May 2011 - Haskell-Cafe

Looking for n-body example with repa.
by Grigory Sarnitskiy 31 May '11

31 May '11

Hello! I'm studying repa package and trying to implement O(N^2) n-body code. I'm using Array DIM2 Double to represent the collection of particles. I've managed to write a function which calculates the energy of one particle in the field of others but I'm struggling to write a code to calculate the energy of the whole system. Maybe someone here has already written such code?

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ANNOUNCE: time-recurrence-0.4.2
by Chris Heller 30 May '11

30 May '11

I've been busy making major changes to my time library: http://hackage.haskell.org/package/time-recurrence Things are still under heavy development. This release is to show the direction the library is heading in. A major shift in the design has been made, and the current work is an attempt at simplifying the recurring time manipulations into the smallest number of function possible, then providing some useful convenience methods on top of that. Version 0.4.2 definitely feels more Haskell-y. Here is a simple example of how things are done: > date = parse822Time "Tue, 02 Sep 1997 09:00:00 -0500" > take 5 $ repeatSchedule' dailyUTC{interval = toInterval 10, moment = date} > [1997-09-02 14:00:00 UTC > ,1997-09-12 14:00:00 UTC > ,1997-09-22 14:00:00 UTC > ,1997-10-02 14:00:00 UTC > ,1997-10-12 14:00:00 UTC] In addition a test suite has been added to the package, it currently is using HUnit test-framework, and the Cabal 1.10 stdio test suite facility. Feedback, suggestions and pull requests are always welcome. -Chris

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Re: [Haskell-cafe] What's the advantage of writing Haskell this way?
by Canhua 30 May '11

30 May '11

sorry, `m a` as an instance of Monoid, not `m' On Tue, May 31, 2011 at 9:30 AM, Canhua <dreameration(a)gmail.com> wrote: > I think you should declare `m' as an instance of Monoid, > rather than as instnaces of Monad and MonadPlus > > On Tue, May 31, 2011 at 8:33 AM, John Ky <newhoggy(a)gmail.com> wrote: >> Hi Brandon, >> Thanks for your suggestion. I'm a little stuck as adding Monad and >> MonadPlus in my instance declaration doesn't seem sufficient. I know >> mconcat comes from Monoid, but I don't know how to put that in. >> data Stream m a >> = Chunks (m a) >> | EOF >> deriving (Show, Eq) >> instance (Monad m, MonadPlus m) => Monoid (Stream m a) where >> mempty = Chunks mempty >> mappend (Chunks xs) (Chunks ys) = Chunks (xs `mappend` ys) >> mappend _ _ = EOF >> instance (Monad m, MonadPlus m) => Monad (Stream m) where >> return = Chunks . return >> Chunks xs >>= f = mconcat (fmap f xs) >> EOF >>= _ = EOF >> Iteratee.hs:28:25: >> No instance for (Monoid (m a)) >> arising from a use of `mempty' >> Possible fix: add an instance declaration for (Monoid (m a)) >> In the first argument of `Chunks', namely `mempty' >> In the expression: Chunks mempty >> In an equation for `mempty': mempty = Chunks mempty >> Iteratee.hs:29:54: >> No instance for (Monoid (m a)) >> arising from a use of `mappend' >> Possible fix: add an instance declaration for (Monoid (m a)) >> In the first argument of `Chunks', namely `(xs `mappend` ys)' >> In the expression: Chunks (xs `mappend` ys) >> In an equation for `mappend': >> mappend (Chunks xs) (Chunks ys) = Chunks (xs `mappend` ys) >> Iteratee.hs:34:43: >> Could not deduce (m ~ []) >> from the context (Monad m, MonadPlus m) >> bound by the instance declaration at Iteratee.hs:32:10-51 >> `m' is a rigid type variable bound by >> the instance declaration at Iteratee.hs:32:17 >> Expected type: [a] >> Actual type: m a >> In the second argument of `fmap', namely `xs' >> In the first argument of `mconcat', namely `(fmap f xs)' >> In the expression: mconcat (fmap f xs) >> Failed, modules loaded: none. >> Cheers, >> -John >> On 31 May 2011 00:38, Brandon Moore <brandon_m_moore(a)yahoo.com> wrote: >>> >>> >From: John Ky <newhoggy(a)gmail.com> >>> >Sent: Monday, May 30, 2011 8:01 AM >>> > >>> >Hi all, >>> > >>> >I'm trying to learn about enumerators by reading this paper and came >>> > across some code on page 2 that I found hard to digest, but I think I >>> > finally got it: >>> >>> >>> Hi John. These programs should behave identically, and I think your >>> version should be preferred. >>> This first code uses some class methods like mconcat, but it seems to >>> always be used on >>> the list in Chunks, so it will only ever use the definition for list, >>> which is equivalent to what >>> you wrote directly in the second code. >>> >>> The result may not be useful, but to understand this more thoroughly you >>> might >>> try parametrizating the definition of Stream so the use of more general >>> operators >>> actually means something. Perhaps >>> >>> data Stream m a = >>> Chunks (m a) >>> | EOF >>> >>> I think you would want Monad and MonadPlus on m. >>> >>> >>> >import Data.Monoid >>> >> >>> >> >>> >>data Stream a >>> >>= Chunks [a] >>> >>| EOF >>> >>deriving (Show, Eq) >>> >> >>> >> >>> >>instance Monad Stream where >>> >>return = Chunks . return >>> >>Chunks xs >>= f = mconcat (fmap f xs) >>> >>EOF >>= _ = EOF >>> >> >>> >> >>> >>instance Monoid (Stream a) where >>> >>mempty = Chunks mempty >>> >>mappend (Chunks xs) (Chunks ys) = Chunks (xs ++ ys) >>> >>mappend _ _ = EOF >>> > >>> > >>> >I guess, it shows my lack of experience in Haskell, but my question is, >>> > why is writing the code this way preferred over say writing it like this: >>> > >>> > >>> >import Data.Monoid >>> >> >>> >> >>> >>data Stream a >>> >>= Chunks [a] >>> >>| EOF >>> >>deriving (Show, Eq) >>> >> >>> >> >>> >>instance Monad Stream where >>> >>return x = Chunks [x] >>> >>Chunks xs >>= f = mconcat (fmap f xs) >>> >>EOF >>= _ = EOF >>> >> >>> >> >>> >>instance Monoid (Stream a) where >>> >>mempty = Chunks [] >>> >>mappend (Chunks xs) (Chunks ys) = Chunks (xs ++ ys) >>> >>mappend _ _ = EOF >>> > >>> > >>> >Cheers, >>> > >>> > >>> >-John >>> > >>> > >>> >_______________________________________________ >>> >Haskell-Cafe mailing list >>> >Haskell-Cafe(a)haskell.org >>> >http://www.haskell.org/mailman/listinfo/haskell-cafe >>> > >>> > >>> > >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >

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Re: [Haskell-cafe] uniplate (was: code review?)
by John Lato 30 May '11

30 May '11

> > From: Neil Mitchell <ndmitchell(a)gmail.com> > > > While I'm on the topic, I recently wrote a tool that wanted to > > traverse deep data structures as produced by haskell-src-exts. ?I > > wound up with about 50 lines of case expressions and around the time > > my hands were literally beginning to hurt decided that enough was > > enough and I should try a generic approach. ?I heard uniplate was > > pretty easy to use, and was pretty pleased to turn the entire thing > > into a single line. ?It took me a little longer to figure out I needed > > to use universeBi since all the examples were monotyped, but once I > > did it Just Worked. ?Amazing. ?So thanks again! ?And maybe you could > > mention universeBi in the instant introduction? > > Yes, I probably should - I'll try and get to that. Of course, I'd also > happily accept a patch against > http://community.haskell.org/~ndm/darcs/uniplate > > I use Uniplate inside HLint, and it's invaluable - there are a lot of > times when List Comp + universeBi really hits the spot. > Does Uniplate include an instance for: > instance Uniplate a => Biplate [a] a ? Since lists are a common case, I was surprised that this instance wasn't included. Is there a reason it shouldn't exist? I was importing Data.Generics.Uniplate.Direct FWIW. Thanks, John Lato

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[Maybe Int] sans Nothings
by michael rice 30 May '11

30 May '11

What's the best way to end up with a list composed of only the Just values,no Nothings? Michael ========================== import Control.Monad.Stateimport Data.Maybe type GeneratorState = State Int tick :: GeneratorState (Maybe Int)tick = do n <- get if ((n `mod` 7) == 0) then return Nothing else do put (n+1) return (Just n) {-*Main> evalState (sequence $ replicate 9 tick) 1[Just 1,Just 2,Just 3,Just 4,Just 5,Just 6,Nothing,Nothing,Nothing]-}

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something between a QQ and Q Exp?
by Nicolas Frisby 30 May '11

30 May '11

This message motivates adding support to Template Haskell for code that can be spliced but can no longer be intensionally analyzed. I'm trying to use the well-known technique of a hidden constructor in order to represent values that satisfy a particular predicate. > module Safe (Safe(), safe, mkSafe) where > > newtype Safe a = Hidden a -- the Hidden constructor is not exported > safe :: Safe a -> a -- the safe observer is exported In my case, the predicate is a restriction on the Haskell syntax used to define the value. In fact, in order to check my predicate, I need some of the information that TH embeds in the result of a [| ... |] quote. Thus I'm using TH to validate the construction of Safe values. > isSafe :: Exp -> Bool > isSafe = ... -- uses TH metadata embedded in the Exp > > mkSafe :: Q Exp -> Q Exp > mkSafe m = do > e <- m > if isSafe e > then return (AppE (ConE 'Hidden) e) > else fail ("not safe:\n\t" ++ pprint e) A user's construction of a Safe value then looks like "$(mkSafe [| ... |])" in some other module. I'd like this to be the only way to build a Safe value. Unfortunately, TH is a double-edged sword. In particular, it can be used to expose the Hidden constructor, thereby invalidating the Safe type encapsulation of my invariant. > abuse (AppE con _) = con > > uhoh :: a -> Safe a > uhoh = $(liftM abuse (safe [| ... |])) On the one hand, I need the user to use TH in order to construct Safe values. On the other, abuse of TH is capable of breaking my security kernel all together. If I could get by with a QuasiQuoter such as "[mkSafe| ... |]", the Hidden constructor would indeed be safe since the result of mkSafe is spliced in immediately and the user has no further access to it. Unfortunately, QuasiQuoters don't have access to the metadata that TH embeds in its quotations, which I need for validation. This inspires my idea for a solution: something between the two mechanisms. My first thought for a solution involved a new core type for Template Haskell. The Splice type would be completely abstract except for constructing it from a splicable type and the ability to splice a Splice into a program. So, TH could splice in either "Q alpha" like normal or "Q (Splice alpha)", for alpha an element of {Exp, Pat, Type, [Dec]}. "mkSafe" could be rewritten to generate a Splice instead of just an Exp, and hence an abusive user could no longer snoop the generated expression to extract the Hidden constructor. Unfortunately, if Splice is just another newtype with a hidden constructor, then it could be subverted in the same way that the "abuse" function exposes the Hidden constructor. Without further support from the Template Haskell system itself, it's turtles all the way down, I suspect. The popularity and convenience of creating a safety kernel by hiding constructors lends importance to plugging this hole, I think. Template Haskell is a useful system, but it currently subverts the most common lightweight approach to enforcing data invariants in the Haskell type system. It'd be nice if both could be used in the same system -- especially since the user can invoke Template Haskell regardless of the library author's intent. Is there already a way to write a security kernel that is robust against this sort of Template Haskell abuse? Or would we need further support from the Template Haskell system? Thanks for your time.

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Template Haskell sometimes sees hidden constructors
by Nicolas Frisby 30 May '11

30 May '11

Whith the three modules at the end of this email, I get some interesting results. Note that none of the constructors are exported, yet Template Haskell can see (and splice in variable occurrences of!) T, C2, W1, and W4. If you load Dump into GHCi, you get to see the Info that TH provides when you reify each of the data types. For T, T2, N1, and N4, their construct is visible in the Info even though M doesn't export it. As a consequence, you can load Unhide with no errors. Thus c = C, c2 = C2, w1 = N1, and w4 = N4, even though those constructors were not supposed to be imported. I couldn't find any mention of this on the GHC Trac for Template Haskell or for a general search of "reify". * http://j.mp/l9Ztjz (Description contains "reify") * http://j.mp/mprUmq (Component = Template Haskell) * Disclaimer: I didn't take the time to inspect this one http://hackage.haskell.org/trac/ghc/ticket/4946 T is isomorphic to (), T2 is like T with a phantom type argument, N1 is a newtype wrapping an Int, and N4 is like N3 with a phantom type variable. This seems too inconsistent to be an intended behavior. Am I missing something? Thanks. ==> M.hs <== module M (T(), T1(), T2(), T3(), T4(), N1(), N3(), N4()) where data T = C data T1 = C1 Int data T2 a = C2 data T3 a = C3 a data T4 a = C4 Int newtype N1 = W1 Int newtype N3 a = W3 a newtype N4 a = W4 Int ==> Dump.hs <== {-# LANGUAGE TemplateHaskell #-} module Dump where import Language.Haskell.TH import M dumpT, dumpT1, dumpT2, dumpT3, dumpT4, dumpN1, dumpN3, dumpN4 :: () dumpT = $(reify ''T >>= fail . show) dumpT1 = $(reify ''T1 >>= fail . show) dumpT2 = $(reify ''T2 >>= fail . show) dumpT3 = $(reify ''T3 >>= fail . show) dumpT4 = $(reify ''T4 >>= fail . show) dumpN1 = $(reify ''N1 >>= fail . show) dumpN3 = $(reify ''N3 >>= fail . show) dumpN4 = $(reify ''N4 >>= fail . show) ==> Unhide.hs <== {-# LANGUAGE TemplateHaskell #-} module Unhide where import Language.Haskell.TH import M c :: T c = $((\(TyConI (DataD _ _ _ [NormalC n _] _)) -> ConE n) `fmap` reify ''T) c2 :: T2 a c2 = $((\(TyConI (DataD _ _ _ [NormalC n _] _)) -> ConE n) `fmap` reify ''T2) w1 :: Int -> N1 w1 = $((\(TyConI (NewtypeD _ _ _ (NormalC n _) _)) -> ConE n) `fmap` reify ''N1) w4 :: Int -> N4 a w4 = $((\(TyConI (NewtypeD _ _ _ (NormalC n _) _)) -> ConE n) `fmap` reify ''N4) ----- for convenience, this is what I get when I load Dump in ghci Dump.hs:9:11: TyConI (DataD [] M.T [] [NormalC M.C []] []) In the expression: $(reify 'T >>= fail . show) In an equation for `dumpT': dumpT = $(reify 'T >>= fail . show) Dump.hs:10:12: TyConI (DataD [] M.T1 [] [] []) In the expression: $(reify 'T1 >>= fail . show) In an equation for `dumpT1': dumpT1 = $(reify 'T1 >>= fail . show) Dump.hs:11:12: TyConI (DataD [] M.T2 [PlainTV a_1627390697] [NormalC M.C2 []] []) In the expression: $(reify 'T2 >>= fail . show) In an equation for `dumpT2': dumpT2 = $(reify 'T2 >>= fail . show) Dump.hs:12:12: TyConI (DataD [] M.T3 [PlainTV a_1627390696] [] []) In the expression: $(reify 'T3 >>= fail . show) In an equation for `dumpT3': dumpT3 = $(reify 'T3 >>= fail . show) Dump.hs:13:12: TyConI (DataD [] M.T4 [PlainTV a_1627390695] [] []) In the expression: $(reify 'T4 >>= fail . show) In an equation for `dumpT4': dumpT4 = $(reify 'T4 >>= fail . show) Dump.hs:14:12: TyConI (NewtypeD [] M.N1 [] (NormalC M.W1 [(NotStrict,ConT GHC.Types.Int)]) []) In the expression: $(reify 'N1 >>= fail . show) In an equation for `dumpN1': dumpN1 = $(reify 'N1 >>= fail . show) Dump.hs:15:12: TyConI (DataD [] M.N3 [PlainTV a_1627390694] [] []) In the expression: $(reify 'N3 >>= fail . show) In an equation for `dumpN3': dumpN3 = $(reify 'N3 >>= fail . show) Dump.hs:16:12: TyConI (NewtypeD [] M.N4 [PlainTV a_1627390693] (NormalC M.W4 [(NotStrict,ConT GHC.Types.Int)]) []) In the expression: $(reify 'N4 >>= fail . show) In an equation for `dumpN4': dumpN4 = $(reify 'N4 >>= fail . show) Failed, modules loaded: M.

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GHCJS/Firefox support for forkIO and MVars
by Hamish Mackenzie 30 May '11

30 May '11

Firstly a big thanks to Neil Mix for this threading library... http://www.neilmix.com/2007/02/07/threading-in-javascript-17/ It is just what GHCJS needs for threading support (at least in Firefox). Here is what I did to get it working in GHCJS... * Added "Jump" so that tail calls don't grow the stack * Implemented a really basic scheduler * Added some thread and MVar primitives * Added a YieldTrampoline calling convention to GHCJS The code is here https://github.com/hamishmack/ghcjs cabal install cd examples runhaskell BuildTestYield.hs ../../ghc-7.0.2 Then open test-yield.html in Firefox. I think you can build it with any GHC 7 (I used 7.0.3), but you need the 7.0.2 source. I get this error when I try to use the 7.0.3 source... ../../ghc-7.0.3/libraries/base/GHC/Unicode.hs:35:0: error: HsBaseConfig.h: No such file or directory

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ANNOUNCE: Clipboard-2.2.0. System clipboard interface.
by Daniel Díaz Casanueva 30 May '11

30 May '11

Hi all, I've just uploaded Clipboard-2.2.0 to Hackage [1]. Clipboard [2] is a library that provides an interface for the system clipboard. The API is very simple, so anyone can use it without knowing of low-level Win32 API. Currently, this library only works on a Windows OS, due to its dependency on the Win32 library. Features for 2.2.0 version: * Added compatibility with the unicode character set, while reading the content of the clipboard. * Unicode character set is the new standard for setting the clipboard content. * Fixed the function 'modifyClipboardString'. * Some changes in documentation. I want to give thanks to Artyom Kazak, who sent me the patch of Unicode compatibilty and the fix for 'modifyClipboardString' function. -- References [1] - Hackage site - http://hackage.haskell.org [2] - Clipboard site - http://sites.google.com/site/dheltadiaz/packages/clipboard

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State Machine and the Abstractions
by Yves Parès 30 May '11

30 May '11

Hello, For the purposes of a simple strategy game, I'd like to build an EDSL that expresses missions. A mission could be represented as a state machine. With basic bricks such as actions (MoveTo, ShootAt...) or tests (EnemiesAround, LowHealth...), I could (ideally dynamically) build some strategic behaviors for the units. I will take the example of a patrol. Applied to a unit (or a group of units), it dictates : go from point 1 to point 2 and then go back and repeat. But when you detect an enemy near, leave the patrol path, destroy it and then resume your patrol where you left it. So if I consider my mission as a monad: data Mission = MoveTo Point | ShootAt Unit patrol = do MoveTo point1 MoveTo point2 patrol So far so good, but there, the only advantage to use a monad instead of a list of MoveTo's is the do-notation. And I lack the expression of tests. Using a GADT it could be: data Mission a where MoveTo :: Point -> Mission () ShootAt :: Unit -> Mission Bool -- If we have destroyed it or not EnemiesAround :: Mission [Unit] -- The enemies that are maybe in sight LowHealth :: Mission Bool -- If I should retreat ... -- (Monad Mission could be nicely expressed using Heinrich Apfelmus' * operational* package) patrol = do MoveTo point1 MoveTo point2 enemies <- EnemiesAround mapM_ ShootAt enemies patrol Aaaaaaaand... here comes the trouble: the actions are done *sequentially*. My units will move and then look at enemies, they will not monitor their environment while they move. So I need a way to say: A is your action of patrolling. B is your action of surveillance. Do both in parallel, but B is preponderant, as if it successes (if enemies are there) it takes over A. So, it is as if I was running two state machines in parallel. Moreover, the last line (the recursive call to patrol) is wrong, as it will restart the patrol from the beginning, and not from where it has been left. But this could be corrected by addind a test like "which point is the closest". So I thought about Arrows, as they can express sequential and parallel actions, but I don't know if it would be a right way to model the interruptions/recoveries. What do you think about it? Do you know of similar situations and of the way they've been solved?

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