Compile-time match expression

[refrogerator]

We currently only have compile-time if statetemts to check for a type in situations like these:

       $if field.typ is string {
            // $(string_expr) produces an identifier
            result.$(field.name) = get_string(data, field.name)
        } $else $if field.typ is int {
            result.$(field.name) = get_int(data, field.name)
        }

A compile-time match expression is way better suited for this and more readable imho.

[ntrel]

I think compile-time match might be quite a burden on the compiler, when it does the same thing as $if. Consider that the condition for $match can be a type, or it can be a compile-time value - that's two fundamentally different forms. Then match has various syntax for patterns, it all gets quite complex.

BTW if we implement #8543 and generalise $(string_expr) -> identifier, your example could be 3 lines, even handling more types:

$if field.Type in string | int | rune | bool {
    result.$(field.name) = $('get_' + field.Type.name)(data, field.name)
}

[medvednikov]

I agree with @ntrel

I feel like having just compile time $ifs will make the language simpler.

[JalonSolov]

Simpler, but a lot more verbose if you have a complex if/else if/else if/else if/else if/...

Just think of how "messy" a large if/else block looks compared to a switch/case block.

[Cons-Cat]

I personally feel like it's "simple" for any runtime expression to be easily factored into a comptime expression assuming that the values it operates on are all compile-time known.

If $if, $else, $for, and $match are all valid, then the rule I need to memorize is "expressions can be made compile-time by prefixing $".

If $match is not valid, then I need to memorize that match is an exception to the above rule, so the rules of V are more complex.