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This exact thing is one feature I like about Perl 6 and (minorly) dislike about Rust.

In P6, 1..10 is exactly the range you'd expect, while ^ is used to exclude either end of the range. For example, 1..^11 is the same as 1..10, as is 0^..^11 (can't say I've ever needed that, but I could see a couple of possible uses).

This also carries over into range syntactic sugar. ^10 is shorthand for 0..^10, and as a result ^@list.elems is a list of valid indices of a list.

I personally find Rust's syntax noisier, but it's a small issue.




A downside of 1..10 is, how do you represent an empty range starting from 1? 1..0? That looks so weird... I'd expect it to mean a backward range starting from 1 and going up to 0 inclusive. A nice thing about end-exclusive notation is you can denote backward ranges, forward ranges, and empty ranges with the same notation.


> In P6, 1..10 is exactly the range you'd expect

That syntax is impressive, handles all four possibilities nicely.

However since reading this short article by Dijkstra, I’ve been convinced that 0..N ranges (inclusive start, exclusive end) are the way to go whenever there’s a choice:

http://www.cs.utexas.edu/users/EWD/ewd08xx/EWD831.PDF

There's certainly a mental benefit of having made a choice and being consistent, which I think is possible with any of the options, but I find the argument of having length = end - start compelling.

It does fall down in some cases, eg when looking for a range over all unsigned ints, as the total number of uints isn’t a uint itself (as uints start at zero), so some way to represent a rage that is inclusive of both ends is still needed.


As a minor nit, it may be better to use `@list.keys` to get a Sequence of indices. Especially if you want it to work when `@list` is `lazy`.




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