<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div><div><br></div>On Feb 19, 2016, at 10:21 AM, Joe Groff via swift-evolution <<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>> wrote:<br><br></div><blockquote type="cite"><div><meta http-equiv="Content-Type" content="text/html charset=utf-8"><br class=""><div><blockquote type="cite" class=""><div class="">On Feb 18, 2016, at 9:01 PM, Curt Clifton <<a href="mailto:curt@curtclifton.net" class="">curt@curtclifton.net</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">I find the naked `initialValue` “declaration" quite odd. It’s not exactly a declaration even. What are the implications for parsing? If we’re already willing to special case`initialValue` in the parser in this context, perhaps we could introduce a binding list instead, like:</div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><br class=""></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">```</div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class=""><div class="">public var behavior lazy<Value>: Value {</div><div class=""> // Behaviors can bind the property's initializer expression with a</div><div class=""> // binding declaration.</div><div class=""> bind initialValue</div><div class=""> …</div></div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">```</div><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;" class="">Besides eliminating the odd naked “declaration”, this has the added advantage that it could be extended to `bind initialValue, propertyName`. :-)</div></div></blockquote></div><br class=""><div class="">Yeah, I'm not happy with it either. An alternative might be to bless a specifically-named `accessor` declaration, e.g.:</div><div class=""><br class=""></div><div class="">var behavior lazy<Value>: Value {</div><div class=""> accessor initialValue() -> Value</div><div class="">}</div><div class=""></div></div></blockquote><blockquote type="cite"><div><br></div></blockquote><br><div>Maybe we're approaching this from the wrong direction. We already have some magic identifiers within behavior declarations as described in Bindings within Behavior Declarations: `Self` for the property's host type, `self` for the property's host instance, and `newValue` as the r-value in the implementation of the `set` core accessor.</div><div><br></div><div>What's to keep us from just treating `initialValue` as similar magic within a behavior declaration? Is it a matter of not wanting to use additional storage if the initial value isn't referenced? If so, it seems like that could be treated as a post-hoc optimization. (Hmm, I suppose this might have resilience issues since changing implementation would change requirements, unless "<span style="background-color: rgba(255, 255, 255, 0);">we consider behaviors to be essentially always fragile" as mentioned in Future Directions.</span>)</div><div><br></div><div>Or is the concern that we must communicate to users of the behavior that a property so annotated must have an in-line initializer? That's implied by the formal grammar in the proposal, where the declaration is named `<span style="background-color: rgba(255, 255, 255, 0);">initial-value-requirement-decl`. </span>If that's the case, then I'd take the blessed accessor alternative over a naked `initialValue` declaration. For regularity, in that world it seems like properties should be able to implement the accessor directly. That is:</div><div><br></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>@lazy var foo: Int = 4</div><div><br></div><div>and</div><div><br></div><div><span class="Apple-tab-span" style="white-space:pre"> </span>@lazy var foo {</div><div><span class="Apple-tab-span" style="white-space:pre"> </span> initialValue { return 4 }</div><div><span class="Apple-tab-span" style="white-space:pre"> </span>}</div><div><br></div><div>should be equivalent.</div><div><br></div><div>If that's distasteful, I'd make a pitch for:</div><div><br></div><div><span style="background-color: rgba(255, 255, 255, 0);"><span class="Apple-tab-span" style="white-space:pre"> </span>initial-value-requirement-decl ::= 'requires' 'initialValue'</span></div><div><span style="background-color: rgba(255, 255, 255, 0);"><br></span></div><div><span style="background-color: rgba(255, 255, 255, 0);">But I'll try to stop bike shedding this syntax now. I continued to be thrilled with the proposal and the directions in which behaviors can evolve.</span></div><div><span style="background-color: rgba(255, 255, 255, 0);"><br></span></div><div><span style="background-color: rgba(255, 255, 255, 0);">Cheers,</span></div><div><span style="background-color: rgba(255, 255, 255, 0);"><br></span></div><div><span style="background-color: rgba(255, 255, 255, 0);">Curt</span></div><div><br></div><div><br></div><div><br></div><div><br></div></body></html>