Oke, But what If I try it on a left folt. Then it must be the other way. let's say we have [1 , 2, 3] so first it's nothing and 1 is taken. so y could be one and nothuing must stay nothing because it's not the last part, The same for 2 , When it's three nothing must changed to just 3. Fold is something I find it very difficult to wrap my head around it. Maybe haskell or any functional proggramming language is not for me, Roelof Kees Bleijenberg schreef op 11-11-2014 15:46:

The program traverses the list from right to left. The first element that acc sees during traversal is the last element of the list. The idea is as follows: Parameter y is the partial calcresult. If y (in your code) is Nothing then acc returns Just x. So the new partial result is the current list element in a Just. The next time acc is called, y is not Nothing but a Just. In that case acc returns the partial calcresult unchanged. So, y is Nothing for the last list element and Just <last element of the list> for all other list elements.

last5 :: [a] -> Maybe a last5 xs = foldr acc initial xs

acc :: a -> Maybe a -> Maybe a acc x Nothing = Just x acc _ aJust = aJust

initial :: Maybe a initial = Nothing

In your code you don’t need the ‘where’ in the acc definition. Define acc for Nothing and for Just.

Kees

--------------- Im now trying to make a rfold solution to this problem.

Till now I have this :

last5 :: [a] -> Maybe a last5 xs = foldr acc initial xs

acc :: a -> Maybe a -> Maybe a acc x y = where acc x Nothing = Just x acc _ j = j

initial :: Maybe a initial = Nothing

So still not working.

Can someone explain to me in small steps how to figure out what the right answer is to the acc part.

Roelof

Roelof Wobben schreef op 11-11-2014 8:20: Thanks, this is working well.

But just for understanding

in the part acc x Nothing x is the input string, and in the part _j means the same as (xs:x)

Roelof

Alex Hammel schreef op 10-11-2014 22:47: It's an indentation issue. Make sure that there are no spaces before the bit about 'last5 = foldr acc' etc.

On Mon, Nov 10, 2014 at 12:48 PM, Roelof Wobben wrote: Roelof Wobben schreef op 10-11-2014 20:59: last5 :: [a] -> Maybe a last5 = foldr acc Nothing where acc x Nothing = Just x acc _ j = j

When I enter this in GHCI I see this error :

Illegal type signature: ‘[a] -> Maybe a last5’

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