[swift-evolution] [proposal draft] new syntax to access a given case's payload

[Joe Groff]

> On Sep 26, 2016, at 8:51 AM, Jérôme Duquennoy via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Summary
> The aim of this proposal is to offer a new syntax to ease some uses of enums with payload.
> 
> Situation to improve:
> Enums makes it possible to have explicate typing where it was not possible before. A classic example of that is filling a dictionary with data coming from a file or a stream (json, plist, …) : the types of possible values is finite : arrays, dicts, int, double, bool or string for json for exemple.
> An enum can represent this finite range of possible types, its make the code and the API more self-documented.
> Then, you have two possibilities to deal with this enum:
> - using switch statements
> - using the if case syntax introduced by swift 2
> 
> The drawback is that those two solutions can lead to writing code with high visual complexity, even though the logic behind is pretty simple.
> 
> Consider this example of a data dictionary, that a web service could have returned:
> - book
>   - title: 
>   - author:
>     - name: Apple
>     - age: 40
> 
> We can decode this in a variable of type [String:Value], where Value is:
> 
> enum Value {
>   case integer(value: Int)
>   case string(value: String)
>   case dict(value: [String:Value])
>   case null
> }
> 
> Here is a snippet of code to access the age of the author:
> 
> if case .dict(let book) = data {
>   if case .dict(let author) = book["author"] ?? .null {
>     if case .integer(let age) = author["age"] ?? .null {
>       // now we have the age
>     }
>   }
> }
> 
> The multiple indentation levels can rapidly make this code unattractive to read, and we need to add a null case to the enum to deal with optional values.
> 
> Proposed solution:
> I suggest to add a new syntax, using the case keyword to ease access to the payload of such enums :
> 
> let payloadContent = case? .enumCase(variable)
> 
> The payloadContent variable will be an optional, that can be either nil, or contain the payload of enumCase.
> If the payload contains multiple variables, payloadContent will be a tupple.
> This syntax can accommodate an optional variable as an input. If the value of variable is nil, then payloadContent will be nil.
> Only enum cases with a payload can be used with this syntax (it would make no sens for cases without a payload).
> 
> With that syntax, the null case of the enum can be removed, and the code to access the age becomes:
> 
> let book = case? .dict(inputData)
> let author = case? .dict(book?["author"])
> let age = case? .integer(author?["age"])
> 
> Advantages:
> - It leverages the well established notion of optional, and similar logic already exists in the language (for the as? operator notably).
> - It does not add a new keyword
> - It promotes the use of enum to enforce explicit typing, which leads to more self-documenting code
> - It reduces the complexity of the code in situations such as the one of the exemple
> 
> Drawbacks:
> - It adds a third use of the case keyword. 
> - In the proposed syntax, the variable between parenthesis is not the payload, but the variable to decode. This might be disturbing, as it differs from the other syntax of enum values.
> - If the payload is an optional, it is not possible to differentiate a non-matching case and a matching case a nil payload.
> 
> Alternatives:
> - Another syntax without parenthesis could be used to avoid the second drawback:
> let payload = case? .enumCase variable
> 
> Impact on existing code:
> None, this is adding a new syntax
> 
> 
> This proposal would have no impact on the ABI, so it probably does not fit the stage 1 of swift 4’s roadmap. But I would be glad to have your feedback, so that I can have a proposal ready once we enter stage 2.
> So what your thoughts on that proposal ?

I think it's reasonable to want an expression for extracting the payload of a enum case as an Optional. Instead of introducing a new operator, though, we could say that the cases themselves behave as Optional properties of an enum, which would allow you to say:

if let book = inputData.dict,
   let author = book["author"].dict,
   let age = author["age"].integer { ... }

-Joe