Chapel: Support first/last methods on enums

[ronawho]

I was writing some code using an enum for timing different stages of code. Something like:

enum TimeStages {A, B, C}
var enumArr: [TimeStages.A..TimeStages.C] real;

Every time I added a new timing stage, I found it a little annoying to update the enum, and the array declaration. I found myself wondering if we could support first()/last() method like we have for ranges/domains.

That way you could have something like:

enum TimeStages {A, B, C}
var enumArr: [TimeStages.first()..TimeStages.last()] real;

I'm open to other ways of writing this sort of pattern, this is just what came to mind first.

[ronawho]

Huh, there's apparently a chpl_enum_first function. That's not the interface we'd like, but it may be a starting point for the compiler implementation.

[cassella]

I think

var enumArr: [TimeStages] real;

or

var enumArr: [{TimeStages}] real;

would be elegant ways to write it.

[bradcray]

it may be a starting point for the compiler implementation.

This will be really easy to implement. If it'd benefit you to have it soon, let me know.

[ronawho]

I'm in no rush for this. I stumbled across chpl_enum_first while looking at something else and I was surprised to see that it was a thing so I wanted to note it here.

[bradcray]

When I said "this will be really easy", I meant "it should be a really easy compiler change." Turns out it's even easier than I was guessing (such that even a performance-tuning programmer could've pulled it of... :D ). See PR #15098

Paul, I feel sortof intrigued by and sortof scared by your suggestion, so did not tackle it here, and am not enough of a champion to fork it off into its own feature request issue myself. But I wouldn't have any qualms if you were to do so.

[BryantLam]

The following code uses the .first and .last keyword identifiers introduced by #15098 as elements in the enum itself. Is this usage intended?

enum E {
    a,
    first,
    b,
    last,
    c,
}

for e in E {
    writeln(e);
}

writeln("----");

for e in E.first..E.last {
    writeln(e);
}

# chpl version 1.23.0 pre-release (8260f8962)

a
first
b
last
c
----
first
b
last

While arguably correct, it could deceive a reader of the code if they don't know the definition of the enum.

[bradcray]

I definitely didn't intend to prevent someone from writing this, in the sense that I didn't want to make the new methods prohibit the use of any specific identifiers within the enum. Here's a similar, pre-existing example that's maybe slightly more worrisome since it doesn't have an obvious workaround:

enum color {red, green, size, blue};

writeln(color.size);