- Haskell-Cafe - Haskell.org

Instances for continuation-based FRP
by Hans Höglund 26 Apr '13

26 Apr '13

Hi everyone, I am experimenting with various implementation styles for classical FRP. My current thoughts are on a continuation-style push implementation, which can be summarized as follows. > newtype EventT m r a = E { runE :: (a -> m r) -> m r -> m r } > newtype ReactiveT m r a = R { runR :: (m a -> m r) -> m r } > type Event = EventT IO () > type Reactive = ReactiveT IO () The idea is that events allow subscription of handlers, which are automatically unsubscribed after the continuation has finished, while reactives allow observation of a shared state until the continuation has finished. I managed to write the following Applicative instance > instance Applicative (ReactiveT m r) where > pure a = R $ \k -> k (pure a) > R f <*> R a = R $ \k -> f (\f' -> a (\a' -> k $ f' <*> a')) But I am stuck on finding a suitable Monad instance. I notice the similarity between my types and the ContT monad and have a feeling this similarity could be used to clean up my instance code, but am not sure how to proceed. Does anyone have an idea, or a pointer to suitable literature. Best regards, Hans

5 7

pattern matching on data families constructors
by Alexey Egorov 25 Apr '13

25 Apr '13

Hi, suppose that there is following data family: > data family D a > data instance D Int = DInt Int > data instance D Bool = DBool Bool it is not possible to match on constructors from different instances: > -- type error > a :: D a -> a > a (DInt x) = x > a (DBool x) = x however, following works: > data G :: * -> * where > GInt :: G Int > GBool :: G Bool > > b :: G a -> D a -> a > b GInt (DInt x) = x > b GBool (DBool x) = x The reason why second example works is equality constraints (a ~ Int) and (a ~ Bool) introduced by GADT's constructors, I suppose. I'm curious - why data families constructors (such as DInt and DBool) doesn't imply such constraints while typechecking pattern matching? Thanks.

3 6

Munich Haskell Meeting
by Heinrich Hördegen 25 Apr '13

25 Apr '13

Dear all, our next regular meeting will take place in Munich on the 29th of April at 19h30 at Cafe Puck. If you plan to join us, please go to http://www.haskell-munich.de/dates and click the button. Until then, have a nice time, Heinrich

1 0

Haskell training in San Francisco Bay Area and New York
by Duncan Coutts 25 Apr '13

25 Apr '13

Well-Typed are offering Haskell courses in the San Francisco Bay Area and New York in early June. They are for professional developers who want to learn Haskell or improve their skills. There is a 2-day introductory course and a 2-day advanced course. Full course and registration details: http://www.well-typed.com/services_training Well-Typed are running these courses in partnership with FP Complete and Skills Matter. Locations, dates ================ San Francisco Bay Area * Introductory Course: June 4-5th, 2013 * Advanced Course: June 6-7th, 2013 New York * Introductory Course: June 10-11th, 2013 * Advanced Course: June 12-13th, 2013 * Early bird discount before April 29th -- Duncan Coutts, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/

1 0

Why the `-ghc7.4.2' suffix on *.SO base names?
by Captain Freako 25 Apr '13

25 Apr '13

In trying to compile a 2 year old, previously working project on a new machine with a fresh Haskell Platform installation, I bumped into this: ghc -o libami.so -shared -package parsec -lHSrts -lm -lffi -lrt AMIParse.o AMIModel.o ami_model.o ExmplUsrModel.o Filter.o /usr/bin/ld: /usr/lib/ghc-7.4.2/libHSrts.a(RtsAPI.o): relocation R_X86_64_32S against `ghczmprim_GHCziTypes_Czh_con_info' can not be used when making a shared object; recompile with -fPIC /usr/lib/ghc-7.4.2/libHSrts.a: could not read symbols: Bad value collect2: error: ld returned 1 exit status When I investigated why the *.A, instead of the *.SO, was being picked up, I found that the *.SO had a `-ghc7.4.2' appended to its base name. Is this new? What is its purpose? Do I need to add a `LIB_SUFFIX' variable to my makefile and append it to all of my `-l<...>'s, or is there a more elegant solution? Thanks! -db

1 0

Fwd: How to do automatic reinstall of all dependencies?
by capn.freako＠gmail.com 25 Apr '13

25 Apr '13

-db ----- Forwarded message ----- From: "Captain Freako" <capn.freako(a)gmail.com> Date: Tue, Apr 23, 2013 9:21 pm Subject: How to do automatic reinstall of all dependencies? To: <haskell-cafe(a)haskell.org>

4 3

Re: [Haskell-cafe] Instances for continuation-based FRP
by Conal Elliott 25 Apr '13

25 Apr '13

The intuition intrigues me. If, upon inspection, it survives morphs into something else, I'd like to hear about it. Good luck! -- Conal "The object of mathematical rigor is to sanction and legitimize the conquests of intuition, and there was never any other object for it." - Jacques Hadamard "I call intuition cosmic fishing. You feel a nibble, then you've got to hook the fish." -- Buckminster Fullero On Wed, Apr 24, 2013 at 7:26 PM, earl obscure <theanswertoproblems(a)gmail.com > wrote: > > His description of the different frp approaches starts at section 2.1 of > the thesis. > http://www.testblogpleaseignore.com/wp-content/uploads/2012/04/thesis.pdfTh… in 3.1 describes implementation of discrete signals. I don't think he > gives a denotational semantics. > I was thinking, the event, is the derivative of the specific continuous > signal it corresponds to, all other continuous signals of the system held > equal. Applying the partial derivative, is like sampling, or discrete time > stepping. But it is samplying the entire state, or multivariate structure > not just the specific symbol. This made more sense unarticulated. I'll > need to think a bit.

1 0

Re: [Haskell-cafe] Instances for continuation-based FRP
by Conal Elliott 25 Apr '13

25 Apr '13

Hi Jared, Oh -- does Elm have a denotational semantics? I haven't heard of one. I just now skimmed the informal description of the Signal type<http://elm-lang.org/docs/Signal/Signal.elm>, and from the reference to "updates" in the description of "merge", it sound like whatever semantics it might have, it couldn't be function-of-time. I'm intrigued with your interpretation. I wonder what it could mean for an event to be a derivative, especially partial one, and for arbitrary types. -- Conal On Wed, Apr 24, 2013 at 1:48 PM, earl obscure <theanswertoproblems(a)gmail.com > wrote: > Hi Conal, > > > Caveat pre-emptor I'm new to haskell, frp, etc.... .. anyway how I was > interpreting Elm's Eventbased strict FRP, was that each event was the > partial derivative of the continuous time variable, and then since it was > being strict, it would evaluate the tangent line or state of the system at > that point, only update when necessary. > > > Now related to Continuations, this is something I've been thinking about > as well,but haven't gotten very far; apparently cont monad, and comonad are > closely related. I was hoping to use the comonad rules, extend/duplicate > to encode different continuations paths, and then extract when, a > continuation path is chosen. Was hoping maybe the analog would be PDE's, > or something more general than my interpretation of Elm's FRP. > > These are just random thoughts that I wanted to get out. Thanks. > > Jared Nicholson. > >

1 0

Request for help with shared object link failure, involving relocation and -fPIC.
by Captain Freako 25 Apr '13

25 Apr '13

Can anyone help me solve this link failure? ghc -o libami.so -shared -package parsec -lHSrts -lm -lffi -lrt AMIParse.o AMIModel.o ami_model.o ExmplUsrModel.o Filter.o /usr/bin/ld: /usr/lib/ghc-7.4.2/libHSrts.a(RtsAPI.o): relocation R_X86_64_32S against `ghczmprim_GHCziTypes_Czh_con_info' can not be used when making a shared object; recompile with -fPIC /usr/lib/ghc-7.4.2/libHSrts.a: could not read symbols: Bad value collect2: error: ld returned 1 exit status Thanks! -db

1 0

Re: [Haskell-cafe] Implementing an embedded language that threads a global structure.
by Ian Bloom 24 Apr '13

24 Apr '13

Incidentally I did find a solution that fits into the type system and also produces the correct result. This was done by making the global parameter change type as well at each lambda so that it becomes a function with the same arrity as the term. Unfortunately to evaluate these terms the program needs fork and do redundant work at every application of lam (so it's a major hack), though it does give me some confidence that a real solution is possible perhaps by using dependent types?: ---- lam :: ((g->(g,a)) -> (g->(g',b))) -> (g->(a->g',a->b)) lam = (\ f -> \ g -> (\x -> fst $ (f (\ gv->(gv,x))) g, \y -> snd $ (f (\ gv->(gv,y))) g )) app :: (g->(a->g1,a->b)) -> (g->(g,a)) -> (g->(g1,b)) app = (\ eA eB -> \ gz -> let (gB, fB) = eB gz in let (lgA, lfA) = eA gB in (lgA fB, lfA fB) ) ext :: ((g1,a)->(g2,b))->(g0->(g1,a))->(g0->(g2,b)) ext = \f -> \a -> \g -> f (a g) idCounter :: Num g => (g,a) -> (g, a) idCounter = \(g, x) -> (g+1, x) -- Example terms trmA = lam (\x -> lam (\y -> app x y)) trmB = (lam (\y -> app trmC y)) trmC = (\g -> (\c->g,\c->c+3)) trmD = (\g -> (g,3)) trmE = app trmC trmD trmF = (ext idCounter) trmD trmG = app trmC (app trmC trmF) trmH = app trmC trmE --- Here's my original statement of the problem http://hastebin.com/raw/bewiqihiyo And here's all my code with new, http://hpaste.org/86273 Unfortunately, I don't know of a way to tell the compiler that essentially the variables x and y in the lam function will always be applied to the same value. Unfortunately, I've reached a limit of my understanding of how pairs integrate with the type system in Haskell, I've considered returning something other than a pair from my base function (such as a function) but I have yet to figure that out. Thanks for your insights, Ian Bloom

1 0