[Haskell-beginners] Are these soloutions all valid and a good use of Haskell

[Kees Bleijenberg]

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 <r.wobben at home.nl> 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’ 




_______________________________________________
Beginners mailing list
Beginners at haskell.org
http://www.haskell.org/mailman/listinfo/beginners




_______________________________________________
Beginners mailing list
Beginners at haskell.org
http://www.haskell.org/mailman/listinfo/beginners