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Deriving type family data
by Tuve Nordius 28 Apr '09

28 Apr '09

If I for some data type wants to derive, in this case Data and Typeable for use with syb code, but the problem is the same regardless what I want to derive. data family Something data Tree = Leaf Something | Fork Something Tree Tree deriving (Data, Typeable) The problem is I want to derive a class for a data type that depends on some non instantiated data type. I can of course rewrite Tree as: data (Data a) => Tree a = Leaf a | Fork a Tree Tree deriving (Data, Typeable) but then data family Something is redundant, I want to think of Something as a not yet instantiated abstract data type Is there anyway to express that either the instance of Something used in Tree should be a member of Data, or even better any instance of Something should be a member of Data or maybe even better that every instance should derive data. data family Something deriving Data //Tuve Nordius

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TraverseAccum: an effectful accumulating map.
by Florent BALESTRIERI 28 Apr '09

28 Apr '09

Hello fellow haskellers. This message is a valid literate haskell file. I ran it with ghc 6.10.1 > module TraverseAccum where > import Control.Applicative > import Data.Traversable A year ago, I read "Why Attribute Grammars Matter". In it we found a function on lists wich combined three traversals into one to compute the differences to the average: > avg_diff_list :: [Double] -> [Double] > avg_diff_list xs = > let nil = const (0,0.0,[]) > cons x f mean = > let (n,s,ds) = f mean > in (n+1,s+x, (x - mean) : ds) > (n,s,ds) = foldr cons nil xs (s/n) > in ds Then I found "Applicative programming with effects" which abstracted the notions of traversal and folding. We can try and generalize avg_diff_list to work over any Traversable data structure. Here is a possible version with two traversals: > avg_diff' :: (Traversable t, Fractional f) > => t f -> t f > avg_diff' t = > let start = (0,0.0) > step (n,s) x = ((n+1, s+x), \mean -> x - mean) > ((n,s), diffs) = mapAccumL step start t > in sequenceA diffs (s/n) But we can do better with only a single traversal. The idea is to define a function traverseAccumL which does the work of mapAccumL but with an effectful map. (mapM) The definition of mapAccumL uses a StateL applicative functor whereas traverseAccumL uses a StateL monad transformer. First a few definitions. > -- left-to-right state transformer > newtype StateLT m s a = StateLT { runStateLT :: s -> m (s, a) } > instance Functor m => Functor (StateLT m s) where > fmap f (StateLT k) = StateLT $ \ s -> fmap f <$> k s > instance (Functor m, Monad m) => Applicative (StateLT m s) where > pure x = StateLT (\ s -> return (s, x)) > StateLT kf <*> StateLT kv = StateLT kfv > where kfv s = do (s', f) <- kf s > (s'', v) <- kv s' > return (s'', f v) Incidently, I always wondered why Monad wasn't a subclass of Functor. > traverseAccumL :: ( Traversable t, Functor m, Monad m) > => (a -> b -> m (a, c)) -> a -> t b -> m (a, t c) > traverseAccumL f s t = runStateLT (traverse (StateLT . flip f) t) s I could have done without StateLT but the signature would have been different: traverseAccumL :: (Traversable t, Monad m) => (a -> b -> m (c, a)) -> a -> t b -> m (t c, a) traverseAccumL f s t = runStateT (mapM (StateT . flip f) t) s Now the definition of avg_diff calls traverseAccumL with the reader monad > avg_diff :: (Traversable t, Fractional f) > => t f -> t f > avg_diff t = > let start = (0,0.0) > step (n,s) x mean = ((n+1, s+x), x - mean) > ((n,s), diffs) = traverseAccumL step start t (s/n) > in diffs The dual definition traverseAccumR is almost the same. The only difference lies in the definition of <*> StateRT kf <*> StateRT kv = StateRT kfv where kfv s = do (s', v) <- kv s (s'', f) <- kf s' return (s'', f v) > -- right-to-left state transformer > newtype StateRT m s a = StateRT { runStateRT :: s -> m (s, a) } > > instance Functor f => Functor (StateRT f s) where > fmap f (StateRT k) = StateRT $ \ s -> fmap f <$> k s > > instance (Functor m, Monad m) => Applicative (StateRT m s) where > pure x = StateRT (\ s -> return (s, x)) > StateRT kf <*> StateRT kv = StateRT kfv > where kfv s = do (s', v) <- kv s > (s'', f) <- kf s' > return (s'', f v) > > traverseAccumR :: ( Traversable t, Functor m, Monad m) > => (a -> b -> m (a, c)) -> a -> t b -> m (a, t c) > traverseAccumR f s t = runStateRT (traverse (StateRT . flip f) t) s -- Florent B.

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Can subclass override its super-class' default implementation of a function?
by siki 28 Apr '09

28 Apr '09

I'm not sure if this is possible at all. I'd like to do something like this: class A a where foo :: a -> Double foo a = 5.0 class (A a) => B a where foo a = 7.0 data Blah = Blah data Bar = Bar instance A Blah instance B Bar let blah = Blah bar = Bar foo blah -- should print out 5 foo bar -- should print out 7 Basically, I have a bunch of instances that have a common functionality but I'd like to be able to group those instances and give each group a different default implementation of that functionality. It's so easy to do this in Java, for example, but I have no idea how to do it in Haskell. The above example will not compile, btw. Thanks a lot -- View this message in context: http://www.nabble.com/Can-subclass-override-its-super-class%27-default-impl… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: [Haskell-cafe] Random number example
by michael rice 28 Apr '09

28 Apr '09

How do I define type Random [Int] for rollNDice in Exercise 1, given the code below? Michael ============ Exercises 1. Implement rollNDice :: Int -> Random [Int] from the previous subsection with >>= and return. NOTE: Since >>= and return are already present in the Prelude, you may want to use import Prelude hiding ((>>=),return) to avoid compilation errors. ================= {-# LANGUAGE NoImplicitPrelude #-} import Prelude hiding ((>>), (>>=), return) type Seed = Int type Random a = Seed -> (a, Seed) randomNext :: Seed -> Seed randomNext rand = if newRand > 0 then newRand else newRand + 2147483647 where newRand = 16807 * lo - 2836 * hi (hi,lo) = rand `divMod` 127773 rollDie :: Random Int rollDie seed = ((seed `mod` 6) + 1, randomNext seed) (>>) :: Random a -> Random b -> Random b (>>) m n = \seed0 -> let (result1, seed1) = m seed0 (result2, seed2) = n seed1 in (result2, seed2) (>>=) :: Random a -> (a -> Random b) -> Random b (>>=) m g = \seed0 -> let (result1, seed1) = m seed0 (result2, seed2) = (g result1) seed1 in (result2, seed2) return :: a -> Random a return x = \seed0 -> (x, seed0) sumTwoDice :: Random Int sumTwoDice = rollDie >>= (\die1 -> rollDie >>= (\die2 -> return (die1 + die2))) rollNDice :: Int -> Random [Int] --- On Thu, 4/23/09, michael rice <nowgate(a)yahoo.com> wrote: From: michael rice <nowgate(a)yahoo.com> Subject: Re: [Haskell-cafe] Random number example To: "Ross Mellgren" <rmm-haskell(a)z.odi.ac> Cc: haskell-cafe(a)haskell.org Date: Thursday, April 23, 2009, 5:49 PM Hi Ross, Thanks for going the extra mile. A lot of what you did I haven't seen before, so it's going to take me some time to go through it. But I'll be back. Michael --- On Thu, 4/23/09, Ross Mellgren <rmm-haskell(a)z.odi.ac> wrote: From: Ross Mellgren <rmm-haskell(a)z.odi.ac> Subject: Re: [Haskell-cafe] Random number example To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Thursday, April 23, 2009, 11:51 AM So there are a couple problems. First is you are trying to rebind prelude functions, when instead you should be creating an instance of Monad. This requires a bit of shuffling because without language extensions you can't instance Monad Random for your type of Random, as it is a type synonym. So, changing the type synonym to a newtype and instancing monad, you get: module Rand9b where import Control.Applicative (Applicative(..), (<$>), (<*>))import Control.Monad (ap, liftM) type Seed = Intnewtype Random a = Rand { unRand :: (Seed -> (a, Seed)) } randomNext :: Seed -> SeedrandomNext rand = if newRand > 0 then newRand else newRand + 2147483647 where newRand = 16807 * lo - 2836 * hi (hi,lo) = rand `divMod` 127773 rollDie :: Random IntrollDie = Rand $ \ seed -> ((seed `mod` 6) + 1, randomNext seed) instance Monad Random where (>>=) = randomBind return = randomReturn instance Functor Random where fmap = liftM instance Applicative Random where pure = return (<*>) = ap randomBind :: Random a -> (a -> Random b) -> Random bm `randomBind` g = Rand $ \seed0 -> let (result1, seed1) = unRand m $ seed0 (result2, seed2) = unRand (g result1) $ seed1 in (result2, seed2) randomReturn :: a -> Random arandomReturn x = Rand $ \ seed0 -> (x, seed0) sumTwoDice :: Random IntsumTwoDice = (+) <$> rollDie <*> rollDie I also threw in instances of Functor and Applicative, so that I could simplify sumTwoDice using applicative form (much nicer, no? Applicative is totally rockin') Now you need one more thing, a way to convert a series of Random actions into a pure function: runRandom :: Seed -> Random a -> arunRandom s f = fst . unRand f $ s which now makes what you want to do in GHCi easy and well wrapped: Prelude> :reload[1 of 1] Compiling Rand9b ( rand9b.hs, interpreted )Ok, modules loaded: Rand9b.*Rand9b> runRandom 0 sumTwoDice3 Hope this helps,-Ross On Apr 23, 2009, at 11:28 AM, michael rice wrote: I pretty much followed the sequence of steps that led to this final code (see below), but will be looking it over for a while to make sure it sinks in. In the meantime, I get this when I try to use it (sumTwoDice) at the command line: [michael@localhost ~]$ ghci rand9 GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( rand9.hs, interpreted ) Ok, modules loaded: Main. *Main> sumTwoDice <interactive>:1:0: No instance for (Show (Seed -> (Int, Seed))) arising from a use of `print' at <interactive>:1:0-9 Possible fix: add an instance declaration for (Show (Seed -> (Int, Seed))) In a stmt of a 'do' expression: print it *Main> Can I employ a 'do' expression from the command line? Also, can I now use functions (>>) (>>=) and 'return' defined in the Prelude and still have this code work? Michael ================== {-# LANGUAGE NoImplicitPrelude #-} import Prelude hiding ((>>), (>>=), return) type Seed = Int type Random a = Seed -> (a, Seed) randomNext :: Seed -> Seed randomNext rand = if newRand > 0 then newRand else newRand + 2147483647 where newRand = 16807 * lo - 2836 * hi (hi,lo) = rand `divMod` 127773 rollDie :: Random Int rollDie seed = ((seed `mod` 6) + 1, randomNext seed) (>>) :: Random a -> Random b -> Random b (>>) m n = \seed0 -> let (result1, seed1) = m seed0 (result2, seed2) = n seed1 in (result2, seed2) (>>=) :: Random a -> (a -> Random b) -> Random b (>>=) m g = \seed0 -> let (result1, seed1) = m seed0 (result2, seed2) = (g result1) seed1 in (result2, seed2) return :: a -> Random a return x = \seed0 -> (x, seed0) sumTwoDice :: Random Int sumTwoDice = rollDie >>= (\die1 -> rollDie >>= (\die2 -> return (die1 + die2))) _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -----Inline Attachment Follows----- _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Converting IO [XmlTree] to [XmlTree]
by rodrigo.bonifacio 28 Apr '09

28 Apr '09

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ghci debugger problem with :continue. is it broken, or is it me?
by Thomas Hartman 28 Apr '09

28 Apr '09

In the program below, can someone explain the following debugger output to me? After :continue, shouldn't I hit the f breakpoint two more times? Why do I only hit the f breakpoint once? Is this a problem in the debugger? thartman@ubuntu:~/haskell-learning/debugger>cat debugger.hs -- try this: -- ghci debugger.hs -- > :break f -- > :trace t -- > :history -- should show you that f was called from h t = h . g . f $ "hey!" t2 = h . g . f $ "heh!" t3 = h . g . f $ "wey!" f = ("f -- " ++) g = ("g -- " ++) h = ("h -- " ++) ts = do putStrLn $ t putStrLn $ t2 putStrLn $ t3 {- Problems using :continue in the ghci debugger? Can someone explain the following debugger output to me? After :continue, shouldn't I hit the f breakpoint two more times? Why do I only hit the f breakpoint once? Is this a problem in the debugger? thartman@ubuntu:~/haskell-learning/debugger>ghci debugger.hs GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( debugger.hs, interpreted ) Ok, modules loaded: Main. *Main> :break f Breakpoint 0 activated at debugger.hs:12:4-15 *Main> ts h -- g -- Stopped at debugger.hs:12:4-15 _result :: [Char] -> [Char] = _ 11 12 f = ("f -- " ++) ^^^^^^^^^^^^ 13 g = ("g -- " ++) [debugger.hs:12:4-15] *Main> :continue f -- hey! h -- g -- f -- heh! h -- g -- f -- wey! *Main> -}

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LondonHUG talk: Engineering Large Projects in Haskell
by Don Stewart 28 Apr '09

28 Apr '09

Slides from last week's London HUG http://www.londonhug.net/ talk are now online. The talk attempts to document some of the tips and tricks Galois has accumulated using Haskell commercially for the past 10 years. You can now read the slides of the talk here: http://www.galois.com/blog/2009/04/27/engineering-large-projects-in-haskell… Cheers, Don

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Problem with installing phooey
by Daryoush Mehrtash 28 Apr '09

28 Apr '09

When I try to install Phooey I get the following error: cabal install --constraint="Stream == 0.3" phooey Resolving dependencies... [1 of 1] Compiling Main ( /tmp/phooey-2.016943/phooey-2.0/Setup.lhs, /tmp/phooey-2.016943/phooey-2.0/dist/setup/Main.o ) Linking /tmp/phooey-2.016943/phooey-2.0/dist/setup/setup ... Configuring phooey-2.0... Warning: No 'build-type' specified. If you do not need a custom Setup.hs or ./configure script then use 'build-type: Simple'. Preprocessing library phooey-2.0... Building phooey-2.0... src/Graphics/UI/Phooey/WinEvents.hs:66:7: Could not find module `Data.Reactive': Use -v to see a list of the files searched for. It seems that the current version of reactive (0.10.5) no longer has Data.Reactive module (it seem to have been removed since 0.5 version of the reactive). Does Phooey really need 0.5 version of the reactive or is there a problem in my installation? Thanks Daryoush

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createProcess problem
by Vasili I. Galchin 28 Apr '09

28 Apr '09

Hello, I am trying to use createProcess. Following is a small fragment. I am trying to run a program called blastall (a bio-informatics program). It is kind of like mv/cp/ln on Unix platforms (where the executable examines the program, i.e. arg[0], and then provides appropriate behavior/semantics). In the case of blastall, it alters its behavior based on the -p(program) CLI argument.WSo in the following it should provide nucleotide behavior ... that is what the 'n' on the end of blastn means. import System.Process main = do handle <- runProcess "blastall" -- executable ["-p blastn"] -- CLI args Nothing -- optional cwd Nothing -- optional env Nothing -- optional stdin Nothing -- optional stdout Nothing -- optional stderr return () When I run this program with ghci, blastall is not a happy camper. It is complaining about "-p blastn": [NULL_Caption] ERROR: Program name undefined: " blastn" <<< the space between the double quote and 'b' is really there!!! [NULL_Caption] ERROR: Program name undefined: " blastn" Yet if I run 'blastall -p blastn' from the DOS prompt no errors(I will try tonight on my Linux machine). If I try 'blastall -p bozo" from the DOS prompt I get the same two errors as when I run my test program using ghci. I feel that it has something to do with double quotes in Haskell ... probably something obviously silly that I am doing. Any ideas? Kind regards, Vasili

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createProcess problem
by Vasili I. Galchin 27 Apr '09

27 Apr '09

Hello, I am trying to use createProcess. Following is a small fragment. I am trying to run a program called blastall (a bio-informatics program). It is kind of like mv/cp/ln on Unix platforms (where the executable examines the program, i.e. arg[0], and then provides appropriate behavior/semantics). In the case of blastall, it alters its behavior based on the -p(program) CLI argument.WSo in the following it should provide nucleotide behavior ... that is what the 'n' on the end of blastn means. import System.Process main = do handle <- runProcess "blastall" -- executable ["-p blastn"] -- CLI args Nothing -- optional cwd Nothing -- optional env Nothing -- optional stdin Nothing -- optional stdout Nothing -- optional stderr return () When I run this program with ghci, blastall is not a happy camper. It is complaining about "-p blastn": [NULL_Caption] ERROR: Program name undefined: " blastn" <<< the space between the double quote and 'b' is really there!!! [NULL_Caption] ERROR: Program name undefined: " blastn" Yet if I run 'blastall -p blastn' from the DOS prompt no errors(I will try tonight on my Linux machine). If I try 'blastall -p bozo" from the DOS prompt I get the same two errors as when I run my test program using ghci. I feel that it has something to do with double quotes in Haskell ... probably something obviously silly that I am doing. Any ideas? Kind regards, Vasili

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