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Re: [Haskell-cafe] Finally tagless - stuck with implementation of "lam"
by Jacques Carette 15 Oct '09

15 Oct '09

(sorry for the slow reply on this topic...) Robert Atkey and Oleg presented some very interesting code in response to your query. But some of you might (and should!) be asking "why on earth did Jacques use unsafePerformIO?", especially when neither Robert nor Oleg did. Simply put: I answered your question exactly as asked, while they answered the question you *should* have asked. At the exact 'type' involved in your question, there is no good answer; if you want an instance of lam at a monadic type 'directly', you need to 'extract' from the monad. But the point is that that isn't really the right question. Rather than requiring complete parametric polymorphism in the answer-type, if you allow a little non-uniformity through a simple type family, the shift in types is sufficient to no longer require a monadic 'extract' at all. But even once you've gotten to that point, that's not enough, because at that point you still have the issue of what calling convention (by value, name or need) to use. And figuring that out is rather fun, so you got detailed answer from Robert and Oleg explaining that part in detail. Robert carefully used IntT and :-> to label the different types, and Oleg's code (http://okmij.org/ftp/tagless-final/CB.hs) showed how these were really just 'labels', which are useful mnemonics for humans, but not essential. If you dig into our JFP paper describing "finally tagless" in all its gory details, you'll see that we use a poor man's version of type families there too (see section 4.3). Jacques

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Relational Algebra
by Günther Schmidt 15 Oct '09

15 Oct '09

Hi all, has anybody already developed an EDSL for relational algebra? HaskellDB does that, but its current implementation only allows for generating SQL, are there other implementations? Günther

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ANNOUNCE: hs-ffmpeg 0.3.2 - Bindings to FFMpeg library
by Vasyl Pasternak 15 Oct '09

15 Oct '09

Hello, I am glad to announce next release of hs-ffmpeg library. Now you could download it from the Hackage along with the ffmpeg-tutorials, which show capabilities of this library. The installation process is a bit tricky now, so welcome to my blog post http://progandprog.blogspot.com/2009/10/video-processing-on-haskell-easy.ht…, where I am describing how to build and install library. Best regards, Vasyl Pasternak

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(no subject)
by Stavenga, G.C. 15 Oct '09

15 Oct '09

Hi, I'm just started to learn Haskell. Coming from a programming contest background (where it is important to be able to solve problems in a small amount of code) I'm wondering what the best way is for simple IO. A typical input file (in a programming contest) is just a bunch of numbers which you want to read one by one (sometimes interspersed with strings). In C/C++ this is easily done with either scanf or cin which reads data separated by spaces. In Haskell I have not found an equally satisfactionary method. The methods I know of 1) Stay in the IO monad and write your own readInt readString functions. A lot of code for something easy. 2) Use interact together with words and put the list of lexemes in a State monad and define getInt where at least you can use read. 3) Use ByteString.Char8 which has readInt (but I couldn't find a readString). But one has to put it also in a State monad. I think that there must be standard function that can do this. What do experienced Haskellers use? Thanks in advance

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Monotype error
by Martijn van Steenbergen 15 Oct '09

15 Oct '09

Dear café, > {-# LANGUAGE Rank2Types #-} > {-# LANGUAGE ImpredicativeTypes #-} > > type Void = forall a. a > > newtype Mono a = Mono { runMono :: [Void] } > > beep :: Mono a -> Mono a > beep (Mono vs) = Mono (map undefined vs) Compiling this with GHC results in: > Monotype.hs:9:28: > Cannot match a monotype with `Void' > Expected type: Void > Inferred type: a What does this error mean and why does the code not compile? Thanks! Martijn.

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newtype deriving Alternative
by Martijn van Steenbergen 15 Oct '09

15 Oct '09

Hello café, I've never written an Alternative instance for a newtype yet that doesn't look like this: > instance Alternative T where > empty = T empty > T x <|> T y = T (x <|> y) Why does newtype deriving not work for Alternative? (It works fine for Monoid.) Thanks, Martijn.

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Fuzzy Logic / Linguistic Variables
by Neal Alexander 15 Oct '09

15 Oct '09

So i was reading "Programming Game AI by Example" by Mat Buckland (http://www.ai-junkie.com/books/toc_pgaibe.html) and decided to rewrite his chapter on Fuzzy logic in haskell (from C++). My initial impression: its one of those scenarios where OOP grossly over complicates things Heres an example taken from the book: An agent needs to decide what weapon to use based on his distance from the target and the amount of ammo each has. The result is in the desirability domain (0-100). http://code.haskell.org/~hexpuem/fuzzyLogic/AI/Logic/Fuzzy/Example.hs An excerpt: ------------------------------------ weapons = [ Weapon "RocketLauncher" 9 rocketLauncher, Weapon "ShotGun" 13 shotgun, Weapon "AssaultRifle" 120 assaultRifle, Weapon "SniperRifle" 7 sniperRifle, Weapon "Knife" 1 knife ] chooseWeapon dist = maximum ratings where ratings = [ (f dist ammo, n) | Weapon n ammo f <- weapons ] ------------------------------------ shotgun :: Double -> Double -> Double shotgun dist ammo = unfuzz desireDom $ rules (fuzz distDom dist) (fuzz ammoDom ammo) where rules :: Fuzzy Distances -> Fuzzy Ammo -> FL Desirability rules distance ammo = do distance `is` SniperSuited =>> Pointless distance `is` Far =>> Undesirable distance `is` Medium =>> Undesirable distance `is` Melee =>> Undesirable (distance `fairly` Near) & (ammo `fairly` High) =>> VeryDesirable (distance `fairly` Near) & (ammo `fairly` Good) =>> Desirable (distance `fairly` Near) & (ammo `is` Low) =>> Undesirable ------------------------------------ Full code at http://code.haskell.org/~hexpuem/fuzzyLogic/. Suggestions welcome - maybe it'd be useful to upload to hackage at some point. It only supports triangle, shoulder, and singleton memberships at the moment.

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Sharing Subexpressions: Memoization of Fibonacci sequence
by SimonK77 14 Oct '09

14 Oct '09

Hallo everyone, the last few weeks I was trying to get used to memoization in haskell. As I found out in a previous post, memoization in haskell is, due to the graph reduction strategy used in haskell, almost always implemented by sharing subexpressions in an expression. In examples as the following this is quite easy to see. fibs = 0:1:zipWith (+) fibs (tail fibs) But as I browsed through the search results from google for this topic, I encountered the following implementation: memoized_fib :: Int -> Integer memoized_fib = let fib 0 = 0 fib 1 = 1 fib n = memoized_fib (n-2) + memoized_fib (n-1) in (map fib [0 ..] !!) You'll find it at Haskell.org. Here's the http://www.haskell.org/haskellwiki/Memoization link Let's assume we have the following implementation of the higher-order-function map: map :: (a -> b) -> [a] -> [b] map _ [] = [] map f (x:xs) = f x : map f xs The reduction sequence of memoized_fib 5 would start as follows: //Reduction of memoized_fib 5 memoized_fib 5 = (map fib [0..] !!) 5 = (fib 0 : map fib [1..] !!) 5 = (0 : fib 1 : map fib [2..] !!) 5 = (0 : 1 : fib 2 : map fib [3..] !!) 5 = (0 : 1 : (memoized_fib 0 + memoized_fib 1) : fib 3 : map fib [4..] !!) 5 = (0 : 1 : (map fib [0..] !! 0 + map fib [0..] !! 1) : (memoized_fib 1 + memoized_fib 2) : map fib [4..] !!) 5 . . . . and so on! I can't see where the sharing of subexpressions happens here. Any help is highly appreciated. Best regards Simon -- View this message in context: http://www.nabble.com/Sharing-Subexpressions%3A-Memoization-of-Fibonacci-se… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Setting environment variables on Windows
by Sönke Hahn 14 Oct '09

14 Oct '09

Hi! I need to set an environment variable from Haskell and i would like to do that cross-platform. There is System.Posix.Env.setEnv, which does exactly, what i want on Linux. There is the module System.Environment, which seems to be cross-platform, but it does not contain functions to manipulate the environment (, just to inspect it). At first glance, I didn't find anything relevant in the sub-modules of System.Win32. Is this just not implemented? How could i implement it myself? Grateful for any comment, Sönke

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Why does replicateM (10^6) $ return 0 produce output in the IO monad, but overflow the maybe monad?
by Thomas Hartman 14 Oct '09

14 Oct '09

-- Why does replicateM (10^6) $ return 0 produce output in the IO monad, but overflow the maybe monad? iterateNTimes i f x = foldr (.) id (replicate i f) $ x tntIO :: IO Int -- same as replicateM (10^6) $ return 0, same as sequence . replicate (10^6) $ return 0 tntIO = return . head =<< (iterateNTimes (10^6) (mcons . return $ 0) (return [])) -- produces output tntMb :: Maybe Int -- overflows tntMb = return . head =<< (iterateNTimes (10^6) (mcons . return $ 0) (return [])) -- stack overflow -- equivalently: mcons m ms = (:) <$> m <*> ms mcons m ms = ap (liftM (:) m) $ ms I guess the maybe version builds up a huge chain of unevaluated thunks somewhere, that's usually the reason. But specifically where and why, and why doesn't IO do the same thing? Equivalent, with 3 element list: (mcons $ return $ 0 ) $ (mcons $ return $ 0) $ (mcons $ return $ 0) $ (return []) (ap . liftM (:) . return $ 0 ) $ (ap . liftM (:) . return $ 0) $ (ap . liftM (:) . return $ 0) $ (return []) (ap $ liftM (:) $ return $ 0 ) $ (ap $ liftM (:) $ return $ 0) $ (ap $ liftM (:) $ return $ 0) $ (return []) t.

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