[swift-evolution] Normalize Enum Case Representation (rev. 2)

[Daniel Duan]

> On Mar 9, 2017, at 12:06 AM, David Hart <david at hartbit.com> wrote:
> 
> 
>> On 9 Mar 2017, at 08:07, Daniel Duan via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> Thanks for the thoughtful feed Xiaodi! Replies are inline. I'm going to incorporate some of the responses into the proposal.
>> 
>>> On Mar 8, 2017, at 9:56 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>> 
>>> The rendered version differs from the text appended to your message. I'll assume the more fully fleshed out version is what you intend to submit. Three comments/questions:
>>> 
>>> Enum Case "Overloading"
>>> 
>>> An enum may contain cases with the same full name but with associated values of different types. For example:
>>> 
>>> enum Expr {
>>>     case literal(Bool)
>>>     case literal(Int)
>>> }
>>> The above cases have overloaded constructors, which follow the same rules as functions at call site for disambiguation:
>>> 
>>> // It's clear which case is being constructed in the following.
>>> let aBool: Expr = .literal(false)
>>> let anInt: Expr = .literal(42)
>>> User must specify an as expression in sub-patterns in pattern matching, in order to match with such cases:
>>> 
>>> case .literal(let value) // this is ambiguous
>>> case .literal(let value as Bool) // matches `case literal(Bool)`
>>> 
>>> Comment/question 1: Here, why aren't you proposing to allow `case .literal(let value: Bool)`? For one, it would seem to be more consistent.
>> 
>> The example in proposal doesn't include any labels. Are you suggesting two colons for sub-patterns with labels? Like `case .literal(value: let value: Bool)`?  This looks jarring. But I'm definitely open to other suggestions.
> 
> No, I think he was proposing replicating variable declaration syntax, where the colon is used preceding the type (instead of using as):
> 
> let value: Bool = ...

We'd still need a way for user to bind a matching value to a name other than the label... I do realize that this would not be a problem if we drop the mandate that label must appear _somewhere_ in the pattern. Hmmm

>>> Second, since we still have some use cases where there's Obj-C bridging magic with `as`, using `as` in this way may run into ambiguity issues if (for example) you have two cases, one with associated value of type `String` and the other of type `NSString`.
>> 
>> Either this should be rejected at declaration, or we need a way to accept a "pre-magic" resolution at pattern matching, when this scenarios is at hand. I'm on the phone so I can't verify. Wouldn't function overloading face a similar problem?
>> 
>>> Also, since enum cases are to be like functions, I assume that the more verbose `as` version would work for free: `case .literal(let value) as (Bool) -> Expr`?
>>> 
>> 
>> This is not being proposed. When a user sees/authors a case, their expectation for the declared case constructor should resemble that of a function. Pattern matching was considered separately since it's not relatable syntactically.
>>> Alternative Payload-less Case Declaration
>>> 
>>> In Swift 3, the following syntax is valid:
>>> 
>>> enum Tree {
>>>     case leaf() // the type of this constructor is confusing!
>>> }
>>> Tree.leaf has a very unexpected type to most Swift users: (()) -> Tree
>>> 
>>> We propose this syntax declare the "bare" case instead. So it's going to be the equivalent of
>>> 
>>> enum Tree {
>>>     case leaf // `()` is optional and does the same thing.
>>> }
>>> 
>>> 
>>> Comment/question 2: First, if associated values are not to be modeled as tuples, for backwards compatibility the rare uses of `case leaf()` should be migrated to `case leaf(())`.
>>> 
>> Yes, and when user uses a arbitrary name when they should have used a label, or when labels are misspelled, the compiler should suggest the correct labels. I wasn't sure how much of migrator related thing should go into a proposal. Perhaps there should be more.
>>> Second, to be clear, you are _not_ proposing additional sugar so that a case without an associated value be equivalent to a case that has an associated value of type `Void`, correct? You are saying that, with your proposal, both `case leaf()` and `case leaf` would be regarded as being of type `() -> Tree` instead of the current `(()) -> Tree`?
>>> 
>> Correct. I'm _not_ proposing implicit `Void`.
>>> [The latter (i.e. `() -> Tree`) seems entirely fine. The former (i.e. additional sugar for `(()) -> Tree`) seems mostly fine, except that it would introduce an inconsistency with raw values that IMO is awkward. That is, if I have `enum Foo { case bar }`, it would make case `bar` have implied associated type `Void`; but, if I have `enum Foo: Int { case bar }`, would case `bar` have raw value `0` of type `Int` as well as associated value `()` of type `Void`?]
>>> 
>>> 
>>> Pattern Consistency
>>> 
>>> (The following enum will be used throughout code snippets in this section).
>>> 
>>> indirect enum Expr {
>>>     case variable(name: String)
>>>     case lambda(parameters: [String], body: Expr)
>>> }
>>> Compared to patterns in Swift 3, matching against enum cases will follow stricter rules. This is a consequence of no longer relying on tuple patterns.
>>> 
>>> When an associated value has a label, the sub-pattern must include the label exactly as declared. There are two variants that should look familiar to Swift 3 users. Variant 1 allows user to bind the associated value to arbitrary name in the pattern by requiring the label:
>>> 
>>> case .variable(name: let x) // okay
>>> case .variable(x: let x) // compile error; there's no label `x`
>>> case .lambda(parameters: let params, body: let body) // Okay
>>> case .lambda(params: let params, body: let body) // error: 1st label mismatches
>>> User may choose not to use binding names that differ from labels. In this variant, the corresponding value will bind to the label, resulting in this shorter form:
>>> 
>>> case .variable(let name) // okay, because the name is the same as the label
>>> case .lambda(let parameters, let body) // this is okay too, same reason.
>>> case .variable(let x) // compiler error. label must appear one way or another.
>>> case .lambda(let params, let body) // compiler error, same reason as above.
>>> Comment/question 3: Being a source-breaking change, that requires extreme justification, and I just don't think there is one for this rule. The perceived problem being addressed (that one might try to bind `parameters` to `body` and `body` to `parameters`) is unchanged whether enum cases are modeled as tuples or functions, so aligning enum cases to functions is not in and of itself justification to revisit the issue of whether to try to prohibit this. 
>>> 
>> To reiterate, here patterns are changed not for any kind of "alignment" with function syntax. It changed because we dropped the tuple pattern (which remains available for matching with tuple values, btw), therefore we need to consider what a first-class syntax for enum case would look like.
>> 
>> The justification for this breaking change is this: with tuples, labels in pattern is not well enforced. User can skip them, bind value to totally arbitrary names, etc. I personally think emulating such rule prevents us from making pattern matching easier to read for experienced devs and easier to learn for new comers. 
>> 
>> It's reasonable to expect existing patterns in the wild either already use the labels found in declaration or they are matching against label-less cases. In other words, existing code with good style won't be affected much. For the code that actually would break, I think the migrator and the compiler can provide sufficient help in form of migration/fixits/warnings.
>> 
>> Ultimately I think requiring appearance of labels one way or another in patterns will improve both the readability of the pattern matching site as well as forcing the author of case declaration consider the use site more.
>>> In fact, I think the proposed solution suffers from two great weaknesses. First, it seems ad-hoc. Consider this: if enum cases are to be modeled as functions, then I should be able to write something intermediate between the options above; namely: `case .variable(name:)(let x)`. Since `.variable` unambiguously refers to `.variable(name:)`, I should also be allowed to write `.variable(let x)` just as I am now.
>>> 
>> Again, patterns are not to be modeled after functions. Only the declaration and usage of case constructors are.
>>> Second, it seems unduly restrictive. If, in the containing scope, I have a variable named `body` that I don't want to shadow, this rule would force me to either write the more verbose form or deal with shadowing `body`. If a person opts for the shorter form, they are choosing not to use the label.
>>> 
>> In fact this (to avoid label conflict in nesting) is the only reason the longer form allows rebinding to other names at all! You say "unduly restrictive", I say "necessarily flexible" :)
>> 
>>> 
>>> Only one of these variants may appear in a single pattern. Swift compiler will raise a compile error for mixed usage.
>>> 
>>> case .lambda(parameters: let params, let body) // error, can not mix the two.
>>> Some patterns will no longer match enum cases. For example, all associated values can bind as a tuple in Swift 3, this will no longer work after this proposal:
>>> 
>>> // deprecated: matching all associated values as a tuple
>>> if case let .lambda(f) = anLambdaExpr {
>>>     evaluateLambda(parameters: f.parameters, body: f.body)
>>> }
>>> 
>>> 
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