🚀 go-pugleaf

In article <6006@mtecv2.mty.itesm.mx>, 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

In article <9224705.24159@mulga.cs.mu.OZ.AU>, fjh@munta.cs.mu.OZ.AU (Fergus James HENDERSON) writes:
...
|> 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
|> 

Sorry, but I tried out this function and it is not correct.
My attempt is (I return a singleton list on success):

middle l = even l l

even halfSpeed []     = []
even halfSpeed (x:xs) = odd halfSpeed xs

odd (midValue:halfSpeed) []     = [midValue]
odd (void    :halfSpeed) (x:xs) = even halfSpeed xs

Test:
Miranda [middle [1..n]|n<-[0..20]]
[[],[1],[],[2],[],[3],[],[4],[],[5],[],[6],[],[7],[],[8],[],[9],[],[10],[]]
Miranda ^D

By the way, I did not understand the constraint completely,
what means CRD?  Do you mean CDR, or as we LISP-haters say: tail?
You have to know the length of the list before you can decide what
element to return.

Mark Ramaer