CODEWITHSUNDEEP
haskell
zaig
zaig

Reputation: 421

Haskell foldr and foldl

The code below is used to generate the intersection of two lists:

unionSet :: Eq a => [a] -> [a] -> [a]
unionSet a b = foldl (\acc x -> if elem x acc then acc else acc ++ [x]) a b

Why does the foldl function work but when I use foldr it generates errors?

Upvotes: 1

Views: 242

Answers (1)

Guvante
Guvante

Reputation: 19221

foldr has the type

(a -> b -> b) -> [a] -> b -> b

while foldl has the type 

(b -> a -> b) -> [a] -> b -> b

Note the order of the two parameters. 

(\x acc -> ...

would fix the error.

Upvotes: 6

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