CODEWITHSUNDEEP
haskellrecursionfunctional-programmingpattern-matching
Ali Toto
Ali Toto

Reputation: 65

remove the element from list

I will have to create a recursive function called removeList.Can anyone help me?

Upvotes: 1

Views: 124

Answers (1)

chepner
chepner

Reputation: 531165

makeList is a regular function, not a data constructor, so you can't pattern match on it. You need to pattern match on Cons and Nil; the fact that the list was returned by makeList isn't relevant.

removeList :: Eq a => a -> List a -> List a
removeList _ Nil = Nil
removeList x (Cons y ys) | x == y = ys -- Don't recurse if you only want to remove the *first* occurrence.
                         | otherwise = Cons y (removeList x ys)

You can work directly with your List type; there is no need to build the result as a built-in list then convert it again.

Compare to the same function implemented on built-in lists, with the type constructor [] and the data constructor (:) used in prefix position rather than infix position.

removeList' :: Eq a => a -> [] a -> [] a
removeList' _ [] = []
removeList' x ((:) y ys) | x == y = ys
                         | otherwise = (:) y (removeList' x ys)

Just for fun, let's generalize this code so that it can be used as the basis for both removeFirst and removeAll:

removeGeneric :: Eq a => (List a -> List a) -> a -> List a -> List a
removeGeneric _ _ Nil = Nil
removeGeneric f x (Cons y ys) | x == y = f y ys
                              | otherwise = Cons y (removeGeneric f x ys)

removeFirst = removeGeneric (\y ys -> ys)
removeAll = removeGeneric removeAll

The first argument to removeGeneric is the function you call on the list the first time you find a match. To remove the first occurrence, you just call the equivalent of tail. To remove all occurrences, you recurse.

Upvotes: 3

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