Destructuring first and last item

[MichalMarsalek]

It's often tempting to write

first, *_, last = collection

to extract the first and the last items from a collection. The intended operation should only fail when the collection is empty, however, when written like this, it requires at least 2 items in the collection. Can Coconut be changed to handle the case where there is exactly one item in which case it should be assigned to both first and last?

[evhub]

No, for a couple of reasons.

First, I think there's good reason to fail the match for the one-element sequence, which is that the actual syntax x, *xs, y (or [x] + xs + [y]) could never be used to construct a one-element tuple/list, so it would be counterintuitive if that syntax as a pattern was able to match a one-element sequence.

Second, since Python added its own match syntax, Coconut has aimed for strict compatibility with Python match syntax everywhere Coconut does pattern-matching, and in Python x, *xs, y can only match two-element sequences and above.

If you'd like a simple match syntax that will match either case, try

(first, *_, last) or (first and last) = collection

which you can even wrap into a view pattern if you want

def first_and_last((first, *_, last) or first and last) = (first, last)

first_and_last -> (first, last) = collection