How to return last element in the list using function in haskell?

问题

My professor gave me an example to get last element in the list using "laste" function: he stated that: definition in the form of “laste xs = …” is not acceptable, whereas definition in the form of “laste = …” is acceptable.

I have tried something like this: Please correct me if my solution is wrong according to problem statement.

laste :: [a] -> Maybe a 
laste [] = Nothing 
laste (x:[]) = Just x 
laste (x:xs) = laste xs

But this gives me answer for example:

ghci>laste[1,2,3,4]
Just 4

I want to get rid of this "Just".

Is there any solution to remove Just?

回答1:

You would need to change the signature of the function to return a simple element.

The thing is that you would need to return an error in case of empty list.

laste :: [a] -> a
laste []     = error "Can't handle empty lists." -- or some other error message
laste [x]    = x
laste (x:xs) = laste xs

回答2:

While Charmini2's answer is functionally correct, it doesn't solve the problem of retrieving the last element in pointfree form. Consider

laste :: [a] -> a
laste = foldr1 (\_ a -> a)

It works according to specs as foldr1 expects a non-empty list. Intuition for why it returns the last element in the list can be gotten from the observation that foldr1 replaces every (:) in the structure of the list with the lambda in the above equation, which basically selects the rightmost of two elements. Repeat, and you get the last.

回答3:

I think your professor meant was that you need to re-implement the Prelude function last in a point-free style.

non point-free example:
filterEven xs = filter even xs

point-free     exapmle:
filterEven    = filter even

point-free examples of last:
lastv1 = (head . reverse)
lastv2 = foldl1 (\acc x -> x)
lastv3 = foldr1 (\x acc -> acc)
lastv4 = \(x:xs) -> if null xs then x else lastv4 xs
lastv5 = \e -> case e of
    [x] -> x
    (_:xs) -> lastv5 xs
    otherwise -> error "empty list"

回答4:

Here a possible working solution:

last' :: [a]    -> a
last'    []     =  error "empty"
last'    (x:[]) =  x
last'    (x:xs) =  last' xs

来源：https://stackoverflow.com/questions/19746639/how-to-return-last-element-in-the-list-using-function-in-haskell

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haskell