January 2009 - Haskell-Cafe

Debugging STM
by John Ky 08 Jan '09

08 Jan '09

Hi, Does anyone have any advice on how to inspect complex TVar data structures that may include cycles? They're opaque as far as Show goes. And sometimes I'd like a more comprehensive view of my data structures at the ghci prompt rather than the dribs and drabs I get by typing "x <- atomically $ readTVar var" and the like all the time. Also, do TVars have a printable identity for debugging purposes? Thanks -John

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Template Haskell question
by Jeff Heard 08 Jan '09

08 Jan '09

Alright... I *think* I'm nearly there, but I can't figure out how to derive a class instance using record accessors and updaters... Can anyone help? There are [| XXXf |] instances at the end of the module and they all need replaced, but I can't figure out what to replace them with. The basic idea of the module is that you define your record type, Q, and that record type contains all the state you're interested in. The Hieroglyph system has other basic state, and the idea is that you use $(additions "QWithState" ''Q) $(deriveUIState ''QWithState) to create your final UIState instance. -- - {-# LANGUAGE TemplateHaskell #-} module Graphics.Rendering.Hieroglyph.TH where import Language.Haskell.TH import Language.Haskell.TH.Syntax import Graphics.Rendering.Hieroglyph.UIState import Graphics.Rendering.Hieroglyph.Primitives import Graphics.UI.Gtk.Types (Widget) import Control.Monad {- output of $( fmap (LitE . StringL . show) [| reify ''BasicUIState |] ) TyConI (DataD [] Graphics.Rendering.Hieroglyph.BasicUIState.BasicUIState [] [RecC Graphics.Rendering.Hieroglyph.BasicUIState.BasicUIState [(Graphics.Rendering.Hieroglyph.BasicUIState.mousePosition,NotStrict,ConT Graphics.Rendering.Hieroglyph.Primitives.Point) ,(Graphics.Rendering.Hieroglyph.BasicUIState.mouseLeftButtonDown,NotStrict,ConT GHC.Base.Bool) ,(Graphics.Rendering.Hieroglyph.BasicUIState.mouseRightButtonDown,NotStrict,ConT GHC.Base.Bool) ,(Graphics.Rendering.Hieroglyph.BasicUIState.mouseMiddleButtonDown,NotStrict,ConT GHC.Base.Bool) ,(Graphics.Rendering.Hieroglyph.BasicUIState.mouseWheel,NotStrict,ConT GHC.Base.Int) ,(Graphics.Rendering.Hieroglyph.BasicUIState.keyCtrl,NotStrict,ConT GHC.Base.Bool) ,(Graphics.Rendering.Hieroglyph.BasicUIState.keyShift,NotStrict,ConT GHC.Base.Bool) ,(Graphics.Rendering.Hieroglyph.BasicUIState.keyAlt,NotStrict,ConT GHC.Base.Bool) ,(Graphics.Rendering.Hieroglyph.BasicUIState.key,NotStrict,ConT Graphics.Rendering.Hieroglyph.UIState.Key) ,(Graphics.Rendering.Hieroglyph.BasicUIState.drawing,NotStrict,AppT (ConT Data.Maybe.Maybe) (ConT Graphics.UI.Gtk.Types.Widget)) ,(Graphics.Rendering.Hieroglyph.BasicUIState.sizeX,NotStrict,ConT GHC.Float.Double) ,(Graphics.Rendering.Hieroglyph.BasicUIState.sizeY,NotStrict,ConT GHC.Float.Double) ,(Graphics.Rendering.Hieroglyph.BasicUIState.imageCache,NotStrict,AppT (ConT Data.Maybe.Maybe) (ConT Graphics.Rendering.Hieroglyph.UIState.ImageCache))]] []) -} -- usage: $(additions "MyTypeName" OldTypeName) {- - Add fields to a record type for handling basic UI state for Hieroglyph. Gives you mouse buttons, etcetera -} additions newtypenamestr nm = do TyConI (DataD _ _ _ [RecC _ fielddefs]) <- reify nm let newtypename = mkName newtypenamestr return $ (DataD [] newtypename [] [RecC newtypename [(mkName "mousePositionf",NotStrict,ConT ''Point) ,(mkName "mouseLeftButtonDownf",NotStrict,ConT ''Bool) ,(mkName "mouseRightButtonDownf",NotStrict,ConT ''Bool) ,(mkName "mouseMiddleButtonDownf",NotStrict,ConT ''Bool) ,(mkName "mouseWheelf",NotStrict,ConT ''Int) ,(mkName "keyCtrlf",NotStrict,ConT ''Bool) ,(mkName "keyShiftf",NotStrict,ConT ''Bool) ,(mkName "keyAltf",NotStrict,ConT ''Bool) ,(mkName "keyf",NotStrict,ConT ''Key) ,(mkName "drawingf",NotStrict,AppT (ConT ''Maybe) (ConT ''Widget)) ,(mkName "sizeXf",NotStrict,ConT ''Double) ,(mkName "sizeYf",NotStrict,ConT ''Double) ,(mkName "imageCachef",NotStrict,AppT (ConT ''Maybe) (ConT ''ImageCache))] ++ fielddefs] []) -- | Apply a Binary type constructor to given type: "t" -> "Binary t" appUIState :: Type -> Type appUIState t = AppT (ConT ''UIState) t -- | Generate from list of type names result of types application: -- appType T [a,b] -> "T a b" appType :: Name -> [Name] -> Type --appType t [] = ConT t -- T --appType t [t1] = AppT (ConT t) (VarT t1) -- T a --appType t [t1,t2] = AppT (AppT (ConT t) (VarT t1)) (VarT t2) -- T a b == (T a) b appType t ts = foldl (\a e -> AppT a (VarT e)) (ConT t) ts -- general definition -- | Generate `n` unique variables and return them in form of patterns and expressions genNames :: Int -> Q ([PatQ],[ExpQ]) genNames n = do ids <- replicateM n (newName "x") return (map varP ids, map varE ids) -- usage: $(deriveUIState ''MyTypeWithUIState) {- - Derive an instance of UIState from some type that has had UIState fields added to it. -} deriveUIState tp = do return [InstanceD [] (appUIState $ appType tp []) [FunD 'mousePosition [| mousePositionf |] ,FunD 'mouseLeftButtonDown [| mouseLeftButtonDownf |] ,FunD 'mouseRightButtonDown [| mouseRightButtonDownf |] ,FunD 'mouseMiddleButtonDown [| mouseMiddleButtonDownf |] ,FunD 'mouseWheel [| mouseWheelf |] ,FunD 'keyCtrl [| keyCtrlf |] ,FunD 'keyShift [| keyShiftf |] ,FunD 'keyAlt [| keyAltf |] ,FunD 'key [| keyf |] ,FunD 'drawing [| drawingf |] ,FunD 'sizeX [| sizeXf |] ,FunD 'sizeY [| sizeYf |] ,FunD 'imageCache [| imageCachef |] ,FunD 'setMousePosition [| \b a -> a{ mousePositionf=b } |] ,FunD 'setMouseLeftButtonDown [| \b a -> a{ mouseLeftButtonDownf=b } |] ,FunD 'setMouseRightButtonDown [| \b a -> a{ mouseRightButtonDownf=b } |] ,FunD 'setMouseMiddleButtonDown [| \b a -> a{ mouseMiddleButtonDownf=b } |] ,FunD 'setMouseWheel [| \b a -> a{ mouseWheelf=b } |] ,FunD 'setKeyCtrl [| \b a -> a{ keyCtrlf=b } |] ,FunD 'setKeyShift [| \b a -> a{ keyShiftf=b } |] ,FunD 'setKeyAlt [| \b a -> a{ keyAltf=b } |] ,FunD 'setKey [| \b a -> a{ keyf=b } |] ,FunD 'setDrawing [| \b a -> a{ drawingf=b } |] ,FunD 'setSizeX [| \b a -> a{ sizeXf=b } |] ,FunD 'setSizeY [| \b a -> a{ sizeYf=b } |] ,FunD 'setImageCache] [| \b a -> a{ imageCachef=b } |] ]

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State Monad - using the updated state in an adhoc manner
by Phil 08 Jan '09

08 Jan '09

Hi, I¹m a newbie looking to get my head around using the State Monad for random number generation. I¹ve written non-monad code that achieves this no problem. When attempting to use the state monad I can get what I know to be the correct initial value and state, but can¹t figure out for the life of me how to then increment it without binding more calls there and then. Doing several contiguous calls is not what I want to do here and the examples I¹ve read all show this (using something like liftM2 (,) myRandom myRandom). I want to be able to do: Get_a_random_number < a whole load of other stuff > Get the next number as defined by the updated state in the first call <some more stuff> Get another number, and so on. I get the first number fine, but am lost at how to get the second, third, forth etc without binding there and then. I just want each number one at a time where and when I want it, rather than saying give 1,2,10 or even n¹ numbers now. I¹m sure it¹s blindly obvious! Note: I¹m not using Haskell¹s built in Random functionality (nor is that an option), I¹ll spare the details of the method I¹m using (NRC¹s ranq1) as I know it works for the non-Monad case, and it¹s irrelevent to the question. So the code is: ranq1 :: Word64 -> ( Double, Word64 ) ranq1 state = ( output, newState ) where newState = ranq1Increment state output = convert_to_double newState ranq1Init :: Word64 -> Word64 ranq1Init = convert_to_word64 . ranq1Increment . xor_v_init -- I¹ll leave the detail of how ranq1Increment works out for brevity. I know this bit works fine. Same goes for the init function it¹s just providing an initial state. -- The Monad State Attempt getRanq1 :: State Word64 Double getRanq1 = do state <- get let ( randDouble, newState ) = ranq1 state put newState return randDouble _________ And then in my main _________ -- 124353542542 is just an arbitrary seed main :: IO() main = do let x = evalState getRanq1 (ranq1Init 124353542542) print (x) As I said this works fine; x gives me the correct first value for this sequence, but how do I then get the second and third without writing the giveMeTenRandoms style function? I guess what I want is a next() type function, imperatively speaking. Many thanks for any help, Phil.

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Tying a simple circularly STM linked list
by John Ky 07 Jan '09

07 Jan '09

Hi, I've written a circularly linked list, but there is some code in it I feel is redundant, but don't know how to get rid of: -- Transactional loop. A loop is a circular link list. data Loop a = ItemLink { item :: a , prev :: TVar (Loop a) , next :: TVar (Loop a) } | InitLink -- Create a new empty transactional loop. newLoop :: a -> STM (TVar (Loop a)) newLoop item = do tLoop <- newTVar InitLink writeTVar tLoop (ItemLink item tLoop tLoop) return tLoop In the above, the InitLink value is only ever used in the newLoop function to create a single one element circular linked list. Is there a way to write newLoop to avoid using this value? Thanks -John

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Linking in GMP as a Windows DLL w/ GHC
by Sigbjorn Finne 07 Jan '09

07 Jan '09

Hi, a number of folks have been asking/looking for ways to avoid statically linking in GMP into GHC binaries under Windows. I've written up some notes on how to go about doing this, which are now available from http://haskell.forkio.com/gmpwindows Let me know if it is useful (or works ;-) ) enjoy --sigbjorn

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template haskell
by brian 07 Jan '09

07 Jan '09

Smart constructors sometimes need to generate errors like "Foo.Bar.Baz.create: invalid value, etc". Can TH generate the module and function names?

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ICFP: Child care at conference
by Matthew Fluet (ICFP Publicity Chair) 07 Jan '09

07 Jan '09

Potential ICFP attendees: The ACM-SIGPLAN Executive Committee, which oversees and sponsors ICFP, is considering expanding its travel grants for event participants in need of child care assistance. (Such expansion would also apply to other major SIGPLAN-sponsored conferences, such as PLDI, POPL, OOPSLA.) The purpose of this message is to solicit feedback from potential attendees regarding the desirability for child care assistance and what forms of assistance be most appropriate. If child care assistance at ICFP (or other SIGPLAN events) is of interest to you, please contact me (at icfp.publicity(a)gmail.com -or- fluet(a)tti-c.org) with your comments. Some questions of particular interest: * What age groups of children are most appropriate to cover? * What types of costs might SIGPLAN cover to facilitate your participation in events? Sincerely, -Matthew Fluet (ICFP Publicity Chair)

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Maintaining laziness
by Jan Christiansen 07 Jan '09

07 Jan '09

This is great. I am working on the very same topic for quite a while now. My aim is to develop a tool that tells you whether a function is least strict. My work is based on an idea by Olaf Chitil: http://www.cs.kent.ac.uk/people/staff/oc/ I think he was the first who introduced the idea of least strictness. In a paper in the pre-proceedings of IFL 2006 he introduced the term least strict and presented the idea behind a tool called StrictCheck. This tool was supposed to check whether a function is least strict. The problem with this tool was that it proposes non-sequential function definitions that are not implementable in Haskell. The main example in the StrictCheck paper is the definition of unzip by using foldr. This one is very similar to your partition example as it is too strict if you do not use lazy pattern matching for the tuple structure. unzip2 :: [(a, b)] -> ([a], [b]) unzip2 = foldr (\(x,y) (xs,ys) -> (x:xs,y:ys)) ([],[]) My favorite example right now is the multiplication of Peano numbers. Suppose the definition of a data type for Peano numbers and addition and multiplication. data Natural = Z | S Natural (+) :: Natural -> Natural -> Natural Z + y = y S x + y = S (x + y) (*) :: Natural -> Natural -> Natural Z * _ = Z S x * y = y + (x * y) Now we can define an infinite Peano number as follows infinity :: Natural infinity = S infinity The evaluation of Z * infinity yields Z while infinity * Z does not terminate. The tool I am working on is supposed to yield the following information. Natural> strictCheck2 (*) 5 100 You have to fulfil f (S _|_) Z = Z (_|_) This states that the evaluation of S _|_ * Z yields _|_ but it could yield Z. Therefore we can improve the function as follows (*) :: Natural -> Natural -> Natural Z * _ = Z S _ * Z = Z S x * y = y + (x * y) Now the evaluation of infinity * Z yields Z as we wanted it to. If we test this new implementation using StrictCheck we get *Natural> strictCheck2 (*) 5 100 The function seems to be least strict The non least strict definition of (*) is taken from the module Data.Number.Natural which was published by Lennart Augustsson in the numbers package at hackage. I mention this here because I think it clearly shows that it is even hard for experienced Haskell programmers to define least strict functions. An even more promising area of application are functional logic programs as least strict functions result in smaller search spaces. For example I was able to improve the enumeration of pythagorean triples in Curry by a factor of 4 only by making a function least strict. Cheers, Jan

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Updating doubly linked lists
by S. Günther 07 Jan '09

07 Jan '09

Hello, I was trying to wrap my head around the stuff at http://haskell.org/haskellwiki/Tying_the_Knot (again) and along the way came a question: Is it possible to change a particular node of the doubly linked list? That is to say, that would like to have a function: update :: DList a -> a -> DList a where update node newValue returns a list where only the value at the node which is passed in is set to the new Value and all other values are the same. All this of course in a pure way, that is without using (M/T/TM)Vars or IORefs. I do apologize if the answer is really obvious and am thankfull for any hints. Thanks in advance Stephan Günther

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Haskell good for parallelism/concurrency on manycore?
by Ahn, Ki Yung 06 Jan '09

06 Jan '09

Dear Haskellers, I just got an inquiry from a research department head of a Korean company seriously considering to adopt functional languages because they heard that functional languages can do good at parallel programming on manycore platforms. They want to know what technologies out there implemented in functional languages, such as Erlang or Haskell, that can help them write more maintainable programs that are better at utilizing parallelism and avoiding bugs related to synchronization. They are also very interested in Cilk, as well as Haskell or Erlang. My first thought is that maybe Cilk would work better for them just because it would not be easy to recruit Erlang or Haskell programmers experienced in network, security, or concurrent/parallel programming. I myself can answer basic inquiries such as what libraries to look for to implenent such and so, but not able to give advice on large scale projects such as unified security solution package. So, it wouldn't be practical form them to launch a project without inviting an Erlang or Haskell expert in their domain as a project manager from overseas, which I don't think they are very willing to do. Are there Haskell consultants or Haskell experts on this subject, who believes that Haskell based approach might work better for them, or Haskell can be useful along with other approaches (e.g. DSL, prototyping, formal modeling of policies)? If so, I would like to recommend them trying contact you, and try my best to help communicating with them, if needed. They know English, of course, but may not be familiar with functional programming orlanguage-oriented programming jargons such as DSLs, oops, I mean language middleware :-) For your information: The company is a network security company whose main products are VPN and firewall appliances and their management software. Their research department is in search of better technologies to implement their future UTM (unified threat management) solutions utilizing the manycore platforms. In Korea, there are some research groups and few companies using OCaml, but almost no Erlang or Haskell communities. This company is preferring local researchers or consultants for advice or consulting, but there's no local group using Haskell seriously, as far as I know, even in research yet. That's why this person contacted me, just because I wrote a small tutorial on Haskell Server Programming while ago. P.S. If you happen to be a local Korean expert on this matter, sorry for my ignorance, and I'd be happy to forward their inquiry to you! -- Ahn, Ki Yung

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