Iterating over an array with different partials should result in different output

[handlebars-lang]

Iterating over an array with different partials should result in different output

[wycats]

The problem here is that the code that invokes blocks is extremely hot so we try to cache as much as we can. In this case, I am seeing something like 2x slowdowns from removing the cache, because it means making a new wrapper to pass to helpers for every block invocation.

Concerningly, this same issue would also exist for passed in helpers. That said, since most people use Handlebars.registerHelper and Handlebars.registerPartial, I am worried about the performance loss here. In general, my rule for Handlebars is that you shouldn't have to pay for features you don't use.

I'm considering requiring a compile-time option if you want to pass in custom helpers or partials, which would never cache the generated wrapper.

Thoughts?

/cc @tomdale @wagenet

[g-w]

Maybe the best solution is to invalidate the cache if new partials or helpers become relevant or irrelevant? If partials or helpers aren't used the behavior is the same, otherwise the closures are recreated once after registering new helpers or partials and the behavior is as expected.

I don't like the idea of a compile time option. It somehow exposes internals to the user that he shouldn't have to be aware of.

[wycats]

That would require keeping track of whether helpers or partials were passed in and passing that information to the code that creates the function wrapper. It would noticeably increase the size of the compiled code, and have an effect on its performance.

The flag would be something like dynamicHelpers: true, not alwaysClearCache: true.

[wagenet]

It sounds like this is difficult to properly implement. I'm closing for now though will glady review any pull requests.

[chrishoage]

Is there any solution to this aside from using Handlebars.registerPartial? It seems messy to have to re-register partials globally rather than pass them into the template function.