- Haskell-Cafe - Haskell.org

Erlang's module discussion
by Tom Murphy 31 May '11

31 May '11

Hi All, I sure love Hackage, but there's a very interesting discussion going on, on the Erlang mailing list, about completely restructuring the module-model. Before you dismiss it as crazy, know that the topic was brought up by Joe Armstrong, one of the creators of the language. Here's the archive: http://erlang.org/pipermail/erlang-questions/2011-May/058769.html Food for thought... Tom

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Student Internships for Parallel Haskell Programming at NII, Tokyo
by Sebastian Fischer 31 May '11

31 May '11

As part of my research fellowship at the National Institute of Informatics in Tokyo, I announce the availability of student internships for up to three months between July and September 2011. Qualified applicants are enrolled in a Masters or Phd program, have a firm grasp of the Haskell programming language, and, ideally, know the different approaches to parallel and concurrent programming available in the Glorious Glasgow Haskell Compilation System (GHC). Their task is to support research on combinators for parallel programming by developing and comparing efficient implementations. The available fund covers sojourn costs but no travel expenses. It is meant to foster collaboration between Germany and Japan but qualified students from other countries may also be funded. If you want to spend the summer hacking Haskell in Tokyo please do not hesitate to contact me off list. Feel free to write even if you are unsure whether you are qualified or have any other questions regarding the internship. Best regards, Sebastian http://research.nii.ac.jp/members/fischer/ P.S. Information about the status after the earthquake is available on the Local Information page of ICFP'11: http://www.biglab.org/icfp11local/index.html Additionally, you may want to follow recent news in English by Mainichi Daily News and NHK World: http://mdn.mainichi.jp/ http://www3.nhk.or.jp/daily/english/

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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 31 May '11

31 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 31 May '11

31 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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How unique is Unique
by Emil Axelsson 30 May '11

30 May '11

Hello! Lacking a proper blog, I've written some notes about Data.Unique here: http://community.haskell.org/~emax/darcs/MoreUnique/ This describes a real problem that makes Data.Unique unsuitable for implementing observable sharing. The document also proposes a solution that uses time stamps to generate symbols that are "more unique". Does anyone have any comments on the proposed solution? Are there any alternatives available? Thanks! / Emil

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Re: [Haskell-cafe] Haskell School of Expression (graphics)
by John Lato 30 May '11

30 May '11

> > From: michael rice <nowgate(a)yahoo.com> > I think this is fixed in the gtk2hs source tree, have you tried building from the repo? > cabal install gtk2hs-buildtools > darcs get --lazy http://code.haskell.org/gtk2hs/ > cd gtk2hs > chmod +x bootstrap.sh > ./bootstrap.sh John Lato > Is this worth chasing down? I loaded the Haskell Platform, which I assume > has some graphics capability (Cairo?). Maybe my time would be better spent > getting familiar with that? > > Michael > --- On Sun, 5/29/11, Andrew Coppin <andrewcoppin(a)btinternet.com> wrote: > > From: Andrew Coppin <andrewcoppin(a)btinternet.com> > Subject: Re: [Haskell-cafe] Haskell School of Expression (graphics) > To: haskell-cafe(a)haskell.org > Date: Sunday, May 29, 2011, 9:45 AM > > On 28/05/2011 08:06 PM, michael rice wrote: > > > /tmp/glib-0.11.22906/glib-0.11.2/Gtk2HsSetup.hs:190:70: > > Couldn't match expected type `[PackageDB]' > > with actual type `PackageDB' > > Expected type: PackageDBStack > > Actual type: PackageDB > > In the sixth argument of `registerPackage', namely `packageDb' > > In the expression: > > registerPackage > > verbosity installedPkgInfo pkg lbi inplace packageDb > > cabal: Error: some packages failed to install: > > cairo-0.11.1 failed during the configure step. The exception was: > > ExitFailure 1 > > There's some sort of glitch with Gtk2hs (on which SOE depends) that causes > it to not build with GHC 7.x (?) unless you hand-edit some of the files (!) > > http://hackage.haskell.org/trac/gtk2hs/ticket/1203

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Detecting overlay-ed signal patterns
by Dmitri O.Kondratiev 30 May '11

30 May '11

Hello, I am trying to detect signal patterns in a system with numerous signals overlaying each other in time. I am aware that there is a whole class of problem solving methods for this task, yet trying my own algorithm, that may be wrong of course :) Next I use a simple example to describe the problem and my algorithm. Maybe someone will find this problem interesting enough to comment. Thanks in advance! In the following example signals from varying sources are represented by chars - unique signal = unique char. Examples of signal patterns (Pk) to detect: P1 = abcd P2 = 12345 Examples of a mixture of signals (pattern signals are underscored): M1 = s s n a_b_c_d_ r t 6 1_ 2_ 3_ 4_ 5_ 8 7 k l t M2 = g h a_ r 1_ w e 2_ j k b_ c_ l m p 3_ d_ 4_ e r 5_ v x M3 = ... MN = ... In the above examples two patterns P1 and P2 are interleaved with each other and other patterns. All mixtures are recorded for 24 hours for 12 month. One mixture corresponds to one 24 period (day). To find signal patterns in all these mixtures (M1, M2) my algorithm goes through these steps: 1) For every day: For every signal 'Y' create a list of 'weight pairs' such as: (Y, X1, 1), (Y, X2, 1), .. (Y, XN, 1) where X1, X2, ... XN are signals 'accessible from' (or following) signal 'Y' during this day So, for example, given the sequence: "abcde", chars in a left column of a table can access a list of chars in the right column: a | b c d e b | c d e c | d e d | e 2) For every signal sum all its 'weight pairs' for all days of the year: W(Y) = Sum((Y, Xk, 1), j=1... 364) For the above example, M1+ M2 will double the value of weight pairs in patterns: P1 = abcd P2 = 12345 such as: (a,b,2) (a,c,2) (a,d,2) (b,c,2) (b,d,2) (b,e,2) (c,d,2) (c,e,2) (d,e,2) and also: (1,2,2) (1,3,2) (1,4,2), (1,5,2) (2,3,2) (2,4,2) (2,5,2) etc... On the other hand, after summation M1+M2, weight pairs for other char sequences, that do not form patterns of often repeated signals, will not increase much. To demonstrate: M_1 and M_2 are sequences with repeating patterns being removed: M_1 = s s n r t 6 8 7 k l t M_2 = g h r w e j k l m p e r v x Here most pair weights will be small, same as before addition: (s,s,2) (g,h,1), (w,e, 1) ... 3) Compute weight of all possible chains in this year. Each chain consists of chars that can be accessed from one to another. Chars are examined, proceeding from the beginning of the sequence to its end, and never in the opposite direction. Going always in one direction and being bound with sequence end, makes chains finite. Chains for 'abcde' example will be: abcde, acde, ade, ae, bcde, bde, be, cde, cd, ce, de Weight of the chain is computed as a sum of its pairs: W(Ci) = Sum(Pj, j=1,..R) where R is a number of pairs that this chain is built from. For example for chain a 'acde' the weight will be: W('acde') = W(a,c)+ W(a,d) + W(a,e) 4) Split chains in groups by its length 5) For each group select chains with maximum weight. With pattern P1 = 'abcd' spread over some signal mixture, my algorithm will find (I hope) the following 4 classes of chains with maximum weight: 1) abcde, acde, ade, ae, 2) bcde, bde, be, 3) cde, cd, ce, 4) de In the same way, chains for pattern P2 = 12345 should be found. Thus, grounding on the idea of 'signal precedence' found in repeating pattern of several signals, and using pair 'weights' to mark signal chains - patterns are detected. Summation of pair weights allows to stress out, or highlight the repeating patterns. Inserting other signals inside a 'pattern pair' does not obscure it because in a long run (after many summations) its weight will make such pair stand out of the crowd. I have not yet fully developed this algorithm and have not tested it on big enough training sets. I hope it will work on training sets exhibiting some regularities - which in this case means repeating patterns. Of course it will not work on randomly generated data, where all pairs have approximately the same weight. Working with pairs and chains will allow for natural parallelism in processing huge sequences of events, I hope. It would be nice to use some analog of Hadoop map-reduce for this task. Any experience with Haskell analog of Apache Hadoop cluster (http://hadoop.apache.org/mapreduce/)? Does such thing exist in Haskell world at all? I would be happy to get criticism of my algorithm and comments on 'route-cause analysis' algorithms in general, thanks! Dmitri

1 0

Please help with double recursion
by Dmitri O.Kondratiev 30 May '11

30 May '11

Hello, I am trying to solve a simple task, but got stuck with double recursion - for some reason not all list elements get processed. Please advice on a simple solution, using plane old recursion :) *** Task: >From a sequence of chars build all possible chains where each chain consists of chars that can be accessed from one to another. For example, given the sequence: "abcde" In table below chars in a left column can access a list of chars in the right column: a | b c d e b | c d e c | d e d | e Then chains for this example will be: abcde, acde, ade, ae, bcde, bde, be, cde, cd, ce, de *** My incomplete result, which I get running my program (see at the end of this message): ('a','b'),('b','c'),('c','d'),('d','e'), -- abcde, ('c','e'), -- ce, ('b','d'),('d','e'), -- bde, ('b','e') -- be, ('a','c'),('c','d'),('d','e'), -- acde ('c','e'), ('a','d'),('d','e'), -- ade, ('a','e') -- ae Sequence 'abcde' contains 'bcde', 'cde', 'cd', 'de'. How to add all these sub-sequences as a separate sequences in my output? And I have 'ce' sequence duplicated, which I don't need. Also, how to output chains as a list of lists? Such as: [[abcde, acde, ade, ae,] [bcde, bde, be,] [cde, cd, ce,] de]] *** my code test = chains testPairs 'a' [] testPairs = pairs testSeq testSeq = "abcde" -- From a list of '((from,to),1)' pairs build char chains -- Char chain is a list of chars: [from1, to1 = from2, to2 = from3, to3 = from4, ...] -- 'pairList' - a list of pairs chains pairList from chainList = chainWrk (nextTo pairList from) from chainList where chainWrk [] from chainList = chainList chainWrk (to:tos) from chainList = chainWrk tos from (chains pairList to (chainList ++ [(from,to)])) -- Find direct neighbors for 'from' nextTo pairList from = toList (findPairs pairList from) [] -- From a list of '((from,to),1)' pairs build a list of 'to'-s toList [] tos = tos toList (((from,to),len):ps) tos = toList ps (tos ++ [to]) -- From 'pairList' find elements with 'from' equal to 'start' -- 'pairList' is a list of '((from,to),1)' pairs findPairs pairList search = filter (flt search) pairList where flt search ((from, to), len) = search == from -- From a sequence of chars buid a list of '((from,to),1)' pairs pairs [] = [] pairs (x:xs) = pairWrk x xs [] ++ pairs xs pairWrk hd [] pairLst = pairLst pairWrk hd (x:xs) pairLst = pairWrk hd xs (pairLst ++ [((hd,x),1)])

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ANN: dtd-types 0.0.0.1
by Yitzchak Gale 30 May '11

30 May '11

The dtd-types package provides types for processing XML DTDs in Haskell. These types are intended to be compatible with and extend the set of types provided by John Millikin's xml-types package. This is a very preliminary first version, pending discussion by the community on the web-devel list about integration with xml-types, and any other issues that might come up. Hackage: http://hackage.haskell.org/package/dtd-types/ Darcs: http://code.haskell.org/dtd/dtd-types Enjoy, Yitz

1 0

Efficient object identity (aka symbols as data)
by Jacek Generowicz 30 May '11

30 May '11

[ TLDR: How do you do Lisp symbols in Haskell? ] What is the Haskell approach to efficient comparison and lookup of objects by their identity? Maybe a toy example would help to explain what I mean. Imagine that I want to use Haskell to maximize happiness in a situation where a bunch of people have to be assigned to certain groups according to their preferences, and some constraints on the group sizes. Conceptually my input data might be structured as follows: data Group = Group GroupName MaxGroupSize data Person = Person Name Group Group Group The program should partition the people into groups, attempting to get everyone into their most favoured groups. Whatever algorithm I choose to use for the optimization, will have to do lots of comparisons of Groups and Persons where their *identity* is all that matters: you don't need to look inside the objects. On the other hand, sometimes we will have to look inside the objects, for example in order to answer queries such as "Which group did Johnny get and how far down his list of choices did that group feature?". I should be able to run the program on data that becomes available at run time. The lisper in me is crying out for (lisp-like-)symbols which I can create from the input data at run time and on which some extra information can be hung. How would I organize something like this in Haskell? Thanks.

10 15

Iteratee stack-consumption
by Tony Morris 30 May '11

30 May '11

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 I have written iteratees on the JVM, which does not include tail-call in its instruction set. They incur a stack frame per iteratee input, which can be expensive and unable to scale beyond a few thousand. Is there some trick to permitting this? Some sort of chunking the enumeratee or something? I was unable to find any good general hints in any of the hackage libraries. Thanks for any pointers. - -- Tony Morris http://tmorris.net/ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAk3jNHEACgkQmnpgrYe6r63QcACfSeZvvy+iYVisZ3e/dhWQd3D6 790AnRZCjJT6eCY21XlsVqcpTuamfuXY =ADo5 -----END PGP SIGNATURE-----

1 0

Re: [Haskell-cafe] State Machine and the Abstractions
by Chuzzle Guevero 30 May '11

30 May '11

For one, you have a kind error. You use Mission as a Monad when it only has kind *. I don't know much of arrows, but I suggest writing the combinators you want to have with specialized types, and see where that takes you. If it happens to lead to an implementation of Arrow, yay. If it doesn't, then you at least still have something that functions. > Message: 13 > Date: Fri, 27 May 2011 21:06:10 +0200 > From: Yves Par?s<limestrael(a)gmail.com> > Subject: [Haskell-cafe] State Machine and the Abstractions > To: Haskell-Cafe<haskell-cafe(a)haskell.org> > Message-ID:<BANLkTikVb-zhyTVR=jT03hFiBAKrERxsLQ(a)mail.gmail.com> > Content-Type: text/plain; charset="iso-8859-1" > > 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?

3 2

Utrecht Summer School in Applied Functional Programming
by João Paulo Pizani Flor 29 May '11

29 May '11

Hi dear Haskellers! I'm a senior undergraduate Computer Science student in Brazil, and have always loved Haskell and wrote most of my lab assignments and course projects in it. Haskellers are a very rare species here in Brazil, but I always tried my best to motivate people around me presenting some seminars, writing some tutorials, etc. My personal Haskell project: http://launchpad.net/presto What joy I had then last Friday, when I found out I've been accepted to participate in the Utrecht Summer School 2011, in the course "Applied Functional Programming in Haskell" (http://www.utrechtsummerschool.nl/index.php?type=courses&code=H9) In fact, I got a scholarship from Utrecht University that covers tuition fees and housing, but not the plane tickets to/from the Netherlands. That's the point where I need your advice: without some sort of financial support I won't be able to buy the tickets and travel to Utrecht. I am already searching from grants here in Brazil, but I think it will be hard to find something, not to say impossible. So, do you know any program/fund/foundation to which I can apply and get the needed financial support? Best regards, João Paulo Pizani Flor joaopizani(a)gmail.com Computer Science Federal University of Santa Catarina - Brazil

1 0

Escaping of string literals
by Michael Snoyman 29 May '11

29 May '11

Hi all, I'm working on a program right now that will involve embedding some static files inside my Haskell program as bytestrings. I've done this in the past with file-embed[1]. In this case, I have a strange requirement: I need to be able to modify the embedded data after the compiler has run. In general, I've gotten the code to work (and will soon be following up with a blog post explaining). But in this process, I ran into something I found interesting. If I run this program: {-# LANGUAGE TemplateHaskell #-} import Data.ByteString.Unsafe (unsafePackAddressLen) import Language.Haskell.TH.Syntax import qualified Data.ByteString as S main = do fromAddr <- unsafePackAddressLen 7 $(return $ LitE $ StringPrimL "123\0\&456") print fromAddr let fromStr = S.pack $ map (toEnum . fromEnum) $(return $ LitE $ StringL "123\0\&456") print fromStr I get the result: "123\192\128\&45" "123\NUL456" My understanding is that GHC is automatically escaping the non-ASCII characters in the strings. When I use StringL, GHC also automatically unescapes the bytes, so everything works as expected. In the case of StringPrimL, there is no point at which the unescaping occurs. I haven't been able to find the rules on how this escaping functions. Can anyone point it out to me? Also, for my case, it would be very useful to be able to bypass the escaping. I can easily do so with a post-process on the executable to embed raw bytes, but I'm wondering if there's some way to convince StringPrimL not to do any escaping. Thanks, Michael [1] http://hackage.haskell.org/package/file-embed

2 2

No fish, please
by Andrew Coppin 28 May '11

28 May '11

Both Hackage and Cabal seem to assume as a matter of course that I want to use Haddock to generate all my documentation. Suppose I decide to violate this assumption. Then what? 1. Is there some way I can include my own HTML documentation in the package tarball and have Hackage/Cabal use it? 2. Is there a way to tell Hackage/Cabal to use some tool besides Haddock?

8 13

representing spreadsheets
by Eric Rasmussen 28 May '11

28 May '11

Hi everyone, I'm hoping someone can point me in the right direction for a project I'm working on. Essentially I would like to represent a grid of data (much like a spreadsheet) in pure code. In this sense, one would need functions to operate on the concepts of "rows" and "columns". A simple "cell" might be represented like this: data Cell = CellStr Text | CellInt Integer | CellDbl Double | CellEmpty The spreadsheet analogy isn't too literal as I'll be using this for data with a more regular structure. For instance, one grid might have 3 columns where every item in column one is a CellStr, every item in column two a CellStr, and every item in column 3 a CellDbl, but within a given grid there won't be surprise rows with extra columns or columns that contain some cell strings, some cell ints, etc. Representing cells in a matrix makes the most sense to me, in order to facilitate access by columns or rows or both, and I'd like to know if there's a particular matrix library that would work well with this idea. However, I'm certainly open to any other data structures that may be better suited to the task. Thanks! Eric

4 6

Enterprise Haskell -- help
by Srinivasan Balram 27 May '11

27 May '11

folks: I was advised to post this request here. This is about needs of daily-grind enterprise development. Enterprise developers need 3 categories of books in Haskell urgently: (i) Haskell (CookBooks / Recipes) (ii) Haskell Enterprise Development i.e. how to connect commercial RDBMS and use Haskell along with SQL effectively (iii) Haskell Web/Network Development Books by Don Stewart & Miran Lipovaca are excellent for beginner/intermediaries. Erik Meijer's tutorial videos were very useful as well. But they are not enough. This may sound boring, but such books are badly needed "today". It's hard to convince clients/devs to start projects "without such resources". I am trying to push for Enterprise Haskell projects. Would appreciate pointers to resources for doing large scale Haskell development Thank you -ram

8 12

Re: [Haskell-cafe] Policy for taking over a package on Hackage
by Mario Blažević 27 May '11

27 May '11

On 11-05-25 08:52 AM, Johan Tibell wrote: > On Wed, May 25, 2011 at 2:01 PM, Ivan Lazar Miljenovic > <ivan.miljenovic(a)gmail.com> wrote: >> With my wl-pprint-text package, Jason Dagit suggested to me on >> #haskell that it would make sense to make such a pretty-printer be >> class-based so that the same API could be used for String, ByteString, >> Text, etc. > I'm a bit skeptical of using type classes to abstract over Unicode > string types and byte sequence types. The only API shared by the two > kind of types is that of a sequence. Things like dot , spaces, etc. > don't make much sense on binary data. You must assume that the > ByteString contains text in some encoding to make sense of such > concepts. You don't necessarily need spaces and dot to abstract over bytestrings and Unicode string types. They both implement IsString and Monoid classes, as well as operations null, length, isPrefixOf, take, and drop. That's sufficient to implement a generic parser that works on any input type supporting these operations (see [1] for example), though there is some performance cost. I don't see why a pretty-printer should be any more difficult. [1] http://hackage.haskell.org/package/incremental-parser

1 0

Parallel compilation and execution?
by michael rice 27 May '11

27 May '11

How do I compile and run this parallel program? Michael =========================== import Control.Parallel nfib :: Int -> Intnfib n | n <= 1 = 1 | otherwise = par n1 (seq n2 (n1 + n2 + 1)) where n1 = nfib (n-1) n2 = nfib (n-2) {-nfib :: Int -> Intnfib n | n <= 1 = 1 | otherwise = nfib (n-1) + nfib (n-2)-} main = do putStrLn $ show $ nfib 39 =========================== [michael@hostname ~]$ ghc --make -threaded nfib.hs[michael@hostname ~]$ ./nfib +RTS -N3nfib: the flag -N3 requires the program to be built with -threadednfib: nfib: Usage: <prog> <args> [+RTS <rtsopts> | -RTS <args>] ... --RTS <args>nfib: nfib: +RTS Indicates run time system options follownfib: -RTS Indicates program arguments follownfib: --RTS Indicates that ALL subsequent arguments will be given to thenfib: program (including any of these RTS flags)nfib: nfib: The following run time system options are available:nfib: nfib: -? Prints this message and exits; the program is not executednfib: --info Print information about the RTS used by this programnfib: nfib: -K<size> Sets the maximum stack size (default 8M) Egs: -K32k -K512knfib: -k<size> Sets the initial thread stack size (default 1k) Egs: -k4k -k2mnfib: nfib: -A<size> Sets the minimum allocation area size (default 512k) Egs: -A1m -A10knfib: -M<size> Sets the maximum heap size (default unlimited) Egs: -M256k -M1Gnfib: -H<size> Sets the minimum heap size (default 0M) Egs: -H24m -H1Gnfib: -m<n> Minimum % of heap which must be available (default 3%)nfib: -G<n> Number of generations (default: 2)nfib: -T<n> Number of steps in younger generations (default: 2)nfib: -c<n> Use in-place compaction instead of copying in the oldest generationnfib: when live data is at least <n>% of the maximum heap size set withnfib: -M (default: 30%)nfib: -c Use in-place compaction for all oldest generation collectionsnfib: (the default is to use copying)nfib: -w Use mark-region for the oldest generation (experimental)nfib: nfib: -t[<file>] One-line GC statistics (if <file> omitted, uses stderr)nfib: -s[<file>] Summary GC statistics (if <file> omitted, uses stderr)nfib: -S[<file>] Detailed GC statistics (if <file> omitted, uses stderr)nfib: nfib: nfib: -Z Don't squeeze out update frames on stack overflownfib: -B Sound the bell at the start of each garbage collectionnfib: nfib: -hT Heap residency profile (output file <program>.hp)nfib: -i<sec> Time between heap samples (seconds, default: 0.1)nfib: nfib: -C<secs> Context-switch interval in seconds.nfib: 0 or no argument means switch as often as possible.nfib: Default: 0.02 sec; resolution is set by -V below.nfib: -V<secs> Master tick interval in seconds (0 == disable timer).nfib: This sets the resolution for -C and the profile timer -i.nfib: Default: 0.02 sec.nfib: nfib: --install-signal-handlers=<yes|no>nfib: Install signal handlers (default: yes)nfib: nfib: RTS options may also be specified using the GHCRTS environment variable.nfib: nfib: Other RTS options may be available for programs compiled a different way.nfib: The GHC User's Guide has full details.nfib: [michael@hostname ~]$

6 10

Strange Type Error
by jean-christophe mincke 27 May '11

27 May '11

Hello, Could anyone help me understand what is wrong with the definition of f2 in the code below? class C a b where convert :: a -> b convertToInt :: (C a Int) => a -> Int convertToInt x = convert x f1 x = convertToInt x f2 = \x -> convertToInt x f3 :: (C a Int) => a -> Int f3 = \x -> convertToInt x Thank you Regards J-C

3 4

Email spam from my account on May 26, 2011
by Gregory Propf 27 May '11

27 May '11

I just discovered that some evil spammer has somehow gotten my contacts list and used it to send out a bunch of spam. This is just to notify you that if you get an email from me on May 26, 2011 (other than this one or one like it - the problem was more extensive than I first thought) it wasn't from me. Please don't add me to your spam filters. I've changed my password and hopefully that's the end of it. At this point I don't know how they did it though. - Greg

1 0

(no subject)
by Gregory Propf 27 May '11

27 May '11

As you my friend I invite you to visit my own site first!. http://prospero.ch/friends_links.php?uGIS=45ru4

1 0

Policy for taking over a package on Hackage
by Ivan Lazar Miljenovic 26 May '11

26 May '11

With my wl-pprint-text package, Jason Dagit suggested to me on #haskell that it would make sense to make such a pretty-printer be class-based so that the same API could be used for String, ByteString, Text, etc. I was thinking of doing so, and in such a case it would probably make the most amount of sense to actually then migrate the package back to the wl-pprint package instead of defining yet another package (and corresponding namespace), especially as I recalled from when I first started hacking on wl-pprint-text at the beginning of the year that the last (and only) upload had been in 2007 by Stefan O'Rear. As such, I was going to email Stefan and asked if he minded if I took over maintaining wl-pprint. However, when I went to the package page to get his email address, I saw that Otakar Smrz had released a new version about a month ago, and that the only change in the package that I could find was that OverlappingInstances was added to the .cabal file (which I thought wouldn't make a difference anyway). Now, for all I know, Otakar had already asked Stefan for permission to take over the package (I've CC'd both of them just in case). But even if he didn't, we don't officially have any policies set in place - especially ones that are enforced by requiring registering a package and specifying a maintainer - to indicate that such a thing shouldn't be done. Furthermore, if packages aren't announced then there's no way for people to know that there _is_ a new maintainer. This situation also arose last year [1], and was resolved by someone volunteering to take over a package. However, no formal policy was set in place (despite one being proposed). As such, I'd like to propose the following policy based upon the one Ben Millwood proposed last time on how to take over maintainership of a package that hasn't been updated for a while: 1. Email the current listed maintainer and wait a specified period of time (e.g. 2 weeks). 2. Email haskell-cafe, explicitly CC'ing maintainers of reverse dependencies (at least those that are themselves still active) and request permission to take over. This way, people who know the maintainer might point out that they are indeed still around, etc. 3. If no-one objects within another two weeks, announce that you have taken over maintainership with a new email (in case people are ignoring the previous thread). 4. Upload a point release of the previous package (assuming it follows the PVP) with yourself as the new maintainer (just to get it out there). Alternatively, if you already have a new version ready to go then upload that. Of course, having a policy like this is useless if there is no way to enforce it. Is it possible at the very least to have Hackage note when a new version of a package is uploaded with a new maintainer and require it to be approved by someone before it iis officially available? I think this would be the bare minimum (as opposed to requiring explicit logins on a per-package basis or some kind of signing) required for this kind of scheme to be effective. [1]: http://www.haskell.org/pipermail/haskell-cafe/2010-August/081353.html and http://www.haskell.org/pipermail/haskell-cafe/2010-August/082319.html -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

10 16

Function application layout
by Jonas Almström Duregård 26 May '11

26 May '11

Hi, Would it be possible to allow this in Haskell (where <applied to> is some new operator or keyword): f <applied to> {x a;y b;z c} As an equivalent to: f (x a) (y b) (z c) Of course my intention is that the new keyword should initiate layout syntax so we can write this: f <applied to> x a y b z c In addition to the case where you have a big function application, this is useful for defining trees: Branch <applied to> Branch <applied to> Leaf Leaf Leaf Has something like this been suggested before? Are there any disadvantages other than a new keyword and some potential confusion for readers? Any suggestions for a good keyword? I suppose some variant of $ makes sense, a textual keyword like "with" would be nice but probably break a lot of code. Regards, Jonas

6 10

Haskell Weekly News: Issue 183
by Daniel Santa Cruz 26 May '11

26 May '11

Welcome to issue 180 of the HWN, a newsletter covering developments in the Haskell community. This release covers the week of May 15 to 21, 2011. Announcements Janis Veigtlander announced that the 20th edition of the Haskell Communities and Activities reports is hot off the press! http://goo.gl/GRi7V Simon Marlow announced that he has put together a tutorial on Parallel and Concurrent programming in Haskell. http://goo.gl/LPst8 Quotes of the Week * Sergey Mironov:I looked further and found that 'Oleg already did it'. * monochrom: just say "yes" Top Reddit Stories * Wolfgang - A Wolfram Alpha clone in Haskell (under heavy load?) Domain: jabberwock.xs4all.nl, Score: 75, Comments: 10 On Reddit: http://goo.gl/okNC7 Original: http://goo.gl/7pSXA * Numeric Haskell: a tutorial about the multi-dimensional, parallel arrays package, Repa Domain: haskell.org, Score: 54, Comments: 7 On Reddit: http://goo.gl/ZRSn3 Original: http://goo.gl/InWB9 * Space leak zoo: a catalog of common space leaks in Haskell programs Domain: blog.ezyang.com, Score: 41, Comments: 7 On Reddit: http://goo.gl/npVLv Original: http://goo.gl/aMbo5 * Exercises in Optimization: A (soon to be) three-part series detailing an adventure in optimizing Haskell code. Domain: csks.wordpress.com, Score: 34, Comments: 0 On Reddit: http://goo.gl/cjWBX Original: http://goo.gl/wPqbl * Tutorial: Parallel and Concurrent Programming in Haskell :: PDF Domain: community.haskell.org, Score: 33, Comments: 7 On Reddit: http://goo.gl/CB1wt Original: http://goo.gl/yzTOU * Anatomy of a thunk leak: How to debug Haskell’s most famous space leak Domain: blog.ezyang.com, Score: 32, Comments: 3 On Reddit: http://goo.gl/f18jX Original: http://goo.gl/PiQ98 * Preview release of diagrams EDSL for declarative drawing Domain: haskell.org, Score: 31, Comments: 9 On Reddit: http://goo.gl/6sykO Original: http://goo.gl/uhbV1 * I/O is not a monad Domain: r6.ca, Score: 31, Comments: 23 On Reddit: http://goo.gl/zAra5 Original: http://goo.gl/mn49r * Eliza in 70 lines of Haskell Domain: github.com, Score: 28, Comments: 7 On Reddit: http://goo.gl/hjgNf Original: http://goo.gl/S7t9m * The "Whole Program" Fallacy Domain: lukepalmer.wordpress.com, Score: 26, Comments: 10 On Reddit: http://goo.gl/Efja3 Original: http://goo.gl/vzXVK Top StackOverflow Questions * What Haskell representation is recommended for 2D, unboxed pixel arrays with millions of pixels? votes: 48, answers: 3 Read on SO: http://goo.gl/gVpwB * Good introductory text about GHC implementation? votes: 24, answers: 2 Read on SO: http://goo.gl/6MZoh * Questions on a Haskell -> C# conversion votes: 13, answers: 3 Read on SO: http://goo.gl/0t3Gu * Can't perform I/O in foldr? votes: 12, answers: 4 Read on SO: http://goo.gl/d9Jg0 * What does "this function will fuse" mean? votes: 11, answers: 3 Read on SO: http://goo.gl/tMdlR * Haskell function type question votes: 10, answers: 3 Read on SO: http://goo.gl/gyTns * Is this always true: fmap (foldr f z) . sequenceA = foldr (liftA2 f) (pure z) votes: 10, answers: 1 Read on SO: http://goo.gl/emKmg * Difference between hsc2hs and c2hs? votes: 8, answers: 2 Read on SO: http://goo.gl/1wnhC * Is it possible to create PHP extensions in Haskell? votes: 8, answers: 2 Read on SO: http://goo.gl/QwkpK * How can configuration tools like sdl-config be used with a cabalized project? votes: 8, answers: 2 Read on SO: http://goo.gl/vHCOu About the Haskell Weekly News To help create new editions of this newsletter, please send stories to dstcruz(a)gmail.com. Until next time, Daniel Santa Cruz

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[ANN] Released HSnippets, Emacs YASnippets for Haskell
by Christophe McKeon 26 May '11

26 May '11

Just released HSnippets, Emacs YASnippet snippets for Haskell https://github.com/polypus74/HSnippets cheers, _c

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lambdabot hoogle
by Jacek Generowicz 26 May '11

26 May '11

I have recenly installed lambdabot. Its response to *each* *and* *every* hoogle command is *always* A Hoogle error occurred. I'm hoping that someone on Cafe might be able to offer a more helpful diagnosis than this ... erm ... terse ... error message.

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lambdabot check
by Jacek Generowicz 25 May '11

25 May '11

I've installed lambdabot, but check within it seems to be broken: the only answer it ever gives is "Terminated". For example: lambdabot> check True Terminated lambdabot> quickCheck works just fine in a GHCi session: Prelude> :m + Test.QuickCheck [...] Prelude> quickCheck True [...] +++ OK, passed 100 tests. (GHC 7.0.3 on Ubuntu Natty, everything freshly installed with cabal install as far as possible.) Any advice?

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Calling a C function that returns a struct by value
by Matthew Steele 25 May '11

25 May '11

From Haskell, I want to call a C function that returns a struct by value (rather than, say, returning a pointer). For example: typedef struct { double x; double y; } point_t; point_t polar(double theta); I can create a Haskell type Point and make it an instance of Storable easily enough: data Point = Point CDouble CDouble instance Storable Point where -- insert obvious code here And now I want to do something like this: foreign import ccall unsafe polar :: CDouble -> IO Point ...but that doesn't appear to be legal (at least in GHC 6.12). Is there any way to import this C function into Haskell _without_ having any additional wrapper C code? I mean, I suppose I could write something like: // C: void polar_wrapper(double theta, point_t* output) { *output = polar(theta); } -- Haskell: foreign import ccall unsafe polar_wrapper :: CDouble -> Ptr Point - > IO () polar :: CDouble -> IO Point polar theta = do -- insert obvious code here ...but that's a lot of extra boilerplate if I have many such functions to wrap, and it seems like GHC should be able to do that sort of thing for me. P-: Cheers, -Matt

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code review?
by Chris Bolton 25 May '11

25 May '11

Hey guys. Pretty new to haskell here. I've started off by writing a small command-line interface to plurk.. I was just wondering if anyone would be willing to give everything a look-over and lemme know what kinds of things I should be looking to improve upon, style-wise. Not sure I'm currently doing things in the 'haskell-way' so to speak . Thanks a bunch! https://github.com/saiko-chriskun/hermes (note: the JSON and Plurkell submodules are also mine.)

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uniplate and haskell-src-exts (was: Re: code review?)
by Evan Laforge 25 May '11

25 May '11

> +1 on that, I use uniplate for pretty much all my haskell-src-exts tasks > these days, works like a charm! > I'd love to include some standard traversal functionality in > haskell-src-exts that depends on uniplate, but hesitate to do so because of > HP aspirations for haskell-src-exts. Neil, what do you reckon the chances of > getting uniplate in the HP are? :-) Or, you could just put a note in the documentation suggesting that uniplate is a good way to traverse parsed output, along with an example or two. And maybe mention a separate package haskell-src-exts-uniplate with utility functions, if there are any. Keeping deps under control by splitting packages this way has some precedent, and makes some logical sense too, since it's not like the parsed data types are trying to be abstract by hiding constructors.

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Status of Haskell + Mac + GUIs & graphics
by Conal Elliott 25 May '11

25 May '11

I still haven't found any way to do GUIs or interactive graphics in Haskell on a Mac that isn't plagued one or more of the following serious problems: * Incompatible with ghci, e.g., fails to make a window frame or kills the process the second time one opens a top-level window, * Goes through the X server, and so doesn't look or act like a Mac app, * Doesn't support OpenGL. A year or two ago, I put my Haskell GUI & graphics work on hold while waiting & hoping for a functioning pathway to open. So far I haven't heard of one. If anyone has found a solution, I'd love to hear! - Conal

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Re: [Haskell-cafe] Policy for taking over a package on Hackage
by Ivan Lazar Miljenovic 25 May '11

25 May '11

On 25 May 2011 23:23, Otakar Smrz <otakar.smrz(a)cmu.edu> wrote: > > I took over the maintenance of wl-pprint since Stefan suggested it > himself. I needed to include OverlappingInstances, because that was > required to make better use of the library (there is a difference having > this flag there, which is consistent with the GHC documentation on > OverlappingInstances). > > I was not aware that I have to announce this. I thought people would > know from checking Hackage that there is this maintainer change and a > new version of the package. Sorry. The whole email wasn't aimed at you specifically: it's just that with Hackage's current infrastructure it's quite easy for someone to "take over" a package and possibly do something malicious with it (so the next time they do a "cabal update" they could inadvertently get a package with said malicious code). Especially if there's no announcement of the change, it's difficult to know that someone has indeed taken over maintaining a library, and thus they could keep addressing queries (or throwing blame) at someone "known" to be the maintainer of a package even if they didn't upload the latest version. > Since I do not have any plans to develop wl-pprint further, I have no > problem giving the maintainer's role to you or anyone interested. OK; if no-one has any objections I'll do so and make it class-based as I've discussed in the threads on -cafe (after considering name clashes with Applicative, etc.). > PS: For the point 4 of your suggested rules, I think uploading a new > version of a package if only the maintainer changes, rather than when > there is some actual change to the library, is inappropriate. You could > easily get package versions increasing without any code/documentation > being actually improved or extended. By "point release" I meant incrementing the fourth digit, which is usually used to refer to bug-fixes. My aim with this was that the new maintainer could immediately "make their mark" so that people looking for a project's maintainer on the hackage page knew who to contact (rather than waiting for them to finish of their new & improved version). -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com IvanMiljenovic.wordpress.com

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Cabal upload failure
by Emil Axelsson 25 May '11

25 May '11

Hello! I'm trying to upload a new version of syntactic, but Hackage gives the error: 500 Internal Server Error <stdin>: hWaitForInput: invalid argument (Invalid or incomplete multibyte or wide character) In fact, I get the same error if I use the "Check" functionality on the earlier version that is already on Hackage. Any idea what causes this? / Emil

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ANNOUNCE: haskell-src-exts 1.11.1
by Niklas Broberg 25 May '11

25 May '11

Fellow Haskelleers, I'm pleased to announce the release of haskell-src-exts-1.11.1! * On hackage: http://hackage.haskell.org/package/haskell-src-exts * Via cabal: cabal install haskell-src-exts * Darcs repo: http://code.haskell.org/haskell-src-exts This major release fixes some long-standing bugs in the fixity handling. Many thanks for Lennart Augustsson for the patches! Changelog: 1.10.2 --> 1.11.1 =============== * API change: the fixities field in ParseMode is now of type Maybe [Fixity]. If the field is Nothing the parsing will not try to do any fixity resolution whatsoever, otherwise it behaves as before. * API change, bug fix: The Fixity type contains a QName rather than an Op to name the operator. The operator must match the given QName exactly (i.e., unqualified names only match unqualified names, and qualified names only match qualified names) for applyFixities to perform fixups. * Bug fix: End-of-file inside an OPTIONS pragma no longer loops. Cheers, /Niklas

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Second CFP: Continuation Workshop 2011
by oleg＠okmij.org 25 May '11

25 May '11

Submission deadline: June 25, 2011 Invited speakers: Mats Rooth (Cornell), Noam Zeilberger (Universite' Paris 7) There will be a tutorial session in the evening before the workshop. ACM SIGPLAN Continuation Workshop 2011 http://logic.cs.tsukuba.ac.jp/cw2011/ co-located with ICFP 2011, Tokyo, Japan Saturday, September 24, 2011 Call for Contributions Continuations have been discovered many times, which highlights their many applications in programming language semantics and program analysis, linguistics, logic, parallel processing, compilation and web programming. Recently, there has been a surge of interest specifically in delimited continuations: new implementations (in Scala, Ruby, OCaml, Haskell), new applications (to probabilistic programming, event-driven distributed processing), substructural and constructive logics, natural language semantics. The goal of the Continuation Workshop is to make continuations more accessible and useful -- to practitioners and to researchers in various areas of computer science and outside computer science. We wish to promote communication among the implementors and users in many fields. We would like to publicize the applications of continuations in academic (logic, linguistics) and practical fields and various programming languages (OCaml, Haskell, Scala, Ruby, Scheme, etc). Continuation Workshop 2011 will be informal. We aim at accessible presentations of exciting if not fully polished research, and of interesting academic, industrial and open-source applications that are new or unfamiliar. The workshop will have no published proceedings; submissions of short abstracts are preferred. Invited speakers ---------------- Mats Rooth, Director of Computational Linguistics Lab, Cornell University, USA http://conf.ling.cornell.edu/mr249/ Noam Zeilberger, Universite' Paris 7 http://www.pps.jussieu.fr/~noam/ Tutorials --------- In the evening before the workshop, there will be a tutorial session on delimited continuations and their main applications. Tutorial date and time: Friday, September 23, 2011, 19:00-21:00 Tutorial place: IIJ (next to NII, the place of the ICFP conference) Tutorial speakers: Kenichi Asai and Oleg Kiselyov Important dates --------------- Submission: June 25, 2011 Notification: August 8, 2011 Tutorials: September 23, 2011 Workshop: September 24, 2011 Format ------ The workshop will consist of presentations by the participants, selected from submitted abstracts. Participants are invited to submit working drafts, source code, and/or extended abstracts for distribution on the workshop homepage and to the attendees, but as the workshop will have no formal proceedings, any contributions may be submitted for publication to other venues. (See the SIGPLAN republication policy for more details.) Scope ----- We seek several types of presentations on topics related to continuations. We especially encourage presentations that describe work in progress, outline a future research agenda, or encourage lively discussion. Research presentations on: - implementations of continuations - semantics - type systems and logics - meta-theory and its mechanization - code generation with continuations or effects - distributed programming - systems programming and security - pearls Research presentations must be broadly accessible and should describe new ideas, experimental results, significant advances in the theory or application of continuations, or informed positions regarding new control operators. Application presentations, or status reports These broadly accessible presentations should describe interesting applications of continuations in research, industry or open source. We encourage presentations of applications from areas outside of programming language research -- such as linguistics, logics, AI, computer graphics, operating systems, etc. These presentations need not present original research, but should deliver information that is new or that is unfamiliar to the general ICFP audience. (A broadly accessible version of research presented elsewhere, with the most recent results and more discussion of future work may be acceptable as a CW 2011 status report.) The abstract submission should justify, to a general reader, why an application is interesting. Demos and work-in-progress reports Live demonstrations or presentations of preliminary results are intended to show new developments, interesting prototypes, or work in progress. In the abstract submission (which need only be about half a page), describe the demo and its technical content, and be sure to include the demo's title, authors, collaborators, references, and acknowledgments. A demonstration should take 10-15 minutes, and a work-in-progress report should take about 5 minutes. The exact time per demo will be decided based on the number of accepted submissions. (Presenters will have to bring all the software and hardware for their demonstration; the workshop organizers are only able to provide a projector.) Submission Guidelines and Instructions -------------------------------------- Unlike the previous Continuation Workshops, we do not require the submission of complete research papers. We will select presentations based on submitted abstracts, up to 2 (A4 or US letter) pages long in the PDF format (with the optional supplementary material, up to 8 PDF pages). Persons for whom this poses a hardship should contact the program chair. Submissions longer than a half a page should include a paragraph synopsis suitable for inclusion in the workshop program. Email submissions to cw2011-submit(a)logic.cs.tsukuba.ac.jp Organizers ---------- Yukiyoshi Kameyama, University of Tsukuba Chung-chieh Shan, Rutgers University Oleg Kiselyov Program Committee ----------------- Kenichi Asai, Ochanomizu University, Japan Malgorzata Biernacka, University of Wroclaw, Poland Hugo Herbelin, PPS - pi.r2, INRIA, France Oleg Kiselyov Julia Lawall, University of Copenhagen, Denmark Tiark Rompf, EPFL, Switzerland Chung-chieh Shan, Rutgers University (Chair) Hayo Thielecke, University of Birmingham, UK

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Re: [Haskell-cafe] [Haskell] ANNOUNCE: time-recurrence-0.1
by Yitzchak Gale 25 May '11

25 May '11

Moving the discussion from haskell to haskell-cafe. Chris Heller wrote: >> http://github.com/hellertime/time-recurrence >> A library for generating and inspecting recurring times. Very nice. Please put it up on hackage so we can see the haddocks, try it out easily, etc. >> hoping to solicit some feedback on its design. Very well done. I have been think about recurrence rules, too. Version 2 Olson timezone files contain POSIX rules to specify clock change rules for times later than the last one listed in the timezone series. Currently, the timezone-series package ignores those. I like your introduction of symbolic months and weekdays. The numeric-only interface of Data.Time for those things leads to messiness like multiple functions depending on how you number the weekdays. That logic should be moved out to a separate interface. You are correct that it is worth creating a separate module for that. Please do so! Include neater versions of the corresponding functions from Data.Time. Another comment - I would have used fromGregorianValid instead of fromGregorian. It's not a good idea for a basic time library to introduce possible crashes. At least you should provide an alternative safe interface. Similarly for toEnum. >> The general direction is to have something that works much >> like the RRULE portion of the iCalendar (RFC 5545) >> specification. I have decided avoid strict RFC compliance >> at this time. Ashley Yakeley wrote: > At one point I investigated a really generalised version of this, including > an abstract syntax type and perhaps a way of parsing some useful > subset of English expressions into it. However I got stuck on such > things as > > "any day after the sixth Sunday of a month" > > In order to prove that today was not such a day, it would have know that > "sixth Sunday of a month" never happens. Simply searching into the past for > one would never terminate. Either I would have to restrict the language, or > assert the beginning of time (not necessarily a bad solution). I don't see why you would need to make any such restrictions. There would only be a finite number of primitive comparison types, and since your language would be total, any expression could be reduced to the primitive ones in a finite number of steps. Then it would just be a matter of arithmetic on sets of ranges; see, for example, http://hackage.haskell.org/package/Ranged-sets That allows for half-infinite and infinite ranges as well. Regards, Yitz

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Re: [Haskell-cafe] Code critique - Was [Maybe Int] sans Nothings
by michael rice 25 May '11

25 May '11

Thanks, Alexander. Not as much red ink as I was expecting. My monitor is acting up, so I may be be dropping off the radar unexpectedly while acquiring a replacement. Each prefix is guaranteed to have at least one following word in its suffix list (possibly more) because the map is built from the actual Psalms text. The only prefix for which this does not occur is the final two words of the text, thus the TRUE case in buildMap puts the sole non_word in the suffix list. The generate function then _always_ builds a prefix from the first two words of the text, "Blessed is," looks up its suffix list, chooses the next word randomly, and creates a new prefix by pairing the second word of the old prefix and the new word. Repeat. I've attached the AWK and C++ sources for the same Markov exercise if you wish to examine them. Both just write a single word on each line so I cobbled a Haskell program (lines.hs) to turn them into lines. Michael --- On Tue, 5/24/11, Alexander Solla <alex.solla(a)gmail.com> wrote: From: Alexander Solla <alex.solla(a)gmail.com> Subject: Re: [Haskell-cafe] Code critique - Was [Maybe Int] sans Nothings To: "michael rice" <nowgate(a)yahoo.com> Cc: "Haskell Cafe" <haskell-cafe(a)haskell.org> Date: Tuesday, May 24, 2011, 10:42 PM My comments are in-line, marked off with >>> On Tue, May 24, 2011 at 4:09 PM, michael rice <nowgate(a)yahoo.com> wrote: The input file: http://dl.dropbox.com/u/27842656/psalms The Markov chain exercise from "The Practice of Programming", Kermighan/Pike. Sample runs at the end. Michael ============================ import System.Environment(getArgs)import System.Randomimport Control.Applicativeimport Control.Monad.Reader import Control.Monad.Stateimport Data.Maybeimport Data.Map type Prefix = (String,String)type GeneratorState1 = State ((Map Prefix [String]),Prefix,[String]) type GeneratorState2 = StateT (Prefix,StdGen) (Reader (Map Prefix [String])) non_word = "\n" f key new old = new ++ old >>> I don't see what f is for, since it doesn't do anything with the key. buildMap :: GeneratorState1 (Map Prefix [String])buildMap = do (mp,(pfx1,pfx2),words) <- get if (Prelude.null words) then {- No more words. Return final map (adding non_word for final prefix). -} return (insertWithKey' f (pfx1,pfx2) [non_word] mp) else do {- Add word to map at prefix & continue. -} put (insertWithKey' f (pfx1,pfx2) [head words] mp, (pfx2,(head words)), tail words) buildMap >>> I'm not a fan of explicit if-then-else's, but my preferred alternative won't win much either. (see http://osdir.com/ml/haskell-cafe@haskell.org/2011-05/msg00612.html for an example of what I'm talking about) generate :: GeneratorState2 (Maybe String)generate = do ((pfx1,pfx2),gen) <- get mp <- ask let suffixList = mp ! (pfx1,pfx2) >>> I'm not sure how you're guaranteed that mp ! (pfx1, pfx2) exists, at first glance. "lookup" uses Maybe semantics, in the case there is no result. let (index,newGen) = randomR (0, (length suffixList)-1) gen >>> I might use a function like:>>> listRange :: [a] -> (Int, Int)>>> listRange list = (0, (length $ l) - 1) >>> This is a common enough pattern to abstract away. let word = suffixList !! index if (word == non_word) then return Nothing else do put ((pfx2,word),newGen) return (Just word) rInt :: String -> IntrInt = read >>> rInt is fair enough, but you can also have the same effect with an explicit type signature ((read n) :: Int) I tend to prefer the latter, personally. main = do (seed:nwords:_) <- (Prelude.map rInt) <$> getArgs contents <- getContents putStrLn $ unwords $ catMaybes $ runReader (evalStateT (sequence $ replicate nwords generate) ((non_word,non_word),mkStdGen seed)) (evalState buildMap (singleton (non_word,non_word) [], (non_word,non_word), words contents)) >>> Nice use of functor application. {-[michael@hostname ~]$ ghc --make markov.hs[1 of 1] Compiling Main ( markov.hs, markov.o ) Linking markov ...[michael@hostname ~]$ cat psalms | ./markov 111 100Blessed is the LORD, in thine own cause: remember how the foolish people have blasphemed thy name. In the courts of the righteous: The LORD taketh pleasure in the desert. And he led them with the wicked, and with the whole earth, is mount Zion, on the sides of thine only. O God, and was troubled: I complained, and my God. My times are in thy praise. Blessed be God, which is full of the LORD is good: for his wondrous works. Now also when I am small and despised: yet do I put my trust: how say ye to[michael@hostname ~]$ cat psalms | ./markov 666 100 Blessed is the LORD, and cried unto thee, Thy face, LORD, will I remember thee from the beginning: and every one that is weaned of his heart to any wicked transgressors. Selah. They return at evening: they make ready their arrow upon the people; and thou hast destroyed all them that fight against them that trust in thee: and let my tongue cleave to the heavens by his power for ever; and thy lovingkindnesses; for they have laid a snare before them: and that my ways were directed to keep thy word. Mine eyes fail while I have said that [michael@hostname ~]$ --- On Tue, 5/24/11, Alexander Solla <alex.solla(a)gmail.com> wrote: From: Alexander Solla <alex.solla(a)gmail.com> Subject: Re: [Haskell-cafe] [Maybe Int] sans Nothings To: "Haskell Cafe" <haskell-cafe(a)haskell.org> Date: Tuesday, May 24, 2011, 5:01 PM > Personally, I find non-functional values without Eq instances to be > degenerate. So I really do not mind superfluous Eq constraints. I > would not hesitate to use filter ((/=) Nothing) in a function whose type > has no free type variables. It's just a bit of plumbing inside of a > more complex function. Sometimes it seems to be better to not allow Eq on Float and Double. Since most algebraic laws do not hold for those types, it is more often an error than an intention to compare two Float values. And how to compare (IO a) values? Floats, Doubles, and IO are all "degenerate" types, for the reasons you outline. (Admittedly, Float and Double have Eq instances, but invalid Eq semantics) Notice how their value semantics each depend on the machine your runtime runs on, as opposed to merely the runtime. Bottom is another one of these degenerate types, since comparisons on arbitrary values are undecidable. Also, by thinking about function types, you often get interesting use cases. Thus I would not assume too quickly that a type will always be instantiated by types other than a function type. Thus I would stick to (filter isJust) and use this consistently for monomorphic and polymorphic types. I am not suggesting (filter ((/=) Nothing)) /over/ (filter isJust). Obviously, once one is aware of a better tool, one should use it. But I am suggesting that for simple cases which are unlikely to change in any substantive way, we should probably just use the tools we already know of, as opposed to searching for the "right" one. Both might involve costs. There is a cost involved in going to Google, thinking up a search term, finding that Data.Maybe has relevant functions, picking the right one. It takes less time to write "filter ..." than to type "haskell removing nothing from list", for example. When dealing with known unknowns, there is a balance to be made, and it is not easy. Michael's choice to ask the list imposed costs. (Not that we mind, we're all volunteers, after all). But it probably took 10 minutes to get the first reply. He could have written a bit of code that worked correctly, given the context of his problem, in 20 seconds. Then again, he probably worked on a different bit of code until somebody sent a solution, so we probably only have to account for the time spent in context switching, for everyone involved. -----Inline Attachment Follows----- _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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ANNOUNCE: time-recurrence-0.2
by Chris Heller 25 May '11

25 May '11

Hot on the heels of the 0.1 release of time-recurrence: http://hackage.haskell.org/package/time-recurrence comes your favorite recurring time library: time-recurrence-0.2. At this point the library can generate many of the examples given in section 3.8.5.3 of the iCalendar RFC. The interface has been reworked to make things more concise, an example: > -- Generate every 10 days, 5 occurrences > startDate = utcGregorianWithTime 1997 9 2 14 0 0 -- Y M D HH MM SS > map moment $ take 5 $ recurBy 10 [] $ Daily startDate > [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] I'm still tweaking the main data structures so things are very unstable right now. There is no support for operators like the iCalendar BYSETPOS, which would allow you to generate "the 30th of the month in June if it is a Week Day otherwise the last Week Day in June". For the future I also am looking at how to enforce some of the more esoteric iCalendar rules, such as using BYMONTHDAY in a MONTHLY frequency when also using BYDAY. Since I am not being strict I can simply punt on the matter, but it would be interesting to support such dependencies. As always I encourage all feedback, suggestions and pull requests. -Chris

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GOA broken with recent lambdabots
by Jacek Generowicz 24 May '11

24 May '11

GOA is broken with recent lambdabots, according to http://hackage.haskell.org/package/goa-3.0.2 It certainly fails to compile for me (like this http://hpaste.org/46994/goa_build_failure ) From the fact that it is known to be broken, should I infer that it is a difficult problem that would take an expert a long time to fix? Or is it broken merely because nobody has got around to fixing it, and would somebody (like me) who is not a Haskell expert and has never looked into the source of lambdabot or GOA, stand a chance of fixing it?

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Cabal: wrapping namespace of a package into top-level module
by max ulidtko 24 May '11

24 May '11

Hi haskell-cafe. I have a package which builds with cabal pretty fine, but there is namespace issue which disturbs me. The problem is that the package exports (to the toplevel namespace!) some modules with fairly general names, like Tests, Basics, Applications. This is probably an oversight of the original package author, and the namespace shouldn't be organised like this... but it is. And I'm looking for a way to avoid potential namespace troubles should I install the package, other than going and reforming the namespace tree myself. What I was thinking about was some cabal option to "wrap" package's namespace into a toplevel module, say "PackageName", so that module Tests could be imported by usual code with `import PackageName.Tests`. Is that possible with cabal? Things are further complicated by the numerous intra-library imports. While the "outside" code refers to a module with PackageName.ModuleName, it would be very desirable that "inside" code used just straight ModuleName. ------ Regards, max ulidtko

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Re: [Haskell-cafe] ANNOUNCE: HaNS-2.1.0.0
by Don Stewart 24 May '11

24 May '11

Yay! The Hackage URL is: http://hackage.haskell.org/package/hans Cheers, Don On Tue, May 24, 2011 at 11:39 AM, Trevor Elliott <trevor(a)galois.com> wrote: > Galois, Inc. is pleased to announce the release of HaNS, the Haskell > Network Stack. HaNS is a lightweight, pure Haskell network stack that > can be used for Haskell networking in the context of the HaLVM, or with > a Linux tap device. Currently, HaNS supports 802.3, IPv4, ARP, DHCP > (partially), ICMP, UDP, and TCP. > > On Hackage: http://hackage.haskell.org/package/HaNS > Git repo: http://code.galois.com/cgi-bin/gitweb > > The TCP component is a port of Peng Li and Stephan Zdancewic's Haskell > TCP library, which in turn was translated from Norrish et al's HOL4 TCP > specification. > > What might you use HaNS for? Developing new and interesting networking > protocols, at any layer of the network stack, or your own variants of > existing protocols. HaNS allows HaLVM nodes to support networking, > meaning you can run experimental network services directly on top of the > HaLVM (without any pesky OS getting in the way). There are probably lots > of other cool applications of HaNS we haven't though of yet. > > HaNS is released under a BSD license, and should be considered > experimental at this stage. > > If you have any questions please contact the HaLVM team at > > halvm-devel(a)community.galois.com. > > -- Trevor Elliott (for the HaLVM team) > > P.S. If you're building with the HaLVM, make sure to add -fhalvm to your > cabal install invocation. > > > = References = > > HaLVM: http://halvm.org > Haskell TCP: http://hackage.haskell.org/package/tcp > > > _______________________________________________ > Libraries mailing list > Libraries(a)haskell.org > http://www.haskell.org/mailman/listinfo/libraries > >

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