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What's the advantage of writing Haskell this way?
by John Ky 31 May '11

31 May '11

Hi all, I'm trying to learn about enumerators by reading this paper<https://john-millikin.com/downloads/enumerator_0.4.10.pdf>and came across some code on page 2 that I found hard to digest, but I think I finally got it: 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

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Please add a method for optimized concat to the Semigroup class
by Yitzchak Gale 31 May '11

31 May '11

Hi Edward, Thanks much for the very useful semigroups package. When using it in practice, it would be very useful to have an analogue to the mconcat method of Monoid. It has the obvious default implementation, but allows for an optimized implementation for specific instances. That turns out to be something that comes up all the time (at least for me) in real life. Thanks, Yitz

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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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