April 2009 - Haskell-Cafe

Issue with IsFunction/Vspace in GHC 6.10.1
by Jacques Carette 02 Apr '09

02 Apr '09

I was playing with some of Oleg's code (at end for convenience). After minor adjustments for ghc 6.10.1, it still didn't work. The error message is quite puzzling too, as it suggests adding exactly the constraint which is present... Any ideas? Jacques -- Oleg's definition of a vector space class, based on IsFunction and -- TypeCast. See http://okmij.org/ftp/Haskell/isFunction.lhs -- for the January 2003 message, which works in GHC 6.2.1 and 6.4 -- code below *works* in 6.8.1 AFAIK {-# LANGUAGE EmptyDataDecls, MultiParamTypeClasses, FunctionalDependencies #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE OverlappingInstances #-} module Q where class Vspace a v | v -> a where (<+>) :: v -> v -> v (*>) :: a -> v -> v instance (IsFunction v f, Vspace' f a v) => Vspace a v where (<+>) = doplus (undefined::f) (*>) = dostar (undefined::f) class Vspace' f a v | f v -> a where doplus :: f -> v -> v -> v dostar :: f -> a -> v -> v instance Num a => Vspace' HFalse a a where doplus _ = (+) dostar _ = (*) -- etc. No problem. instance (IsFunction v f, Vspace' f a v, Vspace a v) => Vspace' HTrue a (c->v) where doplus _ f g = \x -> f x <+> g x dostar _ a f x = a *> (f x) test1 = (1::Int) <+> 2 test2 = ((\x -> x <+> (10::Int)) <+> (\x -> x <+> (10::Int))) 1 test3 = ((\x y -> x <+> y) <+> (\x y -> (x <+> y) <+> x)) (1::Int) (2::Int) test4 = ((\x y -> x <+> y) <+> (\x y -> ((2 *> x) <+> (3 *> y)))) (1::Int) (2::Int) data HTrue data HFalse class IsFunction a b | a -> b instance TypeCast f HTrue => IsFunction (x->y) f instance TypeCast f HFalse => IsFunction a f -- literally lifted from the HList library class TypeCast a b | a -> b, b->a where typeCast :: a -> b class TypeCast' t a b | t a -> b, t b -> a where typeCast' :: t->a->b class TypeCast'' t a b | t a -> b, t b -> a where typeCast'' :: t->a->b instance TypeCast' () a b => TypeCast a b where typeCast x = typeCast' () x instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast'' instance TypeCast'' () a a where typeCast'' _ x = x

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Retargeting the ghc code generator
by Vasili I. Galchin 02 Apr '09

02 Apr '09

Hello, Is there any project to retarget the GHC code generator to generate Common Intermediate Language(CLI) in order to run on Mono or .NET? I assume that Mondrian did precisely that. Thanks, Vasili

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Looking for practical examples of Zippers
by Gü?nther Schmidt 02 Apr '09

02 Apr '09

Hi, my quest for data structures continues. Lately I came across "Zippers". Can anybody point be to some useful examples? Günther

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Re: HXT: desperatedly trying to "concat"
by Steffen Schuldenzucker 02 Apr '09

02 Apr '09

Hello again. I finally got it myself. It was just a matter of parentheses: See http://hpaste.org/fastcgi/hpaste.fcgi/view?id=3229 for the corrected version. Looks like what I was trying to do is not expressable via just an arrow, one needs a function mapping the input arrow to a new one. I'm gonna make a big "WATCH YOUR PARENTHESES!" poster... Yeah, with some arrows on it... Thanks for reading, anyway. Steffen ________________________________________________________________________ Neu bei WEB.DE: Kostenlose maxdome Movie-FLAT! https://register.maxdome.de/xml/order/LpWebDe?ac=OM.MD.MD008K15726T7073a

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Funny type signature question
by Michael Snoyman 02 Apr '09

02 Apr '09

I've butted into this problem multiple times, so I thought it's finally time to get a good solution. I don't even have the terminology to describe the issue, so I'll just post the code I'm annoyed with and hope someone understands what I mean. import Control.Monad.Random import System.Random data Marital = Single | Married | Divorced deriving (Enum, Bounded, Show) randomEnum :: (Enum a, Bounded a, RandomGen g) => Rand g a randomEnum = do let minb = minBound maxb = maxBound randVal <- getRandomR (fromEnum minb, fromEnum maxb) return $ head [toEnum randVal, minb, maxb] -- if I do the obvious thing (return $ toEnum randVal) I get funny errors main = do stdGen <- newStdGen let marital = evalRand randomEnum stdGen :: Marital putStrLn $ "Random marital status: " ++ show marital Any help is appreciated. Thanks! Michael

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ANN: Buster 0.99.1, a library for application orchestration that is not FRP
by Jeff Heard 02 Apr '09

02 Apr '09

Read more about it on its webpage: http://vs.renci.org/jeff/buster Yes, it’s to solve a particular problem. And yes, this is a rough draft of an explanation of how it works. I’ve not even really solidified the vocabulary yet, but I have this module which couches a large, abstract, interactive (both with the user and the system), multicomponent application in terms of a bus, inputs, behaviours, and events. * Time is continuous and infinite. * An event is a static, discrete item associated with a particular time. * The bus is the discrete view of event in time at an instant. * A widget is an IO action that assigns events to a particular time based only upon sampling the outside world (other events and behaviours are irrelevant to it). e.g. a Gtk Button is a widget, a readable network socket is an widget, the mouse is an widget, the keyboard is an widget, a multitouch gesture engine is a widget. * A behaviour is a continuous item — it exists for the entire program and for all times — which maps events on the bus to other events on the bus. It is an IO action as well — where widgets only sample the outside world and are in a sense read only, behaviours encapsulate reading and writing.

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Character I/O
by Andrew Coppin 02 Apr '09

02 Apr '09

I ran the following program: >module Main where > >main = do > putStrLn "Key" > c <- getChar > print c > main Unfortunately, it doesn't do what I was hoping. Specifically, it only returns any data when I hit [return]. It also seems to be giving me a command history somehow. Special keys (e.g., arrow keys, backspace, escape) don't get through to the application. I'm trying to write a program that uses individual keys as commands. What's the correct way to get this to work?

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HXT: desperatedly trying to "concat"
by Steffen Schuldenzucker 02 Apr '09

02 Apr '09

Hi. I've got a problem with the Haskell XML Toolkit (hxt). I want to write a little app that performs REST requests from a certain (rather simple) XML format. A example procedure Call looks like testData defined below. What I'd like to do is to transform this xml tree into a GET variable string using an XmlArrow. The task sounds easy, and it has to be easy, but I've been sitting here for about a day now, staring at my code. It looks like this (the transformation should be done by the arrow "mkGetStr"): -- Rest.hs -- This is also on HPaste: http://hpaste.org/fastcgi/hpaste.fcgi/view?id=3210#a3210 {-# LANGUAGE NoMonomorphismRestriction #-} module Rest where import Text.XML.HXT.Arrow import Data.List getParamsA = hasName "param" >>> getChildren >>> isElem >>> (getName &&& (getChildren >>> getText)) >>> arr2 mkGetPair mkMethodStr = ("method=" ++) mkGetPair k v = k ++ "=" ++ v getMethodA = hasName "method" >>> getChildren >>> getText >>> arr mkMethodStr mkGetStr = isElem >>> (getMethodA <+> getParamsA) >>. intercalate "&" -- Try it with: runX (testData >>> mkGetStr) >>= print testData = xread <<< constA ( "<method>my.Method</method>" ++ "<param>" ++ "<foo_arg>Foo</foo_arg>" ++ "<bar_arg>Bar</bar_arg>" ++ "</param>" ) -- End of Rest.hs What I get out of it is this (in ghci): *Rest> runX (testData >>> mkGetStr) >>= print "method=my.Methodfoo_arg=Foo&bar_arg=Bar" There is an "&" missing right after "method=my.Method"! Why? I've tried many variants of this and they give me either this or a similar result or multiple results (what I don't want either). I'd be really happy if someone could save my day and help me with this issue. Thanks in advance, Steffen ________________________________________________________________________ Neu bei WEB.DE: Kostenlose maxdome Movie-FLAT! https://register.maxdome.de/xml/order/LpWebDe?ac=OM.MD.MD008K15726T7073a

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SciMark2 benchmark
by Jon Harrop 02 Apr '09

02 Apr '09

Has anyone ported the SciMark2 benchmark suite to Haskell? -- Dr Jon Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/?e

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ZipList monad, anyone?
by Patai Gergely 02 Apr '09

02 Apr '09

Does ZipList have any useful monad instance? The thought came up while thinking about higher order dataflows and an ArrowApply interface for Yampa. As a ZipList can be thought of as a function with a discrete domain, I figured its monadic form could be analogous to the reader monad, hence: instance Monad ZipList where return = ZipList . repeat ZipList xs >>= f = ZipList $ zipWith ((!!) . getZipList . f) xs [0..] Correct me if I'm wrong, but it seems to me that f always returning an infinite list is a sufficient condition for the monad laws to hold. However, this is painfully inefficient. I assume that bringing FRP-like switching and some clever data structure (which optimises stateless streams for starters) into the picture can potentially lead to constant-time access at least in the case of sequential sampling. Is there any recent work going in this direction? Gergely -- http://www.fastmail.fm - One of many happy users: http://www.fastmail.fm/docs/quotes.html

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