Pitch: Property Delegates

[Douglas_Gregor]

@Douglas_Gregor

Would you consider introducing custom attributes first even if they are only able to define Property delegates at first?

I couple of people have mentioned this, and it's worth investigating because it could pull together a couple of desired features into a single conceptual framework. I'm going to take a shot at implementing the @Lazy var x = 17 , @UserDefaults(key: "key", default: true) var isSomething: Bool syntax implied by the custom-attributes approach before I make any conclusions here.

I went ahead and prototyped support for using property delegates via attribute syntax. Essentially, this means that one would write:

@Lazy var x = 10

@UserDefault(key: "FOO_FEATURE_ENABLED", defaultValue: false)
static var isFooFeatureEnabled: Bool

which is equivalent to the current proposal's

var x = 10 by Lazy

static var isFooFeatureEnabled: Bool
  by UserDefault(key: "FOO_FEATURE_ENABLED", defaultValue: false)

There are some restrictions that come with this design: we no longer have syntactic space for the by private / by public part of the feature. On the other hand, it dovetails with the discussion of custom attributes. I'll bring up the general idea over there.

Doug