- Haskell-Cafe - Haskell.org

Win32-package and Windows error codes in exceptions
by Esa Ilari Vuokko 28 Aug '06

28 Aug '06

Hi, Recently, I've been annoyed how Win32-package throws user errors, ie String, on failures. The System.Win32-being rather low level, it often makes sense for caller to catch some specific errors. I don't really understand how, or if, I could extend System.IO.Error. unix- package uses it via Foreign.C.Error, which makes me wonder if the underlying functionality in there should be extended. Is the only or best choice to go for Dynamic exceptions? That introduces a subtle difference for (already existing) programs that rely on exceptions to be presentable via Show and default handler. Best regards, --Esa Ilari Vuokko

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Re: [Haskell] Exists any null instruction in Haskell?
by Neil Mitchell 27 Aug '06

27 Aug '06

[Moving to haskell-cafe] Hi, > I want to know if it exists a null instruction (a instruction > that do anything Not really, since instructions don't "do anything" anyway - they only compute values. If you are in the IO monad then "return ()" is the do nothing. In the function world, "id" is the do nothing. In recursive list processing, then something like [] is probably the "base case" in a recursive function, so stops the recursivity. If you give a more concrete example, then maybe we can give you a better answer. Thanks Neil

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OOP vs type classes: Everything is object?
by Bulat Ziganshin 26 Aug '06

26 Aug '06

Hello haskell-cafe, i've added section http://haskell.org/haskellwiki/OOP_vs_type_classes#Everything_is_object.3F which lists cases when we DON'T use type classes to emulate OOP classes features -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Re: [Haskell-cafe] monads once again: a newbie perspective
by Andrea Rossato 26 Aug '06

26 Aug '06

Il Fri, Aug 25, 2006 at 01:13:58PM -0400, Cale Gibbard ebbe a scrivere: > Hey cool, a new monad tutorial! :) > > Just out of interest, have you seen my Monads as Containers article? > http://www.haskell.org/haskellwiki/Monads_as_Containers > > Let me know what you think of it. I find that often newcomers to > monads will find the container perspective easier to grasp before > moving on to treating monads as an abstraction of computation, but > that side of things needs coverage too. :) Sure I've read it! I must confess I find it difficult, though. I mean, the exemplification part is very interesting but, for me, it was too difficult to connect it to the code I was looking at. This is way I decided this approach: let's start building a monad and see what it is by actually looking it at work. The evaluator is a very simple piece of code, you can clearly see what it does. And then you start building up your knowledge by expanding it. Take into account that, for me, *writing* that tutorial is *the* way to get to grasp all the concepts behind the type system (and monads). As you can see in a thread below, I'm studying hard in order to find out the proper continuation of the tutorial, and for my learning. So far I'll be able to describe the code of a statefull evaluator that produces output. Then I'd like to add exception handling. Let's see if I'll be able to get that far... By the way, I'll soon add links to the other important tutorials on monads: yours, IO Inside, All about monads. At the I'd like to be able to link A Gentle introduction, the Haskell 98 Report and, why not, "Write yourself a Scheme in 48 hours". That will be it. Thanks for your kind attention. Andrea

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difference between type and newtype
by Andrea Rossato 26 Aug '06

26 Aug '06

Hello! I cannot understand this piece of code: type Z = Int type T a = Z -> (a, Z) newtype T1 a = T1 (Z -> (a,Z)) mkT :: a -> T a mkT a = \x -> (a, x) mkT1 :: a -> b -> (a,b) mkT1 a = \x -> (a, x) why mkT is a type constructor and mkT1 seems not to be? thanks for your kind attention. Andrea

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Space leak whilst implementing streams
by ephemeral.elusive＠gmail.com 26 Aug '06

26 Aug '06

Hello, I have been using arrows to implement stream processors. At first, I tried using the implementation presented in John Hughes' AFP arrows lectures. However, this appeared to have a space leak in its implementation of the left operator for ArrowChoice. I found a way to remove this space leak, however, I do not really understand why there was a space leak in the first place. I would really appreciate any light that could be shed on this. Below I include the AFP implementation (SF) and a modified implementation that no longer has the space leak (SF'). I am using ghc 6.4.2. Many thanks. import Control.Arrow import Data.Maybe main = test test = print (runSF p (repeat 1)) test' = print (runSF' p (repeat (Just 1))) -- heap profile appears to grow linearly for test, but not test' p :: ArrowChoice a => a Int (Either Int Int) p = arr Right >>> left (arr id) newtype SF a b = SF {runSF :: [a] -> [b]} instance Arrow SF where arr f = SF (map f) SF f >>> SF g = SF (f >>> g) first (SF f) = SF (unzip >>> first f >>> uncurry zip) instance ArrowChoice SF where left (SF f) = SF (\xs -> combine xs (f [y | Left y <- xs])) where combine (Left _:xs) (z:zs) = Left z :combine xs zs combine (Right r:xs) zs = Right r:combine xs zs combine [] _ = [] -- SF' does not exhibit the space leak newtype SF' a b = SF' {runSF' :: [Maybe a] -> [Maybe b]} instance Arrow SF' where arr f = SF' (map (maybe Nothing (Just . f))) SF' f >>> SF' g = SF' (f >>> g) first (SF' f) = SF' (maybe_unzip >>> first f >>> uncurry maybe_zip) where maybe_unzip = foldr mu ([],[]) where mu Nothing ~(xs,ys) = (Nothing:xs, Nothing: ys) mu (Just (x,y)) ~(xs,ys) = (Just x:xs, Just y: ys) maybe_zip = zipWith mz where mz (Just x) (Just y) = Just (x,y) mz Nothing Nothing = Nothing instance ArrowChoice SF' where left (SF' f) = SF' (\xs -> xs `combined` f (map drop_right xs)) where combined = zipWith merge_left drop_right Nothing = Nothing drop_right (Just (Left l)) = Just l drop_right (Just (Right _)) = Nothing merge_left Nothing Nothing = Nothing merge_left (Just (Left _)) (Just x) = Just (Left x) merge_left (Just (Right r)) Nothing = Just (Right r)

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ghc profiling
by Jeff Polakow 25 Aug '06

25 Aug '06

Hello, When I compile my program without profiling: bash-3.1$ ghc --make -auto-all -prof -O -o analysis analysis.hs Chasing modules from: analysis.hs Compiling Main ( analysis.hs, analysis.o ) Linking ... everything works fine. However when I compile with profiling: bash-3.1$ ghc --make -auto-all -prof -O -o analysis analysis.hs Chasing modules from: analysis.hs Could not find module `Data.ByteString.Lazy.Char8': use -v to see a list of the files searched for (imported from analysis.hs) ghc complains about not finding the ByteString library. Is this supposed to hapen, and if so, is there a way to get profiling to work? thanks, Jeff -- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

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USB Drivers in Haskell
by Jason Dagit 25 Aug '06

25 Aug '06

Hello, I recently became the owner a USB gadget that tracks movement via GPS and also tracks heart rate (it's a training device for athletes). This device comes with software that is windows only and...doesn't like up to it's potential (to put it politely). Being a programmer type and someone that prefers to use either linux or osx I quickly realized I should write my own software to use the gadget. Fortunately, the company that makes the device provides documentation to write drivers for the device (including a minimal example USB driver for win32). So then I looked around the net for a few open source things, such as open source drivers for the device or a Haskell library to help me start writing drivers for the gadget. I didn't see anything relevant after a few minutes of searching so I figure that means there isn't much. Did I miss a Haskell library for writing device drivers, specifically USB drivers? Would this be hard to write? I would prefer to support osx and linux at a minimum and I think it would be ideal to shoot for cross platform (win32, osx, linux and *bsd). I have this feeling that it could be done by writing platform specific wrappers using hsc2hs on each platform then bringing them together through a unified 'low' level Haskell api. The Haskell api would then be exposed as a module for application developers. Any thoughts? For the interested, the documentation I spoke of can be found here: http://www.garmin.com/support/commProtocol.html Thanks, Jason

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A possible use for HSFFIG (FFIPKG) was USB Drivers in Haskell
by Dimitry Golubovsky 25 Aug '06

25 Aug '06

Bjorn Bringert wrote: >For cross-platform USB drivers, you may want to have a look at libusb >[1]. I have only used it under Linux, but it seems to support Linux, >*BSD and OS X. There also seems to be a win32 port [2]. A Haskell >binding to libusb would be very welcome. I'm just wondering, would FFIPKG or HSFFIG help out? I just looked at usb.h, and it does not seem to contain anything that would break HSFFIG (mainly C functions that take/return structures rather than pointers to structures, do). I am not very much interested personally in interfacing USB, but if anybody is, feel free to try FFIPKG out and ask questions. PS Sorry for such "self-advertising", but I believe that this tool could be helpful at least for prototyping bindings, so I'd appreciate any feedback. -- Dimitry Golubovsky Anywhere on the Web

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a novice Alex question
by Xiong Yingfei 25 Aug '06

25 Aug '06

Hi, I am trying out Alex. I copied the calculator specification file from Alex's official document and changed the wrapper type from "basic" to "monad". However, after I generated the ".hs" file from the lexical specification and compiled the ".hs" file, I got the message "Variable not in scope: `alexEOF'". I cannot find explanation about this 'alexEOF' function in the document. Can any body be kindly enough to tell me what this function is? Should I write it myself or not? My lexical code is listed as the below. Thanks a lot. { module Lex where } %wrapper "monad" $digit = 0-9 -- digits $alpha = [a-zA-Z] -- alphabetic characters tokens :- $white+ ; "--".* ; let { \s -> Let } in { \s -> In } $digit+ { \s -> Int (read s) } [\=\+\-\*\/] { \s -> Sym (head s) } $alpha [$alpha $digit \_ \']* { \s -> Var s } { -- Each action has type :: String -> Token -- The token type: data Token = Let | In | Sym Char | Var String | Int Int deriving (Eq,Show) } -- Xiong, Yingfei (熊英飞) Ph.D. Student Institute of Software School of Electronics Engineering and Computer Science Peking University Beijing, 100871, PRC. Web: http://xiong.yingfei.googlepages.com

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