Coming from D.learn where someone asked for some automatism to turn runtime format strings to `format()` into the equivalent `format!()` form automatically to benefit from compile-time type checks I started wondering...

The OP wasn't looking for other benefits of the template version other than argument checking and didn't consider the downsides either. So maybe there is room for improvement using runtime arguments.

So let's add some features:
1) compile-time "in" contract, run on the argument list
2) functionality to promote runtime arguments to compile-time


string format(string fmt)
in(ctfe) {
  // Test if argument 'fmt' is based off a compile-time
  // readable literal/enum/immutable
  static if (__traits(isCtfeConvertible, fmt))
  {
    // Perform the actual promotion
    enum ctfeFmt = __traits(ctfeConvert, fmt);
    static assert(ctfeFmt == "%s", "fmt string is not '%s'");
  }
}
body
{
  return "...";
}


Note that this idea is based on existing technology in the front-end. Compare how an alias can stand in for a CT or RT argument at the same time:


void main()
{
	const fmt1 = "%x";
	auto fmt2 = "%s";
	aliasTest!fmt1;
	aliasTest!fmt2;
}

void aliasTest(alias fmt)()
{
	import std.stdio;
	static if (__traits(compiles, {enum ctfeFmt = fmt;}))
		// "Promotion" to compile time value
		enum output = "'fmt' is '" ~ fmt ~ "' at compile-time";
	else
		string output = "'fmt' is '" ~ fmt ~ "' at runtime";
	writeln(output);
}


This prints:
'fmt' is '%x' at compile-time
'fmt' is '%s' at runtime

For technical reasons a compile-time "in" contract can not work in nested functions so all the CTFE contracts need to be on the top level, user facing code. That means in practice when there are several formatting functions, they'd extract the implementation of the compile-time contract into separate functions. I have no idea how exactly that scales as the `static if (__traits(isCtfeConvertible, …))` stuff has to remain in the contract. (It's probably ok.)

Extending the CTFE promotion to any variables that can be const-folded is not part of this idea as it leaves a lot of fuzzyness in language specification documents and results in code that produces errors in one compiler, but not in another. Since some people will still find it beneficial it should be a compiler vendor extension and print a warning only on contract violations.

</braindump>

-- 
Marco

On Monday, 31 July 2017 at 17:54:04 UTC, Marco Leise wrote:
> Coming from D.learn where someone asked for some automatism to turn runtime format strings to `format()` into the equivalent `format!()` form automatically to benefit from compile-time type checks I started wondering...
>
> The OP wasn't looking for other benefits of the template version other than argument checking and didn't consider the downsides either. So maybe there is room for improvement using runtime arguments.
>
> So let's add some features:
> 1) compile-time "in" contract, run on the argument list
> 2) functionality to promote runtime arguments to compile-time

This was the more precise question:
How to "promote runtime arguments to compile-time".

I forgot the exact error, but it was during using vibe.d, that there
was an error popping up at runtime, which I thought should have been detected as simple syntax violation already during compile time.

Making me feeling for some seconds being back at .php. :)

I think it was in a regex, where the use of ctRegex would have prevented me from
the runtime error.

And last but not least: Thank you Marco for sharing your thoughts and expertise!

Regards mt.