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){...})
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