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Forum => Anything else we might add? => Topic started by: ssqq on September 14, 2016, 06:23:16 PM

Title: first or last empty list should return nil
Post by: ssqq on September 14, 2016, 06:23:16 PM
When `rest` or `chop` or `filter` a list, empty list would be create.

But many function that process list would throw error with empty list (first last rest nth).

So newlisper need treate empty list as special list to avoid code crash.



If index empty list return nil, then would simplify coding processing list.


Code Select

(set '@lst (process @lst))
(cond
   ((symbol? (first @lst) (dosth ..))
   (true ..))
Title: Re: first or last empty list should return nil
Post by: rrq on September 15, 2016, 06:15:07 PM
My habit for those cases is to use an or clause, as in
Code Select
(first (or @lst '(-1))) Thus, I use a second or term to provide an appropriate sentinel construct to make the access provide whatever value I want the empty list to result in. And sometimes it works better with an if clause, as in
Code Select
(if @lst (first @lst) -1) It all depends on which access is used, and how "serious" an empty list case is.
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