support formatting durations with format string character rather than a special type

[andrewrk]

#7297 introduced a handy way to print time durations:

test "example" {
 std.debug.print("{}", .{std.fmt.duration(1234)}); 
}

This proposal is to delete std.fmt.Duration and std.fmt.duration and make it format character(s) such as {dt}:

test "example" {
 std.debug.print("{dt}", .{1234}); 
}

[tecanec]

I guess the question here is how we prefer to describe the format of our stringified values. Doing it in the format string may feel more natural; It's where we define the overall shape of our output, so it makes sense to describe the format of the individual values there as well.

However, there are also advantages to describing the format along with the values. It's more flexible to define it there, since fmt.format doesn't have to know every way to format every type. It also means that one can't use parameters for formatting one type when they're actually using another, and it's a bit more flexible at run-time.

There should be a line between where we use one solution and where we use the other. Your current strategy seems to lean towards stuffing the format string with functionality. I personally think that we should lean more towards using things like std.fmt.duration for all type-specific formatting options. We need to support usage of such types, anyway, so it aligns better with your ambition of "only one way to do things". It also has the aforementioned flexibility, and it'll be more consistent with user-defined formatting types. That said, we shouldn't completely remove all parameters from the formatting strings, either, as it's the only place where we can index the values. I also think that parameters like width and alignment would make more sense to have as part of the format string.

[Rocknest]

I dont like a million different format characters, i find it easier to remember a function name than a few characters (how fast can you remember meaning of this: printf("%hhu",).

Edit: function name in the format string?

test "example" {
 std.debug.print("{'std.fmt.duration'}", .{1234}); 
}

[tecanec]

Putting the function name in the format string seems problematic. It means that the function will be referenced by fmt.format rather than the caller, so the name would be relative to the callee's scope, leading to all sorts of problems. Adding the means for fmt.format to access the caller's scope directly is also an arguably bad idea.

[jayschwa]

I can see why this might be appealing because std.fmt.Duration seems very simple right now, but in the future, a full-featured point-in-time and duration API is likely to have custom types with many methods and formatting options (e.g. "3d1m1s" vs "3 days, 1 minute, 1 second" vs "~3 days"). I think the status quo of std.fmt.format only dealing with language-level concepts and compartmentalizing higher-level concepts into types with a custom format method is going to be better in the long run.

[Mouvedia]

Would supporting Epoch/POSIX time warranted?

[andrewrk]

done in #24329