[swift-evolution] Rename 'guard' to 'ensure'

[Greg Parker]

> On Feb 22, 2016, at 10:15 AM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> on Mon Feb 22 2016, Erica Sadun <swift-evolution at swift.org> wrote:
>> 
>> On the other hand, I'd love if assert/precondition would be combined
>> into a single call, with an optional `forReleaseBuild:` arg (better
>> named) that defaults to false.
> 
> I really don't want to do that.  Assert and precondition have different
> use-cases, and I don't want people to ask “do I want this on in a
> release build?” (which is a hard decision to make correctly and
> consistently) when they write them.  I want them to ask, “Am I checking
> whether this method is being called correctly or is this just a
> self-sanity check?”

Conversely, "do I want this on in a release build" is precisely the decision I usually make in my own code. The dichotomy is not parameter validation versus sanity checks. Instead the dichotomy is checks that are fast enough for release builds versus checks that are too slow or have rare false positives and must be left only for non-release builds. "assert" and "precondition" are not useful names for that usage.


-- 
Greg Parker     gparker at apple.com     Runtime Wrangler