[Harmony proxies] thought on the "traps that return arrays of property names" open issue

David Bruant david.bruant at labri.fr
Tue Jun 21 04:47:10 PDT 2011


Hi,

I've recently seen the slides of Dave Mandelin "Know your engine - How 
to make your JavaScript fast" [1]. If I understand slide 121 correctly, 
then a type inference engine can know in some cases if all elements of a 
(dense?) array have the same type.

On the Harmony proxies proposal, one open issue is the following:
"Traps that return arrays of property names, such as 
getOwnPropertyNames: should all values in the returned array be coerced 
to String before being returned to the user? (...)"
Regardless of how the problem is seen, considering such a things would 
require to look at the type of each element and convert it if it's not a 
string.
Assuming that programmers already generate arrays of strings (the 
strength of this assumption would need investigation) and assuming that 
the engine is able to realize it in most cases (needs to be investigated 
as well), then the cost of verifying the invariant is cheap.
My point is that in good cases, enforcing the invariant "has to return 
an array of strings" in good cases may be cheap (at least cheaper than 
with a naive approach).

Of course, the assumed cheapness of the invariant enforcement in good 
cases depends on having a type inference engine able to do such checks. 
If I understand well, Mozilla has one able to do that (tell me if I'm 
wrong) and it's open source. Other may have different opinions, but I 
think that if there was another different open source type inference 
engine with the same capacity, then having such an engine could be 
considered as "not that hard", because anyone who would like to create 
one could at least see 2 different approaches and implementations.

David

[1] http://www.slideshare.net/newmovie/know-yourengines-velocity2011


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