Why are these two Haskell "unfold" functions different?

Question

I'm learning Haskell and I'm doing now an exercise with the Maybe Class. I have to create a function which applies f("Maybe function") to a(and its following results) repeatedly until f a returns Nothing. For example f a0 = Just a1,f a1= Just a2,...,f an = Nothing. Then

unfold f a0 = [a0,a1,...,an]

I've tried to do it and and I've got:

unfold :: (a- > Maybe a) -> a -> [a]
unfold f a = case f a of
                 Just n -> n: unfold f a
                 Nothing -> []

The problem is that the solution is:

unfold' :: ( a -> Maybe a) -> a -> [a]
unfold' f a = a : rest ( f a )
     where rest Nothing = []
           rest ( Just x ) = unfold' f x

And my program doesn't work like the solution. Maybe I'm using wrong "case of" but I'm not sure.

Answer 1

Your use of case is fine, but have a look where you cons the new value on the list, and where the solution does.

testFunc = const Nothing

unfold  testFunc 1 == []  -- your version prepends only if f a isn't Nothing
unfold' testFunc 1 == [1] -- the solution _always_ prepends the current value

Also, you're using the same value all the time.

unfold :: (a -> Maybe a) ->a -> [a]
unfold f a = a : case f a of -- cons before the case
    Just n  -> unfold f n    -- use n as parameter for f
    Nothing -> []