December 2012 - Haskell-Cafe

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 24 Dec '12

24 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 23 Dec '12

23 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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Hint's setImports usage
by Martin Hilbig 22 Dec '12

22 Dec '12

hi, how to use Language.Haskell.Interpreter.setImports? i use it like: setImports ["My.Module"] so that my interpreted modules don't need to: import My.Module But i still get: Not in scope: data constructor `MyType' What am i doing wrong? Thanks in advance. have fun martin

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Troubles with DLLs
by Henk-Jan van Tuyl 22 Dec '12

22 Dec '12

L.S., I am having troubles with DLLs; my application (Haskell, C and C++) gives several messages about DLLs not being found. I added these one by one to the current directory. At a certain point there are just error messages about null pointers and "Segmentation fault/access violation in generated code". When I try to load my application in GHCi, I get more messages about missing DLLs, up to a point where GHCi refuses to load a library because of an unknown section (a bug to be solved in GHC 7.6.2). (Several other programs have the same problem.) Now I am still stuck with a null pointer error message. 1) How can I find out which DLLs my application needs (and which package/library is to blame for it) and 2) how can I find the error location when the crash occurs in C/C++ code (it might be something else than a missing DLL of course)? My system: Windows XP + MinGW/MSYS Haskell platform 2012.4.0.0 The application uses wxHaskell. Regards, Henk-Jan van Tuyl -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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category design approach for inconvenient concepts
by Christopher Howard 21 Dec '12

21 Dec '12

Recently I read this article I happened across, about the categorical design pattern: http://www.haskellforall.com/2012/08/the-category-design-pattern.html Barely understood it, of course, but it was a rather intriguing concept. So now I'm looking at all my programming problems trying to make types that can be composed and that seem to fit the idea of a "category". However, what I'm wondering about is ideas that can be "composed" but that don't seem to fit the idea of "category", because they don't obey the associativity law. To give a specific example (pseudo code like, without any idea here of implementation or proper syntax): Say you created a type called "Component" (C for short), the idea being to compose Components out of other Components. Every C has zero or more connectors on it. Two Cs can be connected to form a new C using some kind of composition operator (say, <.>), provided there are enough connectors (one on each). Presumably you would need a "Fail" constructor of some kind to represent the situation when there is not enough connectors. Say you had a C (coupler) with two connectors, a C (thing) with one connector, and a C (gadget) with one connector. So you could have... (coupler <.> thing) <.> gadget Because the coupler and the thing would combine to create a component with one spare connector. This would then combine with the gadget to make the final component. However, if you did... coupler <.> (thing <.> gadget) Then thing and gadget combine to make a component with no spare connectors. And so the new component and the coupler then fail to combine. Associativity law broken. So, can I adjust my idea to fit the "category" concept? Or is it just not applicable here? Or am I just misunderstanding the whole concept? -- frigidcode.com

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arr considered harmful
by Ryan Ingram 21 Dec '12

21 Dec '12

I know it's a bit of an 'intentionally provocative' title, but with the recent discussions on Arrows I thought it timely to bring this up. Most of the conversion from arrow syntax into arrows uses 'arr' to move components around. However, arr is totally opaque to the arrow itself, and prevents describing some very useful objects as arrows. For example, I would love to be able to use the arrow syntax to define objects of this type: data Circuit a b where Const :: Bool -> Circuit () Bool Wire :: Circuit a a Delay :: Circuit a a And :: Circuit (Bool,Bool) Bool Or :: Circuit (Bool,Bool) Bool Not :: Circuit Bool Bool Then :: Circuit a b -> Circuit b c -> Circuit a c Pair :: Circuit a c -> Circuit b d -> Circuit (a,b) (c,d) First :: Circuit a b -> Circuit (a,c) (b,c) Swap :: Circuit (a,b) (b,a) AssocL :: Circuit ((a,b),c) (a,(b,c)) AssocR :: Circuit (a,(b,c)) ((a,b),c) Loop :: Circuit (a,b) (a,c) -> Circuit b c etc. Then we can have code that examines this concrete data representation, converts it to VHDL, optimizes it, etc. However, due to the presence of the opaque 'arr', there's no way to make this type an arrow without adding an 'escape hatch' Arr :: (a -> b) -> Circuit a b which breaks the abstraction: circuit is supposed to represent an actual boolean circuit; (Arr not) is not a valid circuit because we've lost the information about the existence of a 'Not' gate. The arrow syntax translation uses arr to do plumbing of variables. I think a promising project would be to figure out exactly what plumbing is needed, and add those functions to a sort of 'PrimitiveArrow' class. All of these plumbing functions are trivially implemented in terms of 'arr', when it exists, but if it doesn't, it should be possible to use the arrow syntax regardless. -- ryan

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