November 2010 - Beginners

Lazy list Binary instance
by uu1101＠gmail.com 15 Nov '10

15 Nov '10

Dear haskellers, I need to collect data to a file through a long running computation (the population of each generation in an genetic algorithm) and afterwards read the file and compute some statistics in a single pass. I'm using a Writer monad with Data.DList as its Monoid, telling it 'DList.singleton population', at each generation and 'DList.toList . execWriter ...' at the end. Do you think this is the right approach? As the amount of data is very large I'd like to Data.Binary.serialize and Codec.ZLib.compress when writing to the file, and the opposite when reading. As it happens the default list Binary instance does not serve my purpose, as it needs to retain the whole list prior to writing the file. I tried implementing a different Binary instance for the list wrapping it in a newtype, but without success so far. This issue has been discussed before so I tried using Felipe's code [0] and some variations. newtype List a = List [a] split255 :: [a] -> (Word8, [a], [a]) split255 = s 0 where s 255 xs = (255, [], xs) s n (x:xs) = let (n', ys, zs) = s (n+1) xs in (n', x:ys, zs) s n [] = (n, [], []) instance Binary a => Binary (List a) where put (List l) = let (n, xs, ys) = split255 l in do putWord8 n mapM_ put xs when (n == 255) (put $ List ys) get = do n <- getWord8 xs <- replicateM (fromEnum n) get if n == 255 then get >>= \(List ys) -> return (List $ xs ++ ys) else return (List xs) But in this case the Binary.decode needed to read the whole file prior to accessing the first element. Why does this happen? How can I both write and read the file lazily? My guess is that in the line: then get >>= \(List ys) -> return (List $ xs ++ ys) the pattern in the lambda forces get to retrieve the next chunk, which in turn forces the next... until the end is reached. Could this be solved by making it a irrefutable pattern? (I'm sorry, I tried different variations this afternoon and only now that I can't try it this occurs to me). Best regards, Adrian. [0] http://www.mail-archive.com/haskell-cafe@haskell.org/msg30761.html

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Ralf Laemmel's riddle on surviving without the monad transformation library
by C K Kashyap 15 Nov '10

15 Nov '10

Hi, Can someone provide me the solution to the following riddle that Ralf asked in his lecture at http://channel9.msdn.com/Shows/Going+Deep/C9-Lectures-Dr-Ralf-Lmmel-Advance… Riddle: define a custom made monad (only involving (->) and Either String) to survive without the monad transformation library. -- Regards, Kashyap

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(no subject)
by David Schonberger 13 Nov '10

13 Nov '10

http://ungoalperlavita.org/mainlink.html

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Rewriting using State and/or Reader?
by Paul Sargent 12 Nov '10

12 Nov '10

Hi, My Haskell project has quite a lot of code in it which either: a) Uses a set of common values which are stored in an "environment" data type. b) Takes a set of values, and updates them repetitively. Often the same function will do both. Up to now I've tended to avoid monad based code (except List and Maybe), but I'm thinking that maybe I can simplify some of my code. The code for case a) might look something like: data Env = Env {rateA :: Double, valueB :: Double} deriving (Show) f :: Env -> Double -> Double -> Double f env a b = a * ra + b * g' b where ra = rateA env g' = g env g :: Env -> Double -> Double g env b = (valueB env) + b Basically I'm threading the Env parameter through all the function calls that need it. Am I right in saying this sort of stuff is a good candidate for the Reader monad? What do I gain if I do it? Similarly am I right in saying that this type of code is a good candidate for State (called repeatedly from some loop somewhere)? updateList :: Double -> [Item] -> [Item] updateList t xs = map (updateItemToTime t) xs updateItemToTime :: Double -> Item -> Item updateItemToTime = some function which does an incremental calculation for a time slice (obviously the code is nonsense, I'm just trying to give a feel of the structures I'm using) Then, if I did move the code to these monads, how well do the live together? ...and how about functions which use functions in multiple different state or reader monads? Basically, I haven't seen the advantage of using the monadic way for this code, so what am I missing? Thanks Paul

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Why the length function I wrote has such a type signature?
by 贾旭卿 12 Nov '10

12 Nov '10

This is exercise 3.1 of Real World Haskell. I have my length function like this: myLength [] = 0 myLength (_:xs) = 1 + (myLength xs) And I assumed the type signature is like this: mylength :: [a] -> Num But when I wrote this into the file and reloaded it into ghci, there is an error. > The type signature for `mylength' lacks an accompanying binding > Failed, modules loaded: none. > And the type signature given by ghci is > myLength :: (Num t1) => [t] -> t1 > So how can I modify the function to have a type signature like the first one?

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Using symbolic values in the case expression
by C K Kashyap 11 Nov '10

11 Nov '10

Hi, I'd like to do something like this - Instead of doing this - f x = case x of 1 -> "One" 2 -> "Two" I'd like to do this - v1 = 1 v2 = 2 f x = case of v1 -> "One" v2 -> "Two" Is that possible? -- Regards, Kashyap

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a simple problem
by Anonymous W 11 Nov '10

11 Nov '10

Hi, I'm having trouble with this small exercise. A function maybeA takes a list of pairs. A pair has a variable 'a' and a value 'b'. If any first element of a pair equals to any first element of another pair, the function returns Nothing. If they aren't equal, the function returns a tuple containing the first variable, its value and the value of the second variable. maybeA :: [(a,b)] -> Maybe (a,b,b) For example: [(a1,b1),(a2,b2),(a3,b3)..] returns Nothing if a2 = a3 and returns Just (a2,b2,b3) if a2 /= a3 I'm currently have :maybeA [(a1,b1),(a2,b2)] |a1 == a2 = Nothing |a1 /= a2 = Just (x1,y1,y2) but it only takes 2 pairs. Any helps? Thanks

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Can ghc compiler Optimize the several calls to Same function
by nicholas.ulysses 10 Nov '10

10 Nov '10

It's my code to do some recursive things, focus on function 'group'. ---- cow.hs year1 [y1,y2,y3,y4x] = y1 year2 [y1,y2,y3,y4x] = y2 year3 [y1,y2,y3,y4x] = y3 year4x [y1,y2,y3,y4x] = y4x cow_group_sum year = sum (group year) --look here-- group 1 = [1,0,0,0] group (year+1) = [(year4x (group year)), (year1 (group year)), (year2 (group year)), ((year4x (group year)) + (year3 (group year)))] main = print (cow_group_sum 50) ---- end Every time 'group' was called, the (group year) function was called 4 times . when call (group 30), it takes tens seconds to finish. I have tried 'ghc -O3', but it's still 'lazy' on this thing. Map-Reduce seems to be the answer to the question. But I want to make sure that ghc CAN or CANNOT do the Optimize for Dynamic Programming? ---------------- xingbo wu

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case as guards in lambda functions
by Russ Abbott 10 Nov '10

10 Nov '10

I have often wanted to use a guard in a lambda function and had thought it wasn't possible. But apparently the case construct allows a viable approach. Here is a silly example. testCase = map (\xs -> case xs of [] -> "empty list" [y] | y < 5 -> "small singleton" | otherwise -> "large singleton" _ -> "multi-element list") > testCase [[], [2], [7], [1,2,3]] ["empty","small singleton","large singleton","multi-element list"] It seems particularly useful to be able to include both patterns and guards in case expressions. I haven't seen this usage anywhere. Is it considered bad form? * -- Russ *

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IO loops with tail call
by Yasuyuki Ogawa 10 Nov '10

10 Nov '10

Hello! I wrote a simple echo program which end up with input "q": echo = do str <- getLine if (str == "q") then return () else putStrLn str >> echo then I thought it is not tail call because if expressions return a value. (is it right?) So I changed a code: echo "q" = return () echo xs = do putStrLn xs xs' <- getLine echo xs' but Monad connections are too complicated so that I'm not sure it is tail call. Am I wrong? Thank you! -------------------------------------- Get the new Internet Explorer 8 optimized for Yahoo! JAPAN http://pr.mail.yahoo.co.jp/ie8/

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