Mixin on a bunch of foreach fails.

Timon Gehr timon.gehr at gmx.ch
Thu Jan 12 20:14:38 PST 2012

On 01/13/2012 05:07 AM, Charles McAnany (dlang) wrote:
> Hi, all. So I'm trying to make some very ugly code generic. The main
> ugliness isn't in the code shape, it's in the running time. It's O(n^m)
> Eww! (don't worry, n is only about 6.)
> Anyhoo, Here's what I want:
>     void foo(int size)(int[] arr){
>     mixin(forStart!(size));
>     doStuff(pos0, pos1, pos2,...); // this line is generated by another
>     mixin that works correctly.
>     }
>     //Generates for loop headers.
>     private static string forStart( int sz)(){
>     string forStrings = "";
>     for(int i = 0; i < sz; i++){
>     forStrings ~="foreach(pos"~text(i)~"; 0..arr.length)\n ";
>     }
>     return forStrings;
>     }
> It is my great displeasure to report:
> src\hw06.d(35): found 'EOF' instead of statement
> src\hw06.d(18): Error: template instance hw06.tryCombinations!(5) error
> instantiating
> But here's the wacky part:
> I can execute this function and print it to stdout. If I do, I get...
> foreach(pos0; 0..arr.length)
> foreach(pos1; 0..arr.length)
> foreach(pos2; 0..arr.length)
> foreach(pos3; 0..arr.length)
> foreach(pos4; 0..arr.length)
> and I can copy-paste this exact code into where I currently have the
> mixin, and the code behaves correctly.
> Is there some subtle aspect of mixin that I'm missing here?
> Cheers,
> Charles.

Yes. You can only mixin whole statements or expressions. The easiest way 
to fix your code is to join the two mixins into one using the 
concatenation operator. Eg: mixin(forStart!(size)~generateDoStuff(...));

By the way, you can make the code more clean by making 'int sz' a simple 
function parameter. (static string forStart(int sz){...})

More information about the Digitalmars-d-learn mailing list