March 2014 - Beginners

Data.Stream interleave implementation question
by Bryan Brady 18 Aug '14

18 Aug '14

I'm in the process of learning Haskell and am implementing my own Stream type. ( I'm working my way through homework problems I found at http://www.seas.upenn.edu/~cis194/hw/06-laziness.pdf ) One of the questions is: "Deﬁne the stream ruler :: Stream Integer which corresponds to the ruler function 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 4, . . . where the nth element in the stream (assuming the ﬁrst element corresponds to n = 1) is the largest power of 2 which evenly divides n. " I got pretty close, but ended up looking at Data.Stream to nail down my mistake. My original solution was: ----- data Stream a = Cons a (Stream a) deriving (Eq, Ord) streamRepeat :: a -> Stream a streamRepeat a = Cons a (streamRepeat a) interleaveStreams :: Stream a -> Stream a -> Stream a interleaveStreams (Cons a as) (Cons b bs) = Cons a (Cons b (interleaveStreams as bs)) ruler :: Stream Integer ruler = foldr1 interleaveStreams (map streamRepeat [0..]) ----- Attempting to evaluate 'ruler' in ghci does not finish. I knew something was wrong with my implementation of interleaveStreams because it worked when I expanded it out manually for a few steps. The way Data.Stream implements interleave is: interleaveStreams (Cons a as) bs = Cons a (interleaveStreams bs as) I don't understand why this works and why my original implementation doesn't. I figure if the latter works, the former should as well. Here is my reasoning: In the latter definition, Cons a (interleaveStreams bs as), (interleaveStreams bs as) is a thunk. The thunk should only be evaluated when it is needed. In my original definition, (interleaveStreams as bs) is a thunk. The difference is an extra Cons (e.g., Cons b). It seems like the additional Cons is causing problems, I just don't understand why. Can anyone point out what I'm missing? thanks, bb

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Mapping inside State
by martin 01 Apr '14

01 Apr '14

Hello all, I found myselft writing this ugly piece of code silenceAll :: TI -> State (M.Map CH StringState,[DtzEvent]) TI silenceAll t = silence t 0 >> silence t 1 >> silence t 2 >> silence t 3 >> silence t 4 >> silence t 5 >> silence t 6 >> return t because I couldn't figure out how to map (silence t) over [0..6]. What is the standard way of doing this?

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understanding MTL
by Dennis Raddle 30 Mar '14

30 Mar '14

I had an interaction on #haskell today which left me utterly confused but desiring to understand what happened. I was talking about a program in which I was using stacks of monad transformers, inspiration taken from the paper "Monad Transformers: Step by Step". So for example I wanted to combine state and error handling in a Monadic type called "Er". data ErState = ErState StdGen newtype Er a = Er { runEr :: ErrorT String (State ErState) a } deriving(Monad, MonadError String, MonadState ErState) Note that ErState holds a random generator. I had the idea to make a typeclass of monadic types that hold random generators because I'll be using them in other types all over my application. Something like class Monad m => RandomMonad m where getGen :: m StdGen putGen :: StdGen -> m () Then I can write functions like this: myRandoms :: (Random a, RandomMonad m) => m [a] myRandoms = do g <- getGen let (g1, g2) = split g values = randoms g1 putGen g2 return values So then mm_freak on #haskell noticed that I had written a function with a signature like process :: Int -> Er Int He said, no no no functions should be agnostic with regard to data structure. It should look something like process :: (RandomMonad m, MonadError m, MonadState m) => Int -> m Int He then said I don't want to put StateT and ErrorT in the same newtype declaration, so it would be something like newtype Er m a = Er { runEr :: StateT ErState m a } On this point I'm confused. I don't know how to get from a line like the above to eventually making a type that combines error handling and state. Furthermore I need to define instances for things like MonadError. He got me started with the line instance MonadError e m => MonadError e (Er m) where ... define throwError and catchError which I have no idea how to do ... So where we left it, I was confused and didn't understand anything. Rather than just getting the answers, I want to try to understand this, but I feel like I must need more background. This must be the kind of code that is in the monad transformer library. Is there a guide to understanding that somewhere? D

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Help on first program
by John M. Dlugosz 29 Mar '14

29 Mar '14

Can someone explain what's wrong here? What is the code below _really_ saying, vs what I meant? I don't understand the error messages yet. == input == module Main ( main ) where solve n src dest tmp = do solve (n-1) src tmp -- line 6 solve 1 src dest tmp solve (n-1) tmp dest -- line 8 solve 1 src dest _ = putStrLn "move a disk from " ++ src ++ " to " ++ dest -- line 11 main = do putStrLn "Towers of Hanoi problem" solve 5 "A" "B" "C" == output == Prelude> :reload [1 of 1] Compiling Main ( towers1.hs, interpreted ) Failed, modules loaded: none. Prelude> towers1.hs:6:8: Couldn't match expected type `[a0]' with actual type `[Char] -> [Char]' In the return type of a call of `solve' Probable cause: `solve' is applied to too few arguments In a stmt of a 'do' block: solve (n - 1) src tmp In the expression: do { solve (n - 1) src tmp; solve 1 src dest tmp; solve (n - 1) tmp dest } towers1.hs:8:8: Couldn't match expected type `[Char]' with actual type `[Char] -> [Char]' In the return type of a call of `solve' Probable cause: `solve' is applied to too few arguments In a stmt of a 'do' block: solve (n - 1) tmp dest In the expression: do { solve (n - 1) src tmp; solve 1 src dest tmp; solve (n - 1) tmp dest } towers1.hs:11:5: Couldn't match expected type `[Char]' with actual type `IO ()' In the return type of a call of `putStrLn' In the first argument of `(++)', namely `putStrLn "move a disk from "' In the expression: putStrLn "move a disk from " ++ src ++ " to " ++ dest

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Why is this not a "category"?
by John M. Dlugosz 29 Mar '14

29 Mar '14

on https://en.wikibooks.org/w/index.php?title=Haskell/Category_theory&stable=0… the second exercise in the box (see illustration there) asks "(Harder.) If we add another morphism to the above example, it fails to be a category. Why? Hint: think about associativity of the composition operation." There are no answers-to-exercises. Can someone explain to me why adding another function with the same type causes the Haskell type system to no longer form the Hask category? (scratching head)

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What is operator :| ?
by John M. Dlugosz 28 Mar '14

28 Mar '14

http://www.haskell.org/haskellwiki/Typeclassopedia under §8.1, sconcat = sconcat (a :| as) = go a as where What is the meaning of :| ? I see it is really in the cited module source, not a typo on the page. More generally, is there some effective way to search for non-alphabetical Haskell things? Google just ignores the "punctuation".

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algorithms books or tutorial in haskell
by Nishant 28 Mar '14

28 Mar '14

Hi, Can someone provide me link to some pdf or online tutorial where I can find backtracking / dynamic programming kind of algorithms discussed in details. Regards. Nishant

2 1

Array/Vector like DS in haskell
by Nishant 28 Mar '14

28 Mar '14

Hi, Can someone elaborate on how to create a DS which acts like an array or vector and has same time complexity ? A list though can act like array but getting value at index i is linear time but in actual array it is constant. Regards. Nishant

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typeclass
by Dennis Raddle 28 Mar '14

28 Mar '14

If I write a function like this: f (x1,y1) (x2,y2) = (x1++x2,y1++y2) My understanding of typeclasses is just developing, but I notice this is a bit like "lifting" (++) into a tuple. Maybe there is some existing typeclass and a 2-tuple is already an instance of it? Like arrow? D

2 1

random monad
by Dennis Raddle 28 Mar '14

28 Mar '14

I'm writing a program which uses a lot of pseudorandom numbers, and for that reason it would be nice to put the StdGen in a state monad. like let's say I want to combine error handling and storing the StdGen state. import Control.Monad.Error import Control.Monad.State data RandState = RandState StdGen -- Er is a monad that combines error handling and pseudorandom state type Er a = ErrorT String (State RandState) a -- to access random numbers, I could define things like erRandomR :: Random r => (r,r) -> Er r erRandomR (lo, hi) = do RandState g <- get let (value, g') = randomR (lo, hi) g put $ RandState g' return value erRandoms :: Random r => Er [r] erRandoms = do RandState g <- get let (g1, g2) = split g let values = randoms g1 put $ RandState g2 return values erRandomRs :: Random r => (r,r) -> Er [r] erRandomRs (lo,hi) = do RandState g <- get let (g1, g2) = split g let values = randomRs (lo,hi) g1 put $ RandState g2 return values -- I could define new ways of using random values, like choosing a random element of a list erChooseList :: [a] -> Er a erChooseList xs = do let l = length xs when (l==0) (throwError "in randomChooseList, passed null list") idx <- erRandomR (0,l-1) return $ xs !! idx However, after I got done with that, I realized that I wanted to add additional state, maybe a ReaderT , stuff like that--different in different parts of the program. But I always want access to random numbers with the same functions: erRandomR, erRandoms, etc. So I thought class Monad m => RandMonad m where putGen :: StdGen -> m () getGen :: m StdGen Then I could make Er an instance of RandMonad, like this type Er a = ErrorT String (State RandState) a instance RandMonad Er where putGen g = put (RandState g) getGen = do RandState g <- get return g Clearly I don't know what I'm doing, because when I tried to run this much I got the error "Type synonym Er should have 1 argument, but has been given none." I tried a bunch of variations of this but got nowhere. Can someone explain how I should conceive of this?

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