[swift-evolution] [Draft] Rename Sequence.elementsEqual
swift at nattinger.net
Sun Oct 15 14:32:14 CDT 2017
> Sets, as a mathematical concept, have no intrinsic order. However, instances of `Set`, which can be iterated over, *do* have at least one order which can be said to be intrinsic in the following sense: as long as iteration is possible, no API design can prevent that order from being observed and associated with the instance. Put another way, if you can use an instance of a type in a for...in loop, intrinsic to that functionality is a publicly visible order.
You keep saying this, I keep saying it’s only a technical “order” that is an implementation detail and can’t be relied upon for anything and so we shouldn’t provide methods that rely on it. I think this part of the discussion has reached the “agree to disagree” stage.
> You’re a fan of the principal of least surprise. Tell me, which would be less surprising: Set.dropFirst() actually drops a random element, or Set doesn’t have a dropFirst()? And if you think dropFirst() removing an element at random is not surprising, please explain why.
> I think Set.dropFirst removing the first element that I observe on iteration is the least surprising answer, because Swift tells me that the stdlib Set models a set but that it is also a sequence.
Your logic is backwards. You’re saying it’s “least surprising” because that’s how it’s currently implemented, not that it should be implemented that way because it’s least surprising.
> And that’s PRECISELY why lexicographicallyEqual does not make sense to apply to unordered sets. There is no lexicographical comparison possible, so why do you keep insisting they should have a method that falsely claims to lexicographically compare them?
> I agree! It doesn't make sense if no comparison is possible! But Swift tells me that a `Set` is a `Sequence`!
You keep making the circular argument that a Set should do things because it currently does them. If you want to argue against changing things, argue that things shouldn’t be changed, not that the current implementation is correct because it is the current implementation.
> You will always have to account for this possibility, because Swift's `Equatable` explicitly allows "special values" to be not equal to themselves. This is, at least in part, in order to accommodate the IEEE decree that NaN != NaN:
> let x = [Double.nan]
> x.elementsEqual(x) // false
NaN is special, one-shot and unordered sequences are not. Unless you think that all unordered and single-pass sequences should compare false for `elementsEqual`, this is irrelevant for any sequence that doesn’t contain NaN and well-defined (false) for any that does.
> Changing this behavior is way beyond the scope of this thread (and has been the topic of hours (actually, weeks and weeks) of fun on this list previously).
Yes, I’ve seen the discussion on NaN and Comparable. It’s not the same discussion.
>> It would be better to say that the iteration order is well-defined. That will almost always mean documented, and usually predictable though obviously e.g. RNGs and iterating in random order will not be predictable by design.
> Wouldn't it then suffice to document, say, that a set's iteration order is the insertion order?
Now this actually gave me pause. I guess it does match what I said, but I still take issue with the fact that two Sets could compare `==` but not `elementsEqual`. I think that defining iteration order as insertion order adds a piece of publicly documented state that goes beyond what a Set really is. What you describe is really an OrderedSet, just without the random-access manipulation. I’ll have to mull this over to see if I can come up with a coherent and (more) specific requirement for what makes an Iterable a Sequence, since clearly “documented” isn’t enough. Perhaps something along the lines that any two Sequences that compare equal must iterate the same.
> Apple documentation calls this one of the "order-dependent" methods. It is surely acceptable for a type that conforms to an order-dependent protocol to have methods that are order-dependent; they do, however, have to be clearly order-dependent to avoid confusion on unordered types.
I’m not clear on what you’re trying to get across here. It seems you’re saying unordered types shouldn’t have order-dependent methods, which is exactly what I’ve been arguing.
> Then there are all the methods that imply a specific order of iteration. If the “sequence” is unordered, who knows what you’ll get? It is incredibly easy for an engineer to write a method that implicitly relies on a passed sequence being intrinsically ordered and another engineer to pass it an unordered “sequence.” The first engineer could neglect to document the dependency, or even not notice it; or the second engineer could just fail to read the documentation thoroughly enough. There is currently no way for the compiler to enforce passing only an object that is (or at least claims to be) intrinsically ordered.
> It is also incredibly easy for such an engineer to use `for...in` instead to accomplish the same task, generic over ordered and unordered sequences whatever you name such distinguished protocols. I think your beef really still boils down to Set being compatible with `for...in` at all, as Jon acknowledges.
Not providing ordered functions for unordered collections makes the developers think about what they actually need. If any object will do, they can use for…in, .makeIterator().next(), or an `anyObject()` we provide as a convenience. If they actually need the first from some specific order, it’s a reminder they need to sort the objects first to get the right one. That’s particularly useful for functions that actually need an ordered sequence; using OrderedSequence instead of Iterable (just as placeholders) would be a firm reminder not to pass in an unordered collection.
> As I said, you're welcome to tackle the protocol hierarchy, but I really doubt it's within the realm of realistic endpoints for Swift 5. I'm just trying to propose a narrowly targeted pragmatic solution to one specific limited harm that might be deliverable by the next point release. As a great man once said, Swift is a pragmatic language.
If you want a pragmatic solution, fix the bug in functionality, don’t try and rename the method to something obscure to cover it up.
If you want to limit the harm, override `equalObjects` on unordered sequences to use `==` (very strongly preferred), or always `false` (less desirable, but at least consistent)
> The Swift stdlib deliberately eschews modeling everything in protocol hierarchies with the highest level of granularity. There's some fudging, deliberately, to find a happy medium between obtuse and approachable, between too many/too specialized and not enough. For example, I pushed for protocols such as `Field` and `Ring` at the top of the numeric hierarchy, which might allow complex number types to slot into the hierarchy more sensibly, for example. But we have a compromise protocol `Numeric` which doesn't quite have the same guarantees but is much more approachable. Notice that we also don't break down numeric protocols into `Addable`, `Subtractable`, etc.; we also have that fudge factor built into `Equatable`, as I mentioned.
Eh, one or two corner cases on a protocol is probably fine. What’s not fine is over half (Sequence) or almost all (Collection) the methods not being applicable. There is a very clear gap there. We don’t need to fix everything, but this is something that can and should be addressed.
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