January 2016 - Beginners

haskell platform
by DJ 13 Jan '16

13 Jan '16

Getting back to Haskell after giving up three years ago. I am running Linux Mint 17.1. Why is the Haskell Platform for linux (or my distro anyway) stuck at 2013? I see that ghc is several versions newer than what comes with Haskell Platform. That seems like a rather severe lag. Why is that? Does it matter? Best, - Jake -

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Typeclass question
by Alan Buxton 13 Jan '16

13 Jan '16

What am I doing wrong in this admittedly contrived example? The code below will compile. It works as expected, unless I try to do "toMyParam Nothing". See below: λ: let arr = [P1 3.0, P2 'x'] λ: toMyParam False P2 'F' λ: toMyParam (Just 'x') P2 'x' λ: toMyParam Nothing <interactive>:38:1: No instance for (ToMyParam a0) arising from a use of `toMyParam' The type variable `a0' is ambiguous Code below: data MyParam = P1 Double | P2 Char deriving Show class ToMyParam a where toMyParam :: a -> MyParam instance ToMyParam Bool where toMyParam False = P2 'F' toMyParam True = P2 'T' instance ToMyParam Char where toMyParam = P2 instance ToMyParam Double where toMyParam = P1 instance ToMyParam a => ToMyParam (Maybe a) where toMyParam Nothing = P1 0.0 toMyParam (Just x) = toMyParam x

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Profiling tiny executions in ghc
by pmcilroy＠gmail.com 12 Jan '16

12 Jan '16

Is there a way to count numbers of calls, allocations or constructions to get an estimate of program performance in ghc? I am curious about the performance of the various implementations of blowup in previous discussions. Timing and profile sampling is not convenient.

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questions on evaluation.
by Peter McIlroy 12 Jan '16

12 Jan '16

Fabian, For your problem of bang -> baanngggg You can derive Rein's hairy idiom from a more straightforward technique for getting a recursive definition: First get a basic implementation using ++ and recursion: * add a helper function with any "state variables" as arguments. * Ignore concatenation while thinking about a problem. * Use pattern matching for recursion step. This gives the basic (naïve) implementation: blowup :: [a] -> [a] blowup x = concat (blowup' 1 x) -- returns (e.g.) ["b", "aa", "nnn", "gggg" ] blowup' a b :: Integer a => a -> [b] -> [[b]] blowup' n (x:xs) = (replicate n x : blowup' n+1 xs) blowup' _ _ = [] -- base case You can distribute concatenation into blowup', to get recursion over concatenation, but this really is going the wrong direction: blowup' n (x:xs) = (replicate n x) ++ (blowup' n+1 xs) blowup' _ _ = [] Instead, derive Rein's hairy-looking version by (as always) ignoring concatenation and replacing the state variable in blowup' with zip or zipWith. observe: > zip [1..] "bang" [(1,'b'),(2,'a'),(3,'n'),(4,'g')] So blowup' n x becomes blowup' x: blowup x = concat (blowup' x) blowup' x = map (\(a,b) replicate a b) zip [1..] x ==> definition of zipWith blowup' x = zipWith replicate [1..] x ==> blowup x = concat (zipWith replicate [1..] x) You can always push concat down into a function by unwrapping it with join, getting a more efficient version that creates the result directly. blowup x = concat (zipWith replicate [1..] x) ==> blowup x = (join . zipWith replicate [1..] ) x ==> blowup = join . zipWith replicate [1..]

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question on evaluation
by Fabien R 11 Jan '16

11 Jan '16

Hello, I want to define a function blowup that takes "bang" as input and returns "baannngggg". I come up with these functions; myReverse :: [a] -> [a] myReverse [] = [] myReverse (x:xs) = myReverse xs ++ [x] buildLenList :: String -> [Int] buildLenList "" = [] buildLenList (_:xs) = [1 + length xs ] ++ buildLenList xs myRepeat :: Char -> Int -> String myRepeat x 0 = [] myRepeat x n = [x] ++ myRepeat x (n - 1) blowup :: String -> String blowup [] = [] blowup (x:xs) = myRepeat x (head ( (buildLenList (x:xs)))) ++ blowup xs With this code, blowup "bang" returns "bbbbaaanng". So I thought to insert myReverse between head and buildLenList but in that case, the result is only "bang". It seems that the evaluation of buildLenList is not working as I thought. I tried to debug that using ghci debugger but failed (still learning). Can someone explain how the evaluation is done here ? -- Fabien

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error while setting it up
by Abhijit Patel 08 Jan '16

08 Jan '16

for the below command I am getting errror !! please suggest me a solution $make ===--- building phase 0 make --no-print-directory -f ghc.mk phase=0 phase_0_builds ghc.mk:159: *** dyn is not in $(GhcLibWays), but $(DYNAMIC_GHC_PROGRAMS) is YES. Stop. make: *** [all] Error 2

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stack issues
by frantisek kocun 08 Jan '16

08 Jan '16

Hi, I'm new to cabal/stack. I want to use "stack build" to build my project. I can not use latest gkt >= 0.14.2 because of setup: The pkg-config package 'cairo' version >=1.2.0 is required but it could Neither can I now use older version gtk >=0.12.0 && < 0.13 While constructing the BuildPlan the following exceptions were encountered: -- Failure when adding dependencies: gtk: needed (>=0.12.0 && <0.13), 0.14.2 found (latest version available) needed for package: ships-0.1.0.0 What can i do? Thanks

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wxHaskell: add an external event from another thread
by Puck 08 Jan '16

08 Jan '16

Hello all, how is it possible to process external events, for example a succeeded getLine or a takeMVar with its values in the wxHaskell framework? The reason is, that I want to connect a erlang node to wxHaskell. I have found the functions Graphics.UI.WX.Events.newEvent :: String -> (w -> IO a) -> (w -> a -> IO ()) -> Event w a "Create a new event from a get and set function." Beside from, that I don't know what the get and set function shall do, I don't know, how I can add the new event to the event-loop of wx, and when it "fires". Do you know any possibilities? Thank you in advance

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Parametrizing [] as an instance of the Functor type class
by Olumide 08 Jan '16

08 Jan '16

According to LYH, the list is an instance of the Functor type class: instance Functor [] where fmap = map Why does 'f' not appear in this expression?, considering that Functor is defined as class Functor f where fmap :: (a -> b) -> f a -> f b Overall, I'm a bit confused about the relationship between the type constructor f and []. Thanks, - Olumide

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Pattern Matching for record syntax
by Jeon-Young Kang 07 Jan '16

07 Jan '16

Dear All. Hope your 2016 is off to a great start. I would like to get results from pattern matching or something. Here is the my code. data Person = Person {name :: String, age :: Int} names = ["tom", "sara"] -- list of names, String persons = [Person {name = "tom", age = 10}, Person {name="sara", age=9}, Person {name = "susan", age = 8}]. Is there any solution to get the age of "tom" and "sara"? I have no idea of pattern matching for this one. I've tried to use recursion, but I couldn't find any solution for list of records. Sincerely, Young

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