18876, "bradcray", "Should (some) parallel-safe collection types use 'ref' task intents by default?", "2021-12-16T00:43:31Z"
Currently, all traditional, user-level records use const ref as their default task intent, to prevent inadvertent parallel modifications that may not be safe. Since our collections like list, set, map are implemented using records, they get this default intent by default. Similarly associative domains (though not exactly a typical user record) are passed by const ref by default, like other domains, even though they are parallel-safe by default.
While working on advent of code programs this week, something that tripped me up a few times was that I would create a parallel-safe collection, and then get errors when trying to access it within a forall loop. For example:
var l: list(int, parSafe=true);
forall i in 1..n do
l.append(i); // error, as 'l' is a const ref shadow variable
var s: domain(int); // parSafe by default
forall i in 1..n do
s.add(i); // error, as 's' is a const ref shadow variable
It seems as though it would be more productive to have non-parallel-safe collections use 'const' default intents like this, while having parallel-safe ones use 'ref' by default. However, I'm not confident there aren't other cases in which 'ref' by default might cause issues.
This question seems like it intersects a bit with whether parallel-safe vs. non- collections are unified into a single type (as they currently are) or are accessed via different type names in that some mechanisms that we might use to specify a default task intent for a record may be difficult to control via a param field on the type itself.