- Beginners - Haskell.org

Rewriting using State and/or Reader?
by Paul Sargent 13 Nov '10

13 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 13 Nov '10

13 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 12 Nov '10

12 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 12 Nov '10

12 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 11 Nov '10

11 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 11 Nov '10

11 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 11 Nov '10

11 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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How to select items from one list depending on another
by C K Kashyap 11 Nov '10

11 Nov '10

Hi, I have this problem at hand - Given two lists list1 = [0,1,2,0,1,4] list2 = [1,2,3,4,5,6] I need to take items from the second list only when the corresponding item in list1 is non-zero. list3 = map snd $ filter takeValid $ zip list1 list2 where takeValid (a,b) = a /= 0 Is there a shorter way? -- Regards, Kashyap

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Typeclass Hierarchies on Gtk2Hs
by John Smith 10 Nov '10

10 Nov '10

I'm learning Gtk2Hs, and there is an interesting pattern to the types. Each type of widget has its own typeclass, e.g. Button and ButtonClass, Window and WindowClass. Is this idiomatic Haskell? Are there other ways to achieve such a hierarchy? (OO languages support such inheritance without requiring an interface for every class.)

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Showing control characters
by Russ Abbott 10 Nov '10

10 Nov '10

Is there some way to include control characters in a Show string so that they take effect when the element is Shown? For example, if I define a String "a\nb" and use putStrLn on it, the \n causes a return, but when I simply return the string from a function, the \n is shown as \n. Is there a way to get the \n to execute when it's embedded in a returned value? Thanks * -- Russ Abbott _____________________________________________* * Professor, Computer Science California State University, Los Angeles Google voice: 424-235-5752 (424-cell-rja) blog: http://russabbott.blogspot.com/ vita: http://sites.google.com/site/russabbott/ _____________________________________________*

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