haskelltype-constructor

How to convert my own Data constructor to List in Haskell

I have just started trying Haskell after using Python, and I'm facing a lot of problems understanding it

for example after I tried to make a new type called ListBag and writing a simple function to convert list to ListBag I can't go back and do anything with that data:

data ListBag a = LB [(a,Int)] deriving (Show,Eq)

fromList :: (Ord a) => [a] ->ListBag a
fromList xs = LB [ y| y<- ys]
            where ys = toList(fromListWith (+) [(x, 1) | x <- xs])

a :: [Integer]
a=[1,1,1,1,1,1,2,2,2,3,3,3,3,4,5,5]
b = fromList a

Now b is:

LB [(1,6),(2,3),(3,4),(4,1),(5,2)]

and because b is b :: ListBag Integer it can't be mapped or ... So how can I convert it to List again?

Solution

  • Firstly, your ListBag doesn't save order in which elements was in initial list, but... you can convert ListBag a to [a] like so:

    toList :: ListBag a -> [a]
toList (LB ys) = concat $ (\(y, n) -> replicate n y) <$> ys

    And you can redefine your fromList as

    fromList xs = LB $ toList $ fromListWith (+) [ (x, 1) | x <- xs]

    Because [ y | y <- ys] ~ ys and x where x = y ~ y