices are from different providers with different internal implementations that meets the standard.

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fast image processing in haskell?
by Jeff Briggs 06 Aug '06

06 Aug '06

Hello, I am attempting to process images captured from a webcam. My aim is to do so, in real time, at the frame rate of the camera. I'm using GHC 6.4.2 with -O3. A frame consists of ~100k 24bit colour values. The webcam is interfaced through FFI bindings to some C++ -- these are all labelled 'unsafe'. The image is passed to Haskell as a Ptr Word8. To blit this to the screen (via Gtk2Hs) I do the following: data Cam = Cam { snap_width :: !Int , snap_height :: !Int , snap_bytespp :: !Int , snap_size :: !Int , cam_img :: Ptr Word8 , cam_obj :: ForeignPtr () } do (PixbufData _ dst _) <- (pixbufGetPixels pixbuf :: IO (PixbufData Int Word8)) copyBytes dst (cam_img cam) This achieves the desired throughput (25-29fps.) However, I am at a bit of a loss how to do something similar for preprocessing the data in Haskell before blitting the data (whilst also retaining some semblance of functional programming...) Currently, I have: cam_snap cam f x = do let loop (r:g:b:rest) n x = f r g b n x >>= loop rest (n+3) loop _ _ x = return x px <- peekArray (snap_size cam) (cam_img cam) loop px 0 x cam_snap2 cam f x = let loop ptr n x | n >= snap_size cam = return x | otherwise = do let ptrs = scanl plusPtr ptr [1,1] [r,g,b] <- mapM peek ptrs f r g b n x >>= loop (ptr `plusPtr` 3) (n+3) in loop (cam_img cam) 0 x do ... let sum_px r g b _ (sr,sg,sb) = return (sr+r,sg+g,sb+b) sum <- cam_snap (cam ui) sum_px (0.0,0.0,0.0) print sum cam_snap only processes at 5 fps, whereas cam_snap2 operates at 6fps. Any suggestions? Thanks.

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Exercise in YAHT
by algorithms＠gmx.de 06 Aug '06

06 Aug '06

Dear Hugs users, could you answer the following question from a Haskell novice? In "Yet another Haskell Tutorial" by Hal Daumé III there is the following exercise: Exercise 4.8 Write functions listHead, listTail, listFoldl and listFoldr which are equivalent to their Prelude twins, but function on our List datatype. Don’t worry about exceptional conditions on the first two. My question is solely on listfoldr, in fact I only want to ask whether the solution provided in the tutorial is really so obviously wrong as I perceive it to be: (What follows uses the definition data List a = Nil | Cons a (List a) ) listFoldr f y Nil = y listFoldr f y (Cons x xs) = f x (foldr f z xs) This IS wrong, isn't it? The y does not even appear after all. And what can you say about my attempt for listFoldr? listFoldr f y Nil = y listFoldr f y (Cons x xs) = f y (listFoldr f (f y x) xs) My attempt yields a similar, albeit not identical result compared with the Prelude function foldr when :t-ing it under Hugs: Hugs> :t foldr foldr :: (a -> b -> b) -> b -> [a] -> b Hugs> :t listFoldr listFoldr :: (a -> a -> a) -> a -> List a -> a Thank you very much. Christian -- Echte DSL-Flatrate dauerhaft für 0,- Euro*. Nur noch kurze Zeit! "Feel free" mit GMX DSL: http://www.gmx.net/de/go/dsl

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A problem with Show
by zdenes＠seznam.cz 06 Aug '06

06 Aug '06

Hello, I made a simple datatype called Pair and I'd like to make it an instance of Show class. But when I try to do that I run into troubles: data Pair a b = Pair a b instance Show a b => Show (Pair a b) where show (Pair a b) = show a ++ "_" ++ show b In Hugs I get this error: "Haskell does not support multiple parameter classes" When I run it in ghci I get a kind error "Show is applied to too many type arguments in the instance declaration for Show (Pair a b)" Am I jsut using wrong syntax or is it not possible to do? Thanks in advance Zdenes

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REALLY simple STRef examples
by Chad Scherrer 06 Aug '06

06 Aug '06

I've looked around at the various STRef examples out there, but still nothing I write myself using this will work. I'm trying to figure out how the s is escaping in really simple examples like x = runST $ return 1 y = runST $ do {r <- newSTRef 1; readSTRef r} Neither of these works in ghci - they both say <interactive>:1:0: Inferred type is less polymorphic than expected Quantified type variable `s' escapes Expected type: ST s a -> b Inferred type: (forall s1. ST s1 a) -> a In the first argument of `($)', namely `runST' In the definition of `it': ... I thought maybe I needed to replace 1 with (1 :: Int) so the state representation didn't force the type, but it still gives the same result. Can someone point me to the simplest possible runST example that actually works? Thanks! -- Chad Scherrer "Time flies like an arrow; fruit flies like a banana" -- Groucho Marx

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Monad Imparative Usage Example
by Kaveh Shahbazian 05 Aug '06

05 Aug '06

Haskell is the most powerfull and interesting "thing" I'v ever encountered in IT world. But with an imparative background and lack of understanding (because of any thing include that maybe I am not that smart) has brought me problems. I know this is an old issue. But please help it. Question : Could anyone show me a sample of using a monad as a statefull variable? For example see this code in C# : // public class Test { int var; static void Fun1() { var = 0; Console.Write(var); } static void Fun2() { var = var + 4; Console.Write(var); } static void Main() { Fun1(); Fun2(); var = 10; Console.Write("var = " + var.ToString()); } } // I want to see this code in haskell. Thankyou

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Monad Imparative Usage Example
by Kaveh Shahbazian 05 Aug '06

05 Aug '06

Monad Imparative Usage Example Thanks for your replies. I have not haskell on this computer and I will try this solutions tonight. I must notice that IO computations is not the point here. My target is to have this code for mutable variable 'var'. ############################################################################# Haskell is the most powerfull and interesting "thing" I'v ever encountered in IT world. But with an imparative background and lack of understanding (because of any thing include that maybe I am not that smart) has brought me problems. I know this is an old issue. But please help it. Question : Could anyone show me a sample of using a monad as a statefull variable? For example see this code in C# : // public class Test { int var; static void Fun1() { var = 0; Console.Write(var); } static void Fun2() { var = var + 4; Console.Write(var); } static void Main() { Fun1(); Fun2(); var = 10; Console.Write("var = " + var.ToString()); } } // I want to see this code in haskell. Thankyou #############################################################################

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Why shouldn't variable names be capitalized?
by Martin Percossi 05 Aug '06

05 Aug '06

Hi, I'm wondering what the rationale was for not allowing capitalized variable names (and uncapitalized type names and constructors). I can only think of two arguments, and IMHO both of them are bad: 1. Enforces a naming convention. Fine - but my view is that this doesn't belong in the language definition (it belongs in the user's coding standards). I get annoyed, for example, that when I write code that manipulates matrices and vectors, I can't refer to the matrices with capital letters as is common in the literature. And to anyone who says that it's good to enforce naming consistency, I have this to say: Any language that requires me to learn about category theory in order to write imperative code should treat me like an adult when it comes to the naming of variables as well. ;-) 2. It makes it easier to write the compiler. I don't think I need to explain why this is bad... I imagine that someone is just itching to "sort me out". Do your worst! ;-) Thx Martin

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a bunch of newbie questions
by Imam Tashdid ul Alam 04 Aug '06

04 Aug '06

what's the difference between data and co-data exactly? or between inductive data types and co-inductive data types? can you give me some reference points that explain these? > (read "56")::Integer does it in fact pass the type (Integer) to the function (read)? I guess what we want is for the (Integer) implementation of the (read) function to evaluate, not really to cast the value of (read) function to (Integer). in the regex libraries, (~=) cast this way results in completely different things altogether, should type classes have namespaces associated with them? somewhat like Integer.read "56"? this way of selecting the intended implementation, does it work without trouble with multiparameter type classes as well? theoretically is it possible to do a strictness analysis without any help from the programmer? thanks a bunch in advance. __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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