July 2014 - Haskell-Cafe

Another Cabal question
by Jurriaan Hage 21 Jul '14

21 Jul '14

Dear all, I have another issue: I have a Cabal package (call it A) that runtime depends on another (say B), where B provides (only) an executable that is compiled with the Make build-type. Now, I would like to list B under the build-depends of A (since there is no way to specify having just a run-time dependency), but it seems that when I cabal install B the package is not registered (although the executable does in fact become available to use within the standard directory.) So when I then compile A with Cabal it will complain the package is not registered. Does anyone know whether the behaviour for B is normal, or whether this is a bug of some kind? If the former, what is an easy and clean way of dealing with this situation. best, Jur

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applicative question
by grant 20 Jul '14

20 Jul '14

Hi guys, I am trying to simplify some code. A user provides a combination of boolean expressions and for each expression an async task is run until that expression becomes true. The boolean expressions have type (Event e => e -> Bool) eg (PC.availableFlag .&&. "asmith" .== PC.userId) Unfortunately the way I have coded this, the user needs to include the \r -> r ? for each expression, with a unique number indicating the position in the sequence (so I can keep track of the asyncs when restarting after a failure). The 'r' function just wraps the expression in a Concurrently constructor. r :: (forall e.Event e => (e -> Bool) -> Concurrently e) -> Concurrently a Here is a sample expression that someone might provide: expr1 = runit (\r -> (,) <$> r 1 (PC.availableFlag .&&. "asmith" .== PC.userId) <*> r 2 (22 .== SC.signalNumber)) So, is there a way to rewrite or simplify this to avoid having to pass the r + unique Int to each subexpression? Or, is there a different format that I can write this in and then transform to the Concurrently applicative? Here is the runit method: runit expr = runConcurrently (expr (Concurrently . queryToIOAction)) And some more sample queries: expr1 = runit (\r -> r (PC.availableFlag .&&. "asmith" .== PC.userId)) expr2 = runit (\r -> (,) <$> r 1 (PC.availableFlag .&&. "asmith" .== PC.userId) <*> r 2 (22 .== SC.signalNumber)) expr3 = runit (\r -> (\(a b -> Left (a,b)) <$> r 1 (PC.availableFlag .&&. "asmith" .== PC.userId) -- (Event e => e -> Bool) <*> r 2 (2 .== SC.signalNumber)) <|> Right <$> r 3 (1 .== SC.signalNumber)) Thanks for any help! Grant

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Q about last&init
by Закиров Марат 20 Jul '14

20 Jul '14

I am teaching myself haskell. The first impression is very good. But phrase "haskell is polynomially reducible" is making me sad :(. Anyway I am trying to backport my algorithm written in C. The key to performance is to have ability to remove element from the end of a list in O(1). But the original haskell functions last and init are O(n). My questions are: 1) Is last function is something like "black box" written in C++ which perform O(1)? So I shouldn't even try to imagine some haskell O(1) equivalent. 2) Or will optimizer (llvm?) reduce init&last complexity to 1? 3) Some people suggest to use sequences package, but still how do they implement O(1) init&last sequences equivalent in haskell? -- Regards, Marat.

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Memory consumption issues under heavy network throughput/concurrency loads
by Ben Bangert 20 Jul '14

20 Jul '14

I have been testing solutions in several languages for running a network daemon that accepts hundreds of thousands of websocket connections, and moves messages between them. Originally I used this websocket lib, http://jaspervdj.be/websockets/, but upon discovering a rather severe memory leak issue even when sending just a basic ping, switched to Michael Snoyman's first stab at a websocket impl for Yesod here: https://github.com/yesodweb/yesod/commit/66437453f57e6a2747ff7c2199aa7ad25d…. When under mild load (5k connections, each pinging once every 10+ seconds), memory usage remained stable and acceptable around 205 MB. Switching to higher load (pinging every second or less), memory usage spiked to 560 MB, and continued to 'leak' slowly. When I quit the server with the profile diagnostics, it indicate that hundreds of MB were "lost due to fragmentation". In addition, merely opening the connections and dropping them, repeatedly, made the base memory usage go up. Somewhere, memory is not being fully reclaimed when the connections are dropped. For a few thousand connections doing little, this wouldn't matter much. However, I'm trying to gauge whether its feasible to use Haskell to handle 150-200k connections that regularly come/go and are held open for long periods of time. Such massive memory use with what looks like leaks (or fragmentation issues) is problematic. I have created a very simple TCP based echo client/server here: https://github.com/bbangert/echo The server can be run after compiling with no additional options, and will listen on port 8080. The client can be run like so: ./dist/build/echoclient/echoclient localhost 8080 2000 0.5 The last number is the frequency to ping (every half second), the second to last is how many clients to connect. Under my local tests, when pinging every 5+ seconds, 2k clients will take about 50-75 MB of ram. Pinging every 0.5 seconds jumps to ~ 180 MB of ram, and this is for a mere 2k clients. Starting/stopping the echoclient repeatedly also causes the servers overall memory usage to continue to climb higher and higher. Releasing the connections never quite gets the memory usage down to where it started. The issue seems to occur under both GHC 7.6.3 and 7.8.2. I know there's a variety of ghc options that might be tuned, am I missing some critical option to keep memory usage under control? Is there a better way to build high TCP throughput/concurrent servers in Haskell that I'm missing? Thanks, Ben

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Creating a Point
by Rafael Almeida 20 Jul '14

20 Jul '14

Hello, I was talking to friends about how could you make a Point type in haskell. It's intuitive to make a 2D point such as: type Point2D = (Double, Double) Let's define one operation on this new type: add2D (x1, y1) (x2, y2) = (x1+x2, y1+y2) Let's say now we want a 3D point. Then we'd be tempted to do: type Point3D = (Double, Double, Double) add3D (x1, y1, z1) (x2, y2, z2) = (x1+x2, y1+y2, z1+z2) Although those types work great and you could make a type class so you don't have to suffix each function with 2D and 3D. It feels like we are just repeating code when defining the add function. If we want to go 4D, 5D, etc it would be more repeated code. Using a list would be more general: type Point = [Double] now we have a nice, general add function add = zipWith (+) It's not so fun that we are able to do something like: add [2,3] [5,7,11] We have no type-safety that we can only operate on points with the same dimension. How could we address this? Can we make a general function, yet retain the type safety? I suppose maybe there's something that could be done with TH so that we automatically generate those Point2D, Point3D, etc code. I'm not sure that would be a nice path to follow, though. []'s Rafael

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Obtaining kind of a HsType
by Alejandro Serrano Mena 20 Jul '14

20 Jul '14

Dear Haskell-café, I'm trying to get some information of the kinds of types in a program. In particular, I want to obtain the kinds of arguments to type family instances, that is, given: type family F a b :: Nat type instance F (List k) b = b I would like to obtain the kinds of "k" and "b" in the left-hand side of the type instance. In particular, I'm obtaining type family instance information via LTyFamInstEqn [ http://www.haskell.org/ghc/docs/latest/html/libraries/ghc/HsDecls.html#t:LT…] Inside, I get a list of binders of type HsWithBndrs [LHsType name]. I'm assuming each of the LHsTypes inside is a pattern in the type family instance. Thus, I'm looking at those elements and try to find their kind there. Thus, the specific question is: given a HsType, can I obtain its kind somehow? Thanks in advance

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ANN: loop - fast for loops
by Niklas Hambüchen 20 Jul '14

20 Jul '14

Announcing the first version of the loop package: http://hackage.haskell.org/package/loop This is for iterating over numeric or otherwise iteratable things in a fast fashion. Looks like C, runs like C. It's trivial, but very robust performance-wise and more independent of GHC smartness or non-smartness in regards to optimising alternatives like `forM_ [1..n]`. It comes with a lot of benchmarks. Contributions are very welcome.

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ANN: hackage-diff - Compare the public API of different versions of a Hackage library
by Tim C. Schroeder 20 Jul '14

20 Jul '14

Just a first shot at the problem, hope it’s useful to somebody! https://github.com/blitzcode/hackage-diff Cheers, Tim

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Is there a name for this property: x + x - x == x
by Omari Norman 20 Jul '14

20 Jul '14

It's not quite idempotence, because more than one function is involved. It's a common property and I figure I can write a higher order function to build QuickCheck tests for it. I was just wondering if it has a name.

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Conduit & state
by Павел Гаврилов 19 Jul '14

19 Jul '14

Hi! I have: runUnixServer (serverSettings socket) $ \appData -> appSource appData $$ CL.map handleBS =$ appSink appData I want to store some data in priority queue in Monad.State and be able to access it inside the handleBS (now: handleBS :: ByteString -> ByteString). Can you send me on the right path? Thanx, Pavel.

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