Add is_first/is_last helpers to Position

[xmo-odoo]

They're just convenience method, but avoid having to dereference individual variants, and may mitigate missing the Only case.

[xmo-odoo]

Originally proposed by bluss (#169 (comment))

Do make the first/last cases clearer and somewhat less error prone e.g.

use itertools::Itertools;

let sentence = "The quick brown fox.";
let words = sentence.split(' ');

for (pos, word) in words.with_position() {
    if let itertools::Position::Last | itertools::Position::Only = pos {
        print!("{}", word);
    }
}

becomes

use itertools::Itertools;

let sentence = "The quick brown fox.";
let words = sentence.split(' ');

for (pos, word) in words.with_position() {
    if pos.is_last() {
        print!("{}", word);
    }
}

[phimuemue]

Hi there, I'm unsure that an is_first that checks for First || Only is a good idea (I naively assumed a shorthand for only First). If we accept this, I suggest renaming to is_first_or_only so that the documentation guides the reader to the fact a single element is first, too.

But then, is_first_or_only only sugar coats the match, and I usually advocate against such thin wrappers.

Thus, I'm reluctant to merge this. @jswrenn Opinions?

[xmo-odoo]

Hi there, I'm unsure that an is_first that checks for First || Only is a good idea (I naively assumed a shorthand for only First).

To me that is the entire point, it's a predicate telling you whether an element is the first / last of the sequence regardless of the sequence's dimensions, so you don't need to know / care about the Only case.

I was reminder of this inconvenience upon stumbling (back) on https://stackoverflow.com/a/76223155/8182118

[jswrenn]

In this case, I think the chainability of the method tips the scales towards the shorthand being compelling. (As opposed to .map(...) shorthands, since the long-form is already chainable.) I agree with @phimuemue that is_first_or_only is worth the few extra keystrokes for the extra clarity.