🚀 go-pugleaf

ojuarez@mtecv2.mty.itesm.mx (Octavio Juarez Espinosa) writes:

>I am very interested in to receive some version for funtion middle.
>
>(middle list)  return nil if is even number of elements.
>		return the element if is odd the number of elements
>
>Constrains:
>Only one recursion call and only with the CRD of argument.

I don't quite understand why you want to return "nil" when the
list has an even number of elements. If you are applying the
function to a list of numbers, then "nil" is not a number, so
it won't work for strongly typed languages; if you are applying
the function to a list of lists, then you can't tell whether
the function worked or not. Much better to return either a tagged
value (Ok val | Error message) or to raise an exception for even
length lists. The following code completely ignores the issue for
simplicity.

NB. all these functions are "primitive recursive" in the original poster's
sense.

tail' [] = []
tail' (x:xs) = xs

middle_odd [] ys = tail' ys
middle_odd (x:xs) ys = middle_odd (tail' xs) (tail' ys)

middle l = middle_odd ([] : l) l

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