PR Stanley wrote:
Hi
> The same should apply to head and tail. head or tail of [] should
be [].
>
> What does the list think?
Disagree, strongly. Its not even possible for head, since [a] -> a.
Wadler's theorems for free states that if head is given an empty list
the _only_ thing it can do is crash.
What's the logic behind this?
It's easy enough to define tail [] = [], but you can't write head [] =
[] since [] is a *list* and head doesn't return a list. It returns a
list element. And [] has no elements, so... what are you going to return?
Notice that tail [] = error, yet drop 1 [] = [], which is interesting...
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