Can you include an example? I find it hard to visualize for case? pattern where-clause? in expression code-block
Thanks, Brandon > On Jun 9, 2016, at 3:05 PM, Erica Sadun via swift-evolution > <[email protected]> wrote: > > Gist: https://gist.github.com/erica/86f00c1b8ebf45dcf3507ae6ef642b57 > <https://gist.github.com/erica/86f00c1b8ebf45dcf3507ae6ef642b57> > > Regularizing Where grammar > > Proposal: TBD > Author: Brent Royal-Gordon <https://github.com/brentdax>, Erica Sadun > <http://github.com/erica> > Status: TBD > Review manager: TBD > > <https://gist.github.com/erica/86f00c1b8ebf45dcf3507ae6ef642b57#introduction>Introduction > > This proposal fixes an inconsistency for where clause grammar in Swift > language for-in loops. > > Swift Evolution Discussion: Add a while clause to for loops > <http://thread.gmane.org/gmane.comp.lang.swift.evolution/19772/focus=20143> > > <https://gist.github.com/erica/86f00c1b8ebf45dcf3507ae6ef642b57#motivation>Motivation > > Unlike in switch statements and do loops, a for-in loop's where-clause is > separated from the pattern it modifies. > > for case? pattern in expression where-clause? code-block > > case-item-list → pattern where-clause? | pattern where-clause? , > case-item-list > > catch pattern? where-clause? code-block > This separation makes the clause harder to associate with the pattern, can > confuse users as to whether it modifies the expression or the pattern, and > represents an inconsistency in Swift's grammar. This proposal regularizes the > grammar to match other uses. > > Note where clauses in case conditions and optional bindings have been removed > in SE-0099 > <https://github.com/apple/swift-evolution/blob/master/proposals/0099-conditionclauses.md>. > > > <https://gist.github.com/erica/86f00c1b8ebf45dcf3507ae6ef642b57#detailed-design>Detailed > Design > > Current: > > for case? pattern in expression where-clause? code-block > Proposed: > > for case? pattern where-clause? in expression code-block > > <https://gist.github.com/erica/86f00c1b8ebf45dcf3507ae6ef642b57#impact-on-existing-code>Impact > on Existing Code > > Migration should be easily addressed with a simple fix-it. > > > <https://gist.github.com/erica/86f00c1b8ebf45dcf3507ae6ef642b57#alternatives-considered>Alternatives > Considered > > Not accepting this proposal > > >> On Jun 8, 2016, at 9:23 PM, Brent Royal-Gordon via swift-evolution >> <[email protected] <mailto:[email protected]>> wrote: >> >>> This reads to me as “repeat the following block until this fails to be >>> true”, the conditional binding in this case fails to be true if >>> someCondition(value) isn’t true, so the loop ends. I think the key thing >>> here is that the where clause is for the conditional binding and not the >>> loop itself, so in this respect it behaves exactly like an if or guard >>> statement. Meanwhile: >>> >>> for eachValue in theValues where someCondition(eachValue) { … } >>> >>> Reads as “for everything in theValues do the following if >>> someCondition(eachValue) is also true”, in other words this loop always >>> tries to visit every element of the sequence (a while loop has no implicit >>> awareness of the sequence, it’s really just an if statement that runs over >>> and over). In this case the where clause is part of the loop itself. There >>> may be an argument that where should be renamed on for loops to better >>> distinguish this, but once you consider that there’s no pattern or >>> conditional binding here I think it makes a reasonable amount of sense. >> >> The original sin here was in connecting the `where` clause to the for loop's >> sequence expression, rather than its pattern. If `where` were positioned >> right after the loop variable: >> >> for eachValue where someCondition(eachValue) in theValues { … } >> >> It would be much clearer that `where` constrains the values seen by the loop >> body. >> >> I'm not sure why the `where` clause was placed where it is. I suspect it has >> something to do with the `where` clause potentially being more complex than >> the sequence expression, but I was not in the room where it happened, so >> that's idle speculation. >> >> -- >> Brent Royal-Gordon >> Architechies >> >> _______________________________________________ >> swift-evolution mailing list >> [email protected] <mailto:[email protected]> >> https://lists.swift.org/mailman/listinfo/swift-evolution > > _______________________________________________ > swift-evolution mailing list > [email protected] > https://lists.swift.org/mailman/listinfo/swift-evolution
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