Proposal: __traits(code, ...) and/or .codeof

[Timon Gehr]

On 03/22/2012 08:12 PM, F i L wrote:
> Timon Gehr wrote:
>> I think that would necessitate the addition of AST macros.
>
> You mean passing DMD's actual AST tree object? Only a stripped down
> version?

We have the macro keyword. I envision something like:

macro replaceAggregate(macro newAggregate, macro loop : foreach(x; 
aggr){statements}, macro x, macro aggr, macro statements) {
     foreach(x; newAggregate){statements}
}

void main(){
     int[] a = [1,2,3];
     int[] b = [2,3,4];
     replaceAggregate(b, foreach(x;a){writeln(x);});
}

(The syntax looks horrible, but you get the idea: AST walking by pattern 
matching)

Ideally, such macros would allow introducing local helper variables 
inside the macro that are not accessible from outside and introducing 
variables/enums in the context the macro is used. (prefixing identifiers 
that should interfere with the enclosing context with $ would be an 
option). Another idea:

macro someFancyDSL(string dsl){
     mixin(someFancyDSLtoD(dsl);
}

void main(){
     int n = 5;
     someFancyDSL {
         import n;
         let x = 1 : x
         let y = product (take n) x
         export y;
     }
     writeln(y);
}
// this would also enable less fancy but useful stuff:
macro until(macro condition, string str){
     while(!condition){
         mixin(str); // string and mixin not strictly necessary,
                     // maybe enable the feature on macro params too
     }
}

void main(){
     bool done = false;
     int x;
     until(done){
         done = foo(x++);
     }
}

This is just a very rough sketch though, we would need a much more 
refined design. I think getting the symbol scoping right is most important.