[swift-evolution] [Pitch] Memos for Struct Calculated Properties

Wallacy wallacyf at gmail.com
Sat May 7 09:54:55 CDT 2016

We already have lazy properties to get what you are describing. But we need
to wait for the Properties Behavior for the ability to clean a precomputed
lazy var.

Em sáb, 7 de mai de 2016 11:27, Jonathan Hull via swift-evolution <
swift-evolution at swift.org> escreveu:

> I saw a message come across about Memoization, and I thought of using a
> slightly different approach for similar benefit.
> I commonly want to store the result of a complex calculation in a private
> variable in a struct to avoid recalculation cost… but now I have to mark
> that calculation as *mutating* even though it isn’t really mutating in the
> semantic sense (since it will have the same value). It is an implementation
> detail leaking out… the caller shouldn’t care if I am memoizing.
> Also, my attempt to speed things up may have actually slowed things down,
> since a bunch of other stuff had to be copied...
> It would be nice to mark a calculated property of a struct as “memo" so
> that the calculation result is stored… but the calculation shouldn’t need
> to be marked as mutable (unless it actually mutates other things).  That
> memo should be automatically cleared whenever an actual mutating method is
> called on the struct.
> Ideally, the compiler would eventually be smart enough to only clear the
> memos when properties used in the calculations change, but I think just
> clearing it whenever the struct is mutated is a good first step.
> struct MyStruct {
>     var a:Int
>     var b:Int
>     memo var c {
> //Complex calculation here
>     }
> }
> let s = MyStruct(a:2, b:3)
> print(s.c) // result of c is automatically memoized
> print(s.c) // returns memoized result
> s.a = 5 //This clears the memo for c as it may affect the calculation
> print(s.c) // New result of c is automatically memoized
> The other alternative is to do things manually.  Letting the programmer
> declare special private variables on a struct that are allowed to mutate
> without the “mutating” key word.  That is potentially more powerful, but
> also more error prone.  I prefer the set-it-and-forget-it approach.
> If “memo" is the wrong word for this, I am completely open to calling it
> something else.  I would really like to see this functionality in Swift 3
> though (whatever it is called).
> Thanks,
> Jon
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