1.8 KiB
Format Specification for String Interpolation
Summary
This feature allows us to give format specifiers when printing interpolated strings such as .2f for a float rounded to two decimal places or x for printing in hex. It has all of the format specifiers available in string.format. It is delimited by a , in interpolated strings.
Motivation
Most languages allow for format specifiers such as python f-strings or printf in C. Lua also supports string.format but we want to make it easier to use with interpolated strings.
Design
Under the hood, this will just be syntactic sugar in addition to the current implementation of interpolated strings. Before, interpolated strings were passed to string.format with no format was specified, but now we will also pass the optional format specifier. We decided on , to act as the delimiter for format specification.
To give some examples, this is how it would look like in code:
balance = 100.2035
print(`You have ${balance,.2f} in your bank account`)
You have $100.20 in your bank account
number = 12345
print(`12345 is 0x{number,x} in hex!`)
12345 is 0x3039 in hex!
This will support most additions that could be made to string.format in the future as well.
Drawbacks
There are no clear drawbacks to this.
Alternatives
We have also considered allowing arbitrary format specifiers and not just ones supported by string.format. We would allow __tostring to take a second format argument, and that specifier get passed into it. We determined that we won't do this for now because it could mess up backwards compatibility on existing __tostring calls and a performance regression since we can't assume tostring will only have one argument anymore. Also for specifiers that work with string.format will be slower than just using string.format