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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 09 Jan '16

09 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 09 Jan '16

09 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 09 Jan '16

09 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 08 Jan '16

08 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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Functor instance for ordered lists
by martin 05 Jan '16

05 Jan '16

Hello all, Data.List.Ordered is just a bunch of functions operating on ordinary Lists. Fmapping a function over an ordered list has the potential of blowing the ordering. Would it be possible to define a newtype for ordered lists where the order is guaranteed to be maintained? The functor instance then may have to re-order the elements. The problem I see is that data Ordlist a = ... would certainly require an Ord constraint on a, but where would I put it? I can put it on all the functions manipulating OrdLists, but I still wouldn't know how to define a functor instance, because a Functor a does not require Ord a.

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Tree-like Structure with unique elements on each level
by martin 05 Jan '16

05 Jan '16

Hello all, I recently modeled a tree like this: data Predicate a = Any | Pred (S.Set a) data Product a = Pany | Prod (S.Set(Predicate a, S.Set(Product a))) What I wanted to achieve was to have each element only once on each level. Hence the Sets. But this structure does not achieve this. I can easily duplicate elements on a level as long as their subordinates are different, so I might as well give up working with sets. Is there a way to construct a tree where each element can occur only once on each level?

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infinite type
by Julian Rohrhuber 04 Jan '16

04 Jan '16

The function g, when called with a binary function returns a type error which I’d really like to understand: why is this type “infinite" rather than just incomplete or something similar? I would have expected some kind of partial binding. Can someone help me with an explanation? Prelude> let f g = g . g Prelude> let sum x y = x + y Prelude> f sum <interactive>:14:3: Occurs check: cannot construct the infinite type: a ~ a -> a Expected type: (a -> a) -> a -> a Actual type: a -> a -> a Relevant bindings include it :: (a -> a) -> a -> a (bound at <interactive>:14:1) In the first argument of ‘f’, namely ‘sum’ In the expression: f sum With a similar call using lambda expressions, the error is different: Prelude> (\x y -> x + y) . (\x y -> x + y) <interactive>:32:1: Non type-variable argument in the constraint: Num (a -> a) (Use FlexibleContexts to permit this) When checking that ‘it’ has the inferred type it :: forall a. (Num a, Num (a -> a)) => a -> (a -> a) -> a -> a Prelude>

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Why must inflix constructors begin with a colon?
by Olumide 31 Dec '15

31 Dec '15

I'm new to Haskell and wondering why this restriction? For example (from LYH): infixr 5 :-: data List a = Empty | a :-: (List a) deriving (Show, Read, Eq, Ord) Thanks, - Olumide

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