[swift-evolution] [proposal] Allow "let" for computed properties which only reference immutable data

[Chris Lattner]

On May 11, 2016, at 11:31 AM, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
>>    var combined:[Int64] { return a + b }
>> }
>> 
>> Each of these arrays is 512kb, so storing a fixed value for the combined property would require another megabyte of space per instance of Test, but if the combined property is only infrequently called on some instances of Test, then it would be a waste of memory to precompute it, especially if you’re dealing with thousands of instances, with only a fraction of them calling the computed property. If you want to accelerate usage of the computed property then that’s what lazy variables are for, as you can delay the expensive combination until you actually need it.
> 
> Being constant doesn't need to imply "precomputed". I would expect a computed 'let' to still be computed on-demand. 'let' is more important as an API contract; you're guaranteeing to the user that an API won't produce different values if called on the same object at different times.

Right.  “let” means “always has the same value”, a get-only property means “can not be set” (which is less restrictive).  Computed or not isn’t the issue here.

-Chris