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Re: Why Haskell?
by Hans van Thiel 07 Aug '06

07 Aug '06

Hello All, Thanks for the many helpful replies to my question about the suitability of Haskell for industrial/commercial application. >From those I gather there are no licensing problems regarding the use of the standard functions and modules. Use of proprietary modules may or not be restricted by the authors, but of course this is to be expected and fair. Performance and size of Haskell programs don't seem to be problematic either, at least for small and medium size applications. Tool support, however, is less than for the more established languages. But maybe the Eclipse support which is under development will be a boost. Also, there are good interfaces with open source gui widgets etc. So, the summary conclusion would be that Haskell can be used (and is being used) provided the developers are aware that unexpected problems may arise in practice. Advantages of Haskell seem to be short size of code (and all the advantages that implies) and the high abstraction level and mathematical rigour (which makes for less programming errors). As for the 'popularity' of Haskell, someone made the very common sense remark that all successful languages have been promoted by some large corporation. (Please note 'successful' stands for 'spread of use', not 'quality' here.) If fp is to reach the mainstream, it will probably be because of some centralized support from outside the academic world. Maybe something like a 'Haskell Consortium' or 'Haskell Initiative' to promote Haskell should be set up. An example could be the promotion of SystemC for hardware and software co-design by the Open SystemC Initiative www.systemc.org . Availability of an IEEE standard for Haskell would also be a good thing for its acceptance, I think. That is, assuming 'mainstream' is the goal, of course. Regards, Hans van Thiel

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Variants of a recursive data structure
by tpledger＠ihug.co.nz 07 Aug '06

07 Aug '06

Klaus Ostermann wrote: [...] > 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. One way, that I haven't spotted in any of the replies so far, is to declare a composition type data BComp m n a = BC (m (n a)) as seen in http://web.cecs.pdx.edu/~mpj/pubs/springschool.html , so that type LabelledExp = Mu (BComp Labelled Exp) See http://haskell.cs.yale.edu/pipermail/haskell/2001-May/003942.html for more crafty tricks, including making Eq instances for such Mu-based recursive structures. Regards, Tom

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Type inference problem
by Joel Björnson 07 Aug '06

07 Aug '06

Hi. I have a type inference related problem which causes me some confusion. The following is a simplified example capturing the key aspects. First of all, consider a data type (here called) Elem and a type class IsElem. data Elem = E class IsElem a where toElem :: a -> Elem instance IsElem Elem where toElem = id instance Show a => IsElem a where toElem _ = E Values of type Elem along with all instances of Show, instansitates the IsElem class. What I want to achieve is the enabling of definitions of classes/member functions corresponding to the following. class SetId t v r | t -> r where setId :: t -> v -> r And to instantiate the class, allowing 'setId' to produce values of type Elem as in instance SetId Elem String Elem where setId x _ = x Note that the fundep (t -> r) enables the type checker to uniquely determine the return type of setId, depending on it's first argument. It is now possible to write function definitions according to myElem = E `setId` "id" Checking the type of 'myElem' using ghci confirms that it has type 'Elem'. (Because of the E and the fundep) So far, so good... However, problems arises trying to define the function : test = toElem myElem Yielding the error message : 'No instance for (Show Elem) arising use of `toElem` at ...' For some reason it seems like the type checker picks the *wrong* 'toElem', and that the type of 'myElem' can't be properly determined (without adding explicit type signatures). It seems strange though the the type of myElem really can be determined and that the instance (IsElem Elem) is more specific then the instance for Show types. Anyway, I just realised that turning off the 'monomorphism-restriction' solves the problem. (Don't know why though). Is this *the* recommended solution here and is there any general policy regarding the flag ? Help/explanations are appreciated! Thanks a lot /Joel

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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 06 Aug '06

06 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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