memoize & __traits(compiles...)

[Nicholas Wilson]

On Friday, 23 November 2018 at 10:34:11 UTC, John Chapman wrote:
> I'm doing a fair amount of repeatedly checking if a function 
> compiles with __traits(compiles...), executing the function if 
> so, erroring out if not, like this:
>
>   static if (__traits(compiles, generateFunc1())) {
>     return generateFunc1();
>   } static if (__traits(compiles, generateFunc2())) {
>     return generateFunc2();
>   } else static assert(false);
>
> But it seems inefficient to have to evaluate those functions 
> twice, so I'd like to optimise this so if __traits(compiles...) 
> succeeds, the result is cached and then used when the function 
> is actually called. I wondered if using std.functional.memoize 
> would help?

No, std.functional.memoize uses a hashtable to cache the runtime 
results of calls to expensive functions.

assuming that the example is not oversimplified and generateFunc1 
and generateFunc2 are functions, the compiler doesn't do extra 
semantic analysis so the validity of the functions is effectively 
cached.

If they are templates (with parameters) then the compiler will 
automatically memoize them (it too keeps a hashtable of template 
instances).

When in doubt, profile! 
https://blog.thecybershadow.net/2018/02/07/dmdprof/