Advertisements Pattern-matching is based on Eq instance, right? I see that Data.Set.Set implements Eq and >S.fromList [] == S.fromList [1,2] False >S.fromList [1,2] == S.fromList [2,1] True which is right. Then why does case S.fromList [1,2] of mempty -> True return true? (actually it always evaluates to true). I mean such matching is wrong and… Read More Case on a set ignores equality/inequality

Advertisements The function from a -> m b to m (a -> b) rarely appears in programming, but can be made in the Reader monad. The following code is a tentative implementation. Does such a library exist? class Monad m => MonadShift m where shift :: (a -> m b) -> m (a -> b)… Read More Is there any typeclass that defines the function from `a -> m b` to `m (a -> b)`?

Advertisements I try to make the below Haskell code equal to the Python code, but it doesn’t work and i can’t spot the error? Can somebody spot the error? The Python output is what i want def balanceList(lst): length = len(lst) if length<3: return lst else: middle=length//2 return [lst[middle]] + balance(lst[middle+1:] + balance(lst[:middle])) print( balanceList(list(range(1,10)))… Read More What is the equivalent of this Python code in Hasekell?

Advertisements I newbie in Haskell, when i run scanl function in haskell with these parameters it works fine: ghci> scanl (flip(:))[] [3,2,1] [[],[3],[2,3],[1,2,3]] but if i do the same but using the scanr function i get this error: ghci> scanr (flip(:))[] [3,2,1] <interactive>:129:8: error: • Couldn’t match type ‘a’ with ‘[a]’ Expected: [[a]] -> [a]… Read More question about using scanr function in haskell

Advertisements I have a Haskell file called Types.hs that contains this definition, among others: module Types ( KnapsackItem ) where data KnapsackItem = KnapsackItem { weight :: Int, cost :: Int } deriving (Show, Eq) Then there’s my Main.hs (also shortened here for simplicity): import Types evalTotal :: [KnapsackItem] -> [KnapsackItem] -> (Int, Int, String)… Read More Data type field name from module I defined is "not in scope"

Advertisements there. Relative Haskell newbie here. I have a big bunch of data types and functions on two type variables that require that the types are Eq, Ord, and Show. I found myself writing type Foo n t = (Eq n, Eq t, Ord n, Ord t, Show n, Show t) => Bar n t… Read More In Haskell, is there a way to define a type constructor that includes a bunch of type constraints?

Advertisements trying to understand Continuation monad examples implemented in Haskell. Question: where does fn in 1st line of goto on the right – in out fn part comes from? Is it some omitted argument or some destructuring of out [same with out (fn, num) in gotoC]? Imagine you explain this for a mainstream language coder… Read More Question on Haskell syntax. Continuation monad example

Advertisements I see myself using this pattern a lot: f x = do — predicate :: a -> IO Bool b <- predicate x if b then rest () else return () where rest () = do — rest of my IO operations is there a pattern other people use to remove the if then… Read More How to best handle IO Bool logic?

Advertisements I created a custom List type and I’m trying to implement the zip function for it. But I cant figure it out, it always throw an error on the last line. data List a = Empty | Cons a (List a) deriving (Eq, Ord, Show, Read) listZip :: List a -> List a ->… Read More How to access elements of a custom List data type?

Advertisements class Collection c where empty :: c key value singleton :: key -> value -> c key value insert :: Ord key => key -> value -> c key value -> c key value lookup :: Ord key => key -> c key value -> Maybe value delete :: Ord key => key ->… Read More Don't understand this collection class syntax