Filtering Nothing and unpack Just

Question

I\'m having trouble with this program.

filterJust :: [Maybe a] -> [a]

filterJust [] = []
filterJust x = map fromJust (filter (isJust) x)

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Answer 1

• /= can only be used on values of a type that implements Eq ((/=) :: (Eq a) -> a -> a -> Bool).
• Maybe a supports Eq only if a does (there's an instance (Eq a) => Eq (Maybe a)).
• Your type signature says that filterJust works for all types a, even those that don't implement Eq: [Maybe a] -> [a]

Therefore filterJust can't use /=.