- Haskell-Cafe - Haskell.org

ANNOUNCE: Haskell BOF on 29c3 in Hamburg, Germany
by Matthias Fischmann 27 Dec '12

27 Dec '12

Hi everybody, we will be running a nano-hackathon on the CCC congress in Hamburg on Thursday, 2012-12-27, 5pm German time: https://events.ccc.de/congress/2012/wiki/Haskell_BOF Sorry for the short notice. If you would like to meet up later during the congress please drop us a line. cheers, matthias

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Substituting values
by Radical 26 Dec '12

26 Dec '12

Sometimes I'll need something like: if value == Foo then Bar else value Or some syntactic variation thereof: case value of { Foo -> Bar; _ -> value } Is there a better/shorter way to do it? I'm surprised that it's more complicated to substitute a value on its own than e.g. in a list, using filter. Or perhaps I'm missing the right abstraction? Thanks, Alvaro

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How to fold on types?
by Magicloud Magiclouds 26 Dec '12

26 Dec '12

Say I have things like: data LongDec = LongDef a b c ... x y z values = [ 'a', 'b', 'c', ... 'x', 'y', 'z' ] Now I want them to be "LongDef 'a' 'b' 'c' ... 'x' 'y' 'z'". In form, this is something like folding. But since the type changes, so code like following won't work: foldl (\def value -> def value) LongDef values Is it possible to do this in some way? -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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multi-thread and lazy evaluation
by Corentin Dupont 25 Dec '12

25 Dec '12

Hi all, I have a program where the user can submit his own little programs, which are interpreted using Hint. The user-submitted programs are used to modify a state held in a TVar. As of course those user-submitted programs can't be trusted, I'm trying to protect them, like in Mueval. I installed a watchdog to monitor and kill the user's thread if it doesn't finish. However it doesn't work properly, due to lazy evaluation I believe. I made a little exemple to illustrate the problem. -> The following program doesn't terminate, but if you uncomment the "putStrLn" at the end, it will. Could someone explain me this and how to do it properly?? Merry Christmas to all!!!! Corentin * {-# LANGUAGE ScopedTypeVariables #-} import Control.Concurrent.STM.TVar import Control.Concurrent.MVar import Control.Concurrent import Control.Monad.STM data MyData = MyData { a :: String, b :: String } deriving (Show) main = do tv <- atomically $ newTVar $ MyData "a" "b" protectedExecCommand tv myNewData <- atomically $ readTVar tv putStrLn $ show myNewData protectedExecCommand :: (TVar MyData) -> IO () protectedExecCommand tv = do mv <- newEmptyMVar before <- atomically $ readTVar tv id <- forkIO $ execBlocking mv forkIO $ watchDog' 5 id mv res <- takeMVar mv case res of Nothing -> (atomically $ writeTVar tv before) Just after -> (atomically $ writeTVar tv after) watchDog' :: Int -> ThreadId -> MVar (Maybe x) -> IO () watchDog' t tid mv = do threadDelay $ t * 1000000 killThread tid putStrLn $ "process timeout " tryPutMVar mv Nothing return () execBlocking :: MVar (Maybe MyData) -> IO () execBlocking mv = do let (a::String) = a --If you uncomment the next line, it will work --putStrLn $ show a putMVar mv (Just $ MyData a "toto")*

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ghc-heap-view now with recursive pretty-printing
by Joachim Breitner 25 Dec '12

25 Dec '12

Hi, I get the impression that blogs and planet.haskell.org are the best way to disseminate information about new tools any more. Maybe a part of the earlier importance has been taken over by GooglePlus, but not all, leaving both blogs and GooglePlus less useful individually? Anyways, I’d like to tell you about a feature of ghc-heap-view that I have blogged about¹, namely the possibility to inspect heap values recursively in GHCi or in your programs, including thunks, the references between them and sharing: Prelude> :script /home/jojo/.cabal/share/ghc-heap-view-0.4.0.0/ghci Prelude> let x = [1..10] Prelude> x [1,2,3,4,5,6,7,8,9,10] Prelude> :printHeap x _bh [S# 1,S# 2,S# 3,S# 4,S# 5,S# 6,S# 7,S# 8,S# 9,S# 10] Note that the tools shows us that the list is a list of S# constructors, and also that it is still hidden behind a blackhole. After running System.Mem.performGC, this would disappear. Prelude> let x = Just (1 + 1) Prelude> :printHeap x Just _bco Prelude> x Just 2 Prelude> System.Mem.performGC Prelude> :printHeap x Just (S# 2) Here, we see how the calculation was deferred until forced by showing the value of x. The name _bco stands for a bytecode object as used by the interpreter. Getting useful information from them is a bit harder than for compiled thunks, so for more accurate results put the code in a Haskell source file, compile it and use the GHC.HeapView API to print the interesting parts. Prelude> let a = "hi" Prelude> let partial = (a ++) Prelude> partial "" "hi" Prelude> System.Mem.performGC Prelude> let x = (a, partial) Prelude> :printHeap x let x1 = "hi" in (x1,_fun x1) This demonstrates a partial application. The information which function is called there (++ in this case) is lost at runtime, but we still see that the second element of the tuple is a partial application of some value to the first element. Prelude> let s = "ho" Prelude> let x = cycle s Prelude> length (take 100 (show x)) 100 Prelude> System.Mem.performGC Prelude> :printHeap x let x0 = C# 'h' : C# 'o' : x0 in x0 Prelude> let y = map Data.Char.toUpper x Prelude> length (take 100 (show y)) 100 Prelude> :printHeap y C# 'H' : C# 'O' : C# 'H' : C# 'O' : C# 'H' : C# 'O' : C# 'H' : C# 'O' : C# 'H' : C# 'O' : _bh (C# 'H' : C# 'O' : C# 'H' : C# 'O' : C# 'H' : C# 'O' : C# 'H' : C# 'O' : ... : ...) The cyclic, tying-the-knot structure of cycle is very visible. But can also see how easily it is broken, in this case by mapping a function over the list. Prelude> let {x = 'H' : y ; y = 'o' : x } Prelude> length (show (take 10 x, take 10 y)) `seq` return () Prelude> System.Mem.performGC Prelude> :printHeap (x,y) let x1 = C# 'H' : x3 x3 = C# 'o' : x1 in (x1,x3) If you want to look at multiple variables at once, just pass a tuple to printHeap I hope you’ll find it useful, and I am happy to hear about other features that you might be interested in. Also, if you don’t know about it, a graphical and interactive variant of this is available as ghc-vis². Greetings, Joachim ¹ http://www.joachim-breitner.de/blog/archives/580-GHCi-integration-for-GHC.H… ² http://felsin9.de/nnis/ghc-vis/ -- Joachim "nomeata" Breitner mail(a)joachim-breitner.de | nomeata(a)debian.org | GPG: 0x4743206C xmpp: nomeata(a)joachim-breitner.de | http://www.joachim-breitner.de/

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Re: [Haskell-cafe] How to fold on types?
by oleg＠okmij.org 25 Dec '12

25 Dec '12

Magiclouds asked how to build values of data types with many components from a list of components. For example, suppose we have data D3 = D3 Int Int Int deriving Show v3 = [1::Int,2,3] How can we build the value D3 1 2 3 using the list v3 as the source for D3's fields? We can't use (foldl ($) D3 values) since the type changes throughout the iteration: D3 and D3 1 have different type. The enclosed code shows the solution. It defines the function fcurry such that t1 = fcurry D3 v3 -- D3 1 2 3 gives the expected result (D3 1 2 3). The code is the instance of the general folding over heterogeneous lists, search for HFoldr in http://code.haskell.org/HList/Data/HList/HList.hs {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, FlexibleContexts #-} {-# LANGUAGE TypeFamilies, DataKinds, PolyKinds, ScopedTypeVariables #-} {-# LANGUAGE UndecidableInstances #-} -- `Folding' over the data type: creating values of data types -- with many components from a list of components -- UndecidableInstances is a bit surprising since everything is decidable, -- but GHC can't see it. -- Extensions DataKinds, PolyKinds aren't strictly needed, but -- they make the code a bit nicer. If we already have them, -- why suffer avoiding them. module P where -- The example from MagicCloud's message data D3 = D3 Int Int Int deriving Show v3 = [1::Int,2,3] type family IsArrow a :: Bool type instance IsArrow (a->b) = True type instance IsArrow D3 = False -- add more instances as needed for other non-arrow types data Proxy a = Proxy class FarCurry a r t where fcurry :: (a->t) -> [a] -> r instance ((IsArrow t) ~ f, FarCurry' f a r t) => FarCurry a r t where fcurry = fcurry' (Proxy::Proxy f) class FarCurry' f a r t where fcurry' :: Proxy f -> (a->t) -> [a] -> r instance r ~ r' => FarCurry' False a r' r where fcurry' _ cons (x:_) = cons x instance FarCurry a r t => FarCurry' True a r (a->t) where fcurry' _ cons (x:t) = fcurry (cons x) t -- Example t1 = fcurry D3 v3 -- D3 1 2 3 -- Let's add another data type data D4 = D4 Int Int Int Int deriving Show type instance IsArrow D4 = False t2 = fcurry D4 [1::Int,2,3,4] -- D4 1 2 3 4

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Foreign.Erlang help
by Junior White 25 Dec '12

25 Dec '12

Hi all, I have a game server programming in erlang, but now i want to write some game logic in haskell, how to write a haskell node for erlang? ps:I have read the introduce of Foreign.Erlang,I use haskell send a message to a erlang echo server, but the server not receive anything.

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Is OpenAL unqueueBuffers implemented correctly?
by alex＠tapdancinggoats.com 25 Dec '12

25 Dec '12

Hi, there's no maintainer listed for the OpenAL package, so I'm posting here in hopes of finding the right person. The unqueueBuffers function in Sound.OpenAL.AL.Source has the type Source -> [Buffer] -> IO () but I think the type should be Source -> Int -> IO [Buffer] This wraps the function alSourceUnqueueBuffers which uses the array of buffers as an output argument. With the current implementation, it's impossible to get the list of buffers that were unqueued. This implementation works the way I expect: unqueueBuffers :: AL.Source -> Int -> IO [AL.Buffer] unqueueBuffers source nbuffers = allocaArray nbuffers $ \ptr -> do alSourceUnqueueBuffers source (fromIntegral nbuffers) ptr peekArray nbuffers ptr I just started using OpenAL, so I might be misunderstanding how this is supposed to work. Thanks, Alex Midgley

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Layer on a layer of record syntax in the type synonym?
by Christopher Howard 24 Dec '12

24 Dec '12

Using a simple type I gave earlier from my monadic type question... code: -------- data Socket3 a b c = Socket3 a b c deriving (Show) -------- Is it possible somehow to layer on record syntax onto a synonym of the type? The idea would be something like this... code: -------- type SpaceShip = Socket3 { engine :: Last Engine , hull :: Last Hull , guns :: [Guns] } -------- ...purely for the convenience. But this doesn't seem to work with "type" as it assumes you are referring to already made constructors, and evidently "newtype" only allows use of a single record. I could wrap it in a normal "data" declaration but that would add an extra layer of complexity I think. -- frigidcode.com

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Fighting research paper bit-rot
by cheater cheater 23 Dec '12

23 Dec '12

Hi guys, after yet another episode of trying to figure out why library code doesn't make any sense when reading the related paper, I decided to start a small wiki just for the purpose of describing differences between what's in the paper and what's in the code. The first article can be found at: http://functionalpapersupdated.wikia.com/wiki/Transactional_memory_with_dat… This one was tricky: it was the "check" from stm-invariants.pdf. There is a "check" in the STM library which is a completely different function. The "check" from the paper is in another module and library and is called "alwaysSucceeds". Everyone's more than welcome to add their favourite papers and describe the differences. The wiki is freely editable. Hopefully it can, with time, grow to be of help to anyone trying to learn about Haskell or category theory or functional programming in general. I can't promise a huge amount of updates on my side (I'm just a guy learning how to use Haskell, not a researcher) but hopefully this great community can make it happen :) If you're a publishing author, and you know of such updates to your papers, please consider starting a page for your paper. It's also a good place to track the implementations of ideas described in such papers, especially in case there are multiple ones or the implementation hasn't been discussed in the paper itself.

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