January 2008 - Haskell-Cafe

ANN / CFC: minimalistic haskell blogging web application (perpubplat)
by Paul Brown 04 Jan '08

04 Jan '08

All -- I've spent some time on cleaning up my hobby blog publishing software and setting up a darcs repository, so I'll throw it out there for criticism or suggestions: darcs get http://datapr0n.com/repos/perpubplat It's running my blog (http://mult.ifario.us) It's relatively lightweight in that it doesn't use a framework but still weighs in at <1500 LOC with comments, but it does rely on the (Thanks, Bjorn!) FastCGI and XHTML combinator libraries. I have some plans for handling comments and the like, but things I'd like to figure out: - Turn the blog data model into a monad. - An idiomatic approach to handling conditional GET and E-Tag, i.e., send a 304 response if things aren't modified. Thanks in advance. -- paulrbrown(a)gmail.com http://mult.ifario.us/

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ANN / CFP - LLVM bindings for Haskell
by Bryan O'Sullivan 04 Jan '08

04 Jan '08

This is an early release of Haskell bindings for the popular LLVM compiler infrastructure project. If you don't know what LLVM is, it's a wonderful toybox of compiler components, from a complete toolchain supporting multiple architectures through a set of well-defined APIs and IR formats that are designed for building interesting software with. The official LLVM home page is here: http://llvm.org/ The Haskell bindings are based on Gordon Henriksen's C bindings. The C bindings are almost untyped, but the Haskell bindings re-add type safety to prevent runtime crashes and general badness. Currently, the entire code generation system is implemented, with most LLVM data types supported (notably absent are structs). Also plugged in is JIT support, so you can generate code at runtime from Haskell and run it immediately. I've attached an example. Please join in the hacking fun! darcs get http://darcs.serpentine.com/llvm If you want a source tarball, fetch it from here: http://darcs.serpentine.com/llvm/llvm-0.0.2.tar.gz (Hackage can't host code that uses GHC 6.8.2's language extension names yet.) There's very light documentation at present, but it ought to be enough to get you going. <b

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An interesting monad: "Prompt"
by Ryan Ingram 04 Jan '08

04 Jan '08

(This message is a literate haskell file. Code for the "Prompt" monad is preceded by ">"; code for my examples is preceded by "]" and isn't complete, but intended for illustration.) I've been trying to implement a few rules-driven board/card games in Haskell and I always run into the ugly problem of "how do I get user input"? The usual technique is to embed the game in the IO Monad: ] type Game = IO ] -- or ] type Game = StateT GameState IO The problem with this approach is that now arbitrary IO computations are expressible as part of a game action, which makes it much harder to implement things like replay, undo, and especially testing! The goal was to be able to write code like this: ] takeTurn :: Player -> Game () ] takeTurn player = do ] piece <- action (ChoosePiece player) ] attack <- action (ChooseAttack player piece) ] bonusTurn <- executeAttack piece attack ] when bonusTurn $ takeTurn player but be able to script the code for testing, allow undo, automatically be able to save replays, etc. While thinking about this problem earlier this week, I came up with the following solution: > {-# OPTIONS_GHC -fglasgow-exts -fallow-undecidable-instances #-} > -- undecidable instances is only needed for the MonadTrans instance below > > module Prompt where > import Control.Monad.Trans > import Control.Monad.Identity > class Monad m => MonadPrompt p m | m -> p where > prompt :: p a -> m a "prompt" is an action that takes a prompt type and gives you a result. A simple example: ] prompt [1,3,5] :: MonadPrompt [] m => m Int This prompt would ask for someone to pick a value from the list and return it. This would be somewhat useful on its own; you could implement a "choose" function that picked randomly from a list of options and gave non-deterministic (or even exhaustive) testing, but on its own this wouldn't be much better than the list monad. What really made this click for me was that the prompt type could be built on a GADT: ] newtype GamePrompt a = GP (GameState, GameChoice a) ] data GameChoice a where ] -- pick a piece to act with ] ChoosePiece :: Player -> GameChoice GamePiece ] -- pick how they should attack ] ChooseAttack :: Player -> GamePiece -> GameChoice AttackType ] -- etc. Now you can use this type information as part of a "handler" function: ] gameIO :: GamePrompt a -> IO a ] gameIO (GP (state, ChoosePiece player)) = getPiece state player ] gameIO (GP (state, ChooseAttack player piece)) = attackMenu player piece ] -- ... The neat thing here is that the GADT specializes the type of "IO a" on the right hand side. So, "getPiece state player" has the type "IO GamePiece", not the general "IO a". So the GADT is serving as a witness of the type of response wanted by the game. Another neat things is that, you don't need to embed this in the IO monad at all; you could instead run a pure computation to do AI, or even use it for unit testing! > -- unit testing example > data ScriptElem p where SE :: p a -> a -> ScriptElem p > type Script p = [ScriptElem p] > > infix 1 --> > (-->) = SE ] gameScript :: ScriptElem GameChoice -> GameChoice a -> Maybe a ] gameScript (SE (ChoosePiece _) piece) (ChoosePiece _) = Just piece ] gameScript (SE (ChooseAttack _ _) attack) (ChooseAttack _ _) = Just attack ] gameScript _ _ = Nothing ] ] testGame :: Script GameChoice ] testGame = ] [ ChoosePiece P1 --> Knight ] , ChooseAttack P1 Knight --> Charge ] , ChoosePiece P2 --> FootSoldier ] , ... ] ] So, how to implement all of this? > data Prompt (p :: * -> *) :: (* -> *) where > PromptDone :: result -> Prompt p result > -- a is the type needed to continue the computation > Prompt :: p a -> (a -> Prompt p result) -> Prompt p result This doesn't require GADT's; it's just using existential types, but I like the aesthetics better this way. Intuitively, a (Prompt p result) either gives you an immediate result (PromptDone), or gives you a prompt which you need to reply to in order to continue the computation. This type is a MonadPrompt: > instance Functor (Prompt p) where > fmap f (PromptDone r) = PromptDone (f r) > fmap f (Prompt p cont) = Prompt p (fmap f . cont) > > instance Monad (Prompt p) where > return = PromptDone > PromptDone r >>= f = f r > Prompt p cont >>= f = Prompt p ((>>= f) . cont) > > instance MonadPrompt p (Prompt p) where > prompt p = Prompt p return > > -- Just for fun, make it work with StateT as well > -- (needs -fallow-undecidable-instances) > instance (Monad (t m), MonadTrans t, MonadPrompt p m) => MonadPrompt p (t m) where > prompt = lift . prompt The last bit to tie it together is an observation function which allows you to run the game: > runPromptM :: Monad m => (forall a. p a -> m a) -> Prompt p r -> m r > runPromptM _ (PromptDone r) = return r > runPromptM f (Prompt pa c) = f pa >>= runPromptM f . c > > runPrompt :: (forall a. p a -> a) -> Prompt p r -> r > runPrompt f p = runIdentity $ runPromptM (Identity . f) p > > runScript :: (forall a. ScriptElem p -> p a -> Maybe a) > -> Script p -> Prompt p r -> Maybe r > runScript _ [] (PromptDone r) = Just r > runScript s (x:xs) (Prompt pa c) = case s x pa of > Nothing -> Nothing > Just a -> runScript s xs (c a) > runScript _ _ _ = Nothing > -- script & computation out of sync My original goal is now achievable: ] type Game = StateT GameState (Prompt GamePrompt) ] ] action :: GameChoice a -> Game a ] action p = do ] state <- get ] prompt $ GP (state, p) ] runGameScript :: Script GameChoice -> GameState -> Game a -> Maybe (GameState, a) ] runGameScript script initialState game ] = runScript scriptFn script' (runStateT game initialState) ] where ] script' = map sEmbed script ] scriptFn s (GP (s,p)) = gameScript (sExtract s) p ] sEmbed (SE p a) = SE (GP (undefined, p)) a ] sExtract (SE (GP (_,p)) a) = SE p a Any comments are welcome! Thanks for reading this far. -- ryan

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isFunction
by john lask 04 Jan '08

04 Jan '08

Oleg provided the following code to test whether a is a function, this code will not type check in hugs, due to the overlapping instances in IsFunction (x->y) f IsFunction x f and the functional dependency | a-> b ERROR "isfunction.lhs":43 - Instances are not consistent with dependencies *** This instance : IsFunction a b *** Conflicts with : IsFunction (a -> b) c *** For class : IsFunction a b *** Under dependency : a -> b is there a way to acheive the same effect in hugs ? > data HTrue > data HFalse > > class IsFunction a b | a -> b > instance TypeCast f HTrue => IsFunction (x->y) f > instance TypeCast f HFalse => IsFunction x f > > -- literally lifted from the HList library > class TypeCast a b | a -> b, b->a where typeCast :: a -> b > class TypeCast' t a b | t a -> b, t b -> a where typeCast' :: t->a->b > class TypeCast'' t a b | t a -> b, t b -> a where typeCast'' :: t->a->b > instance TypeCast' () a b => TypeCast a b where typeCast x = typeCast' () x > instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast'' > instance TypeCast'' () a a where typeCast'' _ x = x _________________________________________________________________ New music from the Rogue Traders - listen now! http://ninemsn.com.au/share/redir/adTrack.asp?mode=click&clientID=832&refer…

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Instance classes and error (also, related to Data.Binary.GET)
by bbrown 03 Jan '08

03 Jan '08

I am using the Data.Binary module and having some issues reading big endian files (actually, just reading the file). I can read the header but not the rest of the data which contains a set of row information. Also, go ahead and make fun my style of coding. Anyway, This is the my code and the error at the bottom. The issue stems from here, it says I didn't define an instance, but I did: instance Binary URLSet where put _ = do BinaryPut.putWord8 0 get = do remainingByteData <- BinaryGet.getRemainingLazyByteString i :: URLInfo <- decode remainingByteData j :: TitleInfo <- decode remainingByteData k :: DescrInfo <- decode remainingByteData x :: KeywordsInfo <- decode remainingByteData return (URLSet {urlinfo=i, titleinfo=j, descrinfo=k, keywordsinfo=x}) ----- module Main where import Data.Word import Data.Binary import qualified Data.ByteString.Lazy.Char8 as BSLC8 import Data.ByteString.Lazy (ByteString) import Data.Binary.Get as BinaryGet import Data.Binary.Put as BinaryPut import IO import Text.Printf import System.Environment import Control.Monad (replicateM, liftM) {- ********************************************************* Define the Database Data Types SpiderDatabase represents a singleton wrapper for an entire database. ********************************************************* -} data SpiderDatabase = SpiderDatabase { magicNumberA :: Word16, magicNumberB :: Word16, majorVers :: Word16, minorVers :: Word16, headerTag :: Word16, poolLen :: Word16, spiderpool :: [URLSet] } data URLSet = URLSet { urlinfo :: URLInfo, titleinfo :: TitleInfo, descrinfo :: DescrInfo, keywordsinfo :: KeywordsInfo } data URLInfo = URLInfo { tag :: Word8, urlid :: Word16, urllen :: Word16, url :: ByteString } data TitleInfo = TitleInfo { titletag :: Word8, titlelen :: Word16, title :: ByteString } data DescrInfo = DescrInfo { descrtag :: Word8, descrlen :: Word16, descr :: ByteString } data KeywordsInfo = KeywordsInfo { keywordstag :: Word8, keywordslen :: Word16, keywords :: ByteString } {- ********************************************************* Class instances ********************************************************* -} instance Show SpiderDatabase where show db = let magicb = (magicNumberB db) header = (headerTag db) poolct = (poolLen db) in "<<<Database Content>>>\n" ++ (((printf "Magic: %X %X\n") (magicNumberA db)) (magicNumberB db)) ++ printf "URL Pool Count: %d\n" poolct ++ "<<<End>>>" instance Binary URLInfo where put _ = do BinaryPut.putWord8 0 get = do urltag <- getWord8 idx <- getWord16be len <- getWord16be strdata <- BinaryGet.getLazyByteString (fromIntegral len) return (URLInfo {tag=urltag, urlid=idx, urllen=len, url=strdata}) instance Binary DescrInfo where put _ = do BinaryPut.putWord8 0 get = do tag <- getWord8 len <- getWord16be strdata <- BinaryGet.getLazyByteString (fromIntegral len) return (DescrInfo {descrtag=tag, descrlen=len, descr=strdata}) instance Binary TitleInfo where put _ = do BinaryPut.putWord8 0 get = do tag <- getWord8 len <- getWord16be strdata <- BinaryGet.getLazyByteString (fromIntegral len) return (TitleInfo {titletag=tag, titlelen=len, title=strdata}) instance Binary KeywordsInfo where put _ = do BinaryPut.putWord8 0 get = do tag <- getWord8 len <- getWord16be strdata <- BinaryGet.getLazyByteString (fromIntegral len) return (KeywordsInfo {keywordstag=tag, keywordslen=len, keywords=strdata}) instance Binary URLSet where put _ = do BinaryPut.putWord8 0 get = do remainingByteData <- BinaryGet.getRemainingLazyByteString i :: URLInfo <- decode remainingByteData j :: TitleInfo <- decode remainingByteData k :: DescrInfo <- decode remainingByteData x :: KeywordsInfo <- decode remainingByteData return (URLSet {urlinfo=i, titleinfo=j, descrinfo=k, keywordsinfo=x}) instance Binary SpiderDatabase where put _ = do BinaryPut.putWord8 0 get = do magicnumbera <- BinaryGet.getWord16be magicnumberb <- BinaryGet.getWord16be major <- BinaryGet.getWord16be minor <- BinaryGet.getWord16be header <- BinaryGet.getWord16be poolct <- BinaryGet.getWord16be -- ******************************* -- Get the remaining byte string data, -- So that we can use lazy bytestring to load to load the -- the data types. -- ******************************* remainingByteData <- BinaryGet.getRemainingLazyByteString -- pool <- (replicate (fromIntegral poolct) (decode remainingByteData)) z :: URLSet <- decode remainingByteData return (SpiderDatabase {magicNumberA=magicnumbera, magicNumberB=magicnumberb, majorVers=major, minorVers=minor, headerTag=header, poolLen=poolct }) main = do putStrLn "Running Spider Database Reader" args <- getArgs db :: SpiderDatabase <- decodeFile (args !! 0) putStrLn $ show db putStrLn "Done" *** *** Error: DbReader.hs:119:22: No instance for (Binary (Get URLInfo)) arising from a use of `decode' at DbReader.hs:119:22-45 Possible fix: add an instance declaration for (Binary (Get URLInfo)) In a 'do' expression: i :: URLInfo <- decode remainingByteData In the expression: do remainingByteData <- getRemainingLazyByteString i :: URLInfo <- decode remainingByteData j :: TitleInfo <- decode remainingByteData k :: DescrInfo <- decode remainingByteData .... In the definition of `get': get = do remainingByteData <- getRemainingLazyByteString i :: URLInfo <- decode remainingByteData j :: TitleInfo <- decode remainingByteData .... DbReader.hs:120:24: No instance for (Binary (Get TitleInfo)) arising from a use of `decode' at DbReader.hs:120:24-47 Possible fix: add an instance declaration for (Binary (Get TitleInfo)) In a 'do' expression: j :: TitleInfo <- decode remainingByteData In the expression: do remainingByteData <- getRemainingLazyByteString i :: URLInfo <- decode remainingByteData j :: TitleInfo <- decode remainingByteData k :: DescrInfo <- decode remainingByteData .... In the definition of `get': get = do remainingByteData <- getRemainingLazyByteString i :: URLInfo <- decode remainingByteData j :: TitleInfo <- decode remainingByteData .... DbReader.hs:121:24: No instance for (Binary (Get DescrInfo)) arising from a use of `decode' at DbReader.hs:121:24-47 Possible fix: add an instance declaration for (Binary (Get DescrInfo)) In a 'do' expression: k :: DescrInfo <- decode remainingByteData In the expression: do remainingByteData <- getRemainingLazyByteString i :: URLInfo <- decode remainingByteData j :: TitleInfo <- decode remainingByteData k :: DescrInfo <- decode remainingByteData .... In the definition of `get': get = do remainingByteData <- getRemainingLazyByteString i :: URLInfo <- decode remainingByteData j :: TitleInfo <- decode remainingByteData .... DbReader.hs:122:27: No instance for (Binary (Get KeywordsInfo)) arising from a use of `decode' at DbReader.hs:122:27-50 Possible fix: add an instance declaration for (Binary (Get KeywordsInfo)) In a 'do' expression: x :: KeywordsInfo <- decode remainingByteData In the expression: do remainingByteData <- getRemainingLazyByteString i :: URLInfo <- decode remainingByteData j :: TitleInfo <- decode remainingByteData k :: DescrInfo <- decode remainingByteData .... In the definition of `get': get = do remainingByteData <- getRemainingLazyByteString i :: URLInfo <- decode remainingByteData j :: TitleInfo <- decode remainingByteData .... DbReader.hs:142:21: No instance for (Binary (Get URLSet)) arising from a use of `decode' at DbReader.hs:142:21-44 Possible fix: add an instance declaration for (Binary (Get URLSet)) In a 'do' expression: z :: URLSet <- decode remainingByteData In the expression: do magicnumbera <- getWord16be magicnumberb <- getWord16be major <- getWord16be minor <- getWord16be .... In the definition of `get': get = do magicnumbera <- getWord16be magicnumberb <- getWord16be major <- getWord16be .... make: *** [dbreader] Error 1 -- Berlin Brown [berlin dot brown at gmail dot com] http://botspiritcompany.com/botlist/?

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Consensus about databases / serialization
by Peter Verswyvelen 03 Jan '08

03 Jan '08

As I'm a selfmade man, I never really studied relational databases in detail. My intuition told me that the "relational" part was not really suitable for the 3D data, 2D images, animation curves, state machines, and other data I encountered in the videogame and animation business. I could always get away with files, and for the applications I needed to deploy, plugging in a couple of extra gigabytes of RAM and serializing the "object" state to disk was more practical, cheaper and faster. However, a couple of years ago I started studying computer science (I seem to do the theory after the practice, weird behavior ;-) at the Open University, and one of the exams I did was about databases. Initially this course convinced me that databases are actually very nice, but the course ended with a topic on object oriented databases, which were designed to make storing data like "3D models, graphs, networks, and complex datastructures" more practical. Duh. Since then, I did deploy a few commercial applications for customers using databases, which worked fine for the typical "simple/flat" database data. I hated embedding a dynamic untyped language like SQL, as much as I hated embedding code in HTML or XML. IMHO it feels UGLY and unsafe. Regarding the other popular data storage format - XML - I did use that a lot, but it seems like going back to the stone ages, when hierarchical stores/databases got invented (and ditched?) Now, initially after an introduction to Microsoft's LINQ, and recently having read a very brief overview of HAppS, it seems I'm not the only one with those "feelings". Ouch, this introduction got way to long, sorry about that ;-) Finally some practical questions: . regarding Haskell and databases, the page http://haskell.org/haskellwiki/Libraries_and_tools/Database_interfaces describes a few, but which are the ones that are stable and practical? Any user experiences? . HApps is not listed in the page above, because it does not use databases? Is HApps reliable or experimental, and does it scale well? Any success stories? . regarding Haskell and serialization, I don't think that implementing Read/Show is a good way for real serialization, so what other options exist? I could find some libraries at http://hackage.haskell.org/packages/archive/pkg-list.html#cat:Data, but again which are the most practical and stable? When programming in C++/MFC and C#/.NET, I tended to develop my own serialization frameworks because I used that for many things, like logging commands to disk, performing undo/redo, intra and inter process cut/copy/paste, save/load, etc. . Regarding serialization, I'm kinda curious how ADTs and even GADTs are stored and retrieved in a relational database? I guess it could be done using BLOBs and serialization to ByteStrings, so bypassing a lot of the database table structures? . If I would want to experiment with say HAppS, the way I understand it, I first would first have to study "Scratch your boilerplate" and Template Haskell, and maybe some other language features? I'm still new to Haskell, and the road to understanding all language elements and extensions is very long, so sequentially learning it would be insane I guess. I have no practical experience with TH, but I spent a long time trying to do "aspect oriented programming" in C# without success, so TH looks uber to me. Thanks a lot and best wishes for 2008? Peter

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Supporting both cabals?
by Magnus Therning 03 Jan '08

03 Jan '08

GHC 6.8 has just made it into Debian in a usable form. (w00t!) Due to the library split my old cabal files don't work any longer. updating them isn't the problem, the problem is keeping them compatible with both versions of Cabal. I searched the wiki (quickly) but didn't find anything on this topic. Is there a cookbook out there for it? /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus．therning＠gmail．com http://therning.org/magnus What if I don't want to obey the laws? Do they throw me in jail with the other bad monads? -- Daveman

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Importing Data.Char speeds up ghc around 70%
by Joost Behrends 03 Jan '08

03 Jan '08

Hi, while still working on optimizing (naively programmed) primefactors i watched a very strange behavior of ghc. The last version below takes 2.34 minutes on my system for computing 2^61+1 = 3*768614,336404,564651. Importing Data.Char without anywhere using it reduces this time to 1.34 minute - a remarkable speed up. System is WindowsXP on 2.2GHZ Intel, 512MB Ram. I give the complete code here - hopefully all tabs are (4) blanks. Can this be reproduced ? I compile just with --make -O2. module Main where import IO import System.Exit import Data.Char main = do hSetBuffering stdin LineBuffering putStrLn "Number to decompose ?" s <- getLine if s == [] then exitWith ExitSuccess else do putStrLn (show$primefactors$read s) main data DivIter = DivIter {dividend :: Integer, divisor :: Integer, bound :: Integer, result :: [Integer]} intsqrt m = floor (sqrt $ fromInteger m) primefactors :: Integer -> [Integer] primefactors n | n<2 = [] | even n = o2 ++ (primefactors o1) | otherwise = if z/=1 then result res ++[z] else result res where res = divisions (DivIter {dividend = o1, divisor = 3, bound = intsqrt(o1), result = o2}) z = dividend res -- is 1 sometimes (o1,o2) = twosect (n,[]) twosect :: (Integer,[Integer]) -> (Integer,[Integer]) twosect m |odd (fst m) = m |even (fst m) = twosect (div (fst m) 2, snd m ++ [2]) found :: DivIter -> DivIter found x = x {dividend = xidiv, bound = intsqrt(xidiv), result = result x ++ [divisor x]} where xidiv = (dividend x) `div` (divisor x) d2 :: DivIter -> DivIter d2 x |dividend x `mod` divisor x > 0 = x { divisor = divisor x + 2} |otherwise = found x d4 :: DivIter -> DivIter d4 x |dividend x `mod` divisor x > 0 = x { divisor = divisor x + 4} |otherwise = found x d6 :: DivIter -> DivIter d6 x |dividend x `mod` divisor x > 0 = x { divisor = divisor x + 6} |otherwise = found x divisions :: DivIter -> DivIter divisions y |or[divisor y == 3, divisor y == 5] = divisions (d2 y) |divisor y <= bound y = divisions (d6$d2$d6$d4$d2$d4$d2$d4 y) |otherwise = y

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Escape Codes
by H. Mayer 03 Jan '08

03 Jan '08

Hello, Some days ago I needed escape codes on Win32. I didn't find any library for that, so I wrote a simple one. I thought, perhaps someone else will have the same problem, so I put it on my website:http://0hc.net/?what=downloads_sup_AWin32Console I've tested it with the ghc 6.6.1, and everything works as I expected it. I would be happy if someone else could test it and/or report if some important escape codes are missing. --Sincerely H. _________________________________________________________________ Erweitere dein digitales Leben. Hol dir jetzt gratis das neue Windows Live. http://get.live.com

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Basic question concerning data constructors
by Joost Behrends 03 Jan '08

03 Jan '08

Hello, perhaps i will make a wishlist of topics not dealt in the tutorials. Here is something i miss in each of them: notes at the semantics of data constructors. We read data Pair a b = Pair a b in YetAnotherHaskellTutorial. And that is all ! If we omit "data" here, this would be a silly pleonasm. And no single word about this strange behavior of "data" in every tutorial i read. A similar point: The tutorials teach, that "=" has a similar meaning than "=" in mathematics. But there is a big difference: it is not reflexive. The the right side is the definition of the left. Thus "x=y" has still some kind of temporality, which mathematics doesn't have. Wadler himself describes bunches of lazily computed equations as "dataflows" somewhere. Ok, so much on theory. Here a concrete question: For adapting hws (one of the reasons for me to be here, not many languages have a native web server) to Windows i must work on time. In System.Time i found data ClockTime = TOD Integer Integer 2 questions arise here: Does this define "TOD" (which i do not find elsewhere) together with ClockTime also ? And: Why is this not: data ClockTime Integer Integer = TOD Integer Integer ? Is it just an abbreviation for the first? Or is there a connection to ClockTime as an "abstract data type" (a notion, which would have a subtle different meaning than in OOP - since "instance" is such different thing here). Thanks for your attention, Joost

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