Re: Dlang + compile-time contracts

[Martin Tschierschke via Digitalmars-d]

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.

Dlang + compile-time contracts

[Marco Leise via Digitalmars-d]

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.



-- 
Marco