On Sat, Oct 14, 2017 at 16:22 Ole Begemann <o...@oleb.net> wrote:
>
> On 14. Oct 2017, at 21:55, Xiaodi Wu via swift-evolution <
> swift-evolution@swift.org> wrote:
>
> On Sat, Oct 14, 2017 at 14:36 Benjamin G <benjamin.garrig...@gmail.com>
> wrote:
>
>> I think what you're saying and what Kevin is saying are in way not
>> contradictory :
>>
>> You're saying the "SpongeBob" protocol functions make sense and are
>> coherent, Kevin is saying the consequence is that it creates weird
>> functions for Sets, and i think you're both right.
>>
>> an "UnorderedSequence" may very well have a "first" element. The problem
>> is that when you apply it to sets, you're not calling:
>>
>> mySet.generatedRandomOrderSequence().first()
>>
>> but
>>
>> mySet.first()
>>
>> That's where the issue stands, and that's IMHO, a symptom that the
>> protocol hierarchy is a bit misleading.
>> What you would like for sets isn't "first()", but "any()".
>>
>
> 'first' is just a shorthand for 'let first; for element in sequence {
> first = element; break }'. Any type that offers iterated access in a
> for...in loop must, by construction, have a definable 'first'.
>
> Swift Sequences are deliberately allowed to be single-pass, infinite, or
> unordered. A multi-pass, finite Sequence is a Collection. It's not an
> oversight that 'first' is defined on Sequence and not Collection. It's not
> 'weird' to ask what the first element you get on iteration must be; by
> definition, iteration gives you elements sequentially and something must be
> first.
>
>
> I don't think this really changes the point you're making, but the 'first'
> property is in fact defined on Collection and not Sequence. Sequence has
> 'first(where:)', 'dropFirst', and 'prefix' methods, and essentially the
> same argument applies to them.
>
> One rationale I heard (I think from Ben Cohen) for why 'first' is *not*
> defined on Collection is that Sequences can be single-pass, and it would
> feel weird for a seemingly innocuous property like 'first' to have side
> effects.
>
Good point and an important correction. It’s quite sensible to say that a
*property* with side effects is more surprising. But yes, the point here is
that the methods provided on Sequence all explicitly accommodate
single-pass types.
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