Number literal syntax - allow underscores or other way to make number more readable

[rakudrama]

This is likely a 2.x feature.

I would like to be able to use underscores inside number literals to break up long runs of digits.
For example 1000000000 is harder to understand than 1_000_000_000.

Today I saw x & 0x3FFFFFF with a comment saying the result fits in 30 bits.
If you count the Fs, you will see there is an error, but the error has gone unnoticed for several years.
If underscores were permitted, the following difference might be more apparent to the developer or reviewer:

x & 0x_3FF_FFFF     vs     x & 0x_3FFF_FFFF

I would want the exact placement of underscores not be prescribed, the same literal might warrant different structure depending on context. In some byte-oriented code 0x3F_FF_FF_FF might make more sense.

This feature would also make binary literals (0b_1111_1111 = 255) more useful.

[zoechi]

See also #2995

[lrhn]

That feature is on my (rather long) list of conveniences I'd like too.

My design would only allow underscores between digits, so 0x3FFF_FFFF, not 0x_3FFF_FFFF, but apart from that I agree.

[rakudrama]

I found another example of the initial problem of a wrong literal: fc67a19

[lrhn]

I reasonably often see code like 100 * 1000 * 1000 to dodge the issue. That could just have been 100_000_000.

[srawlins]

I think we can close this in favor of dart-lang/language#2 in the new language repo; 20 upvotes today.

[lrhn]

Now that you have linked the two, I agree :)