Question

I’ve written a haskell function which splits a list xs into (init xs, last xs) like so:

split xs = split' [] xs
    where
        split' acc (x:[]) = (reverse acc, x)
        split' acc (x:xs) = split' (x:acc) xs

Since an empty list can not be split in this way, there is no match for the empty list. However, I did not want to simply error ... the function. Thus I defined the following:

split [] = ([], undefined)

Thanks to lazy evaluation I can thus define a safe init which simply returns the empty list for the empty list:

init' = fst . split

Is there some way how I could detect the undefined if I tried to access it, such that

last' xs
  | isUndefined (snd xs) = ...
  | otherwise            = ...

I do know about Maybe and Either, and that those are a better choice for expressing what I want. However I wondered if there is a way to detect an actual value of undefined, i.e. in terms of catching errors, like catching exceptions.

Answer 1

Because bottom subsumes non-termination, the function isUndefined would have to solve the halting problem and thus cannot exist.

But note that even if it existed, you still could not tell if the undefined value in the 2nd element of your tuple was put there through your split function or if the last element of the list was already undefined.