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

[Thorsten Seitz]

> Am 27.09.2016 um 01:04 schrieb Robert Widmann via swift-evolution <swift-evolution at swift.org>:
> 
> Being a power user of this feature, I don’t think the existing syntax is cumbersome enough to warrant this kind of shortcut.  I really like being able to
> 
> guard case let .dict(book) = data else {
> 	// bail out
> }
> guard case let .dict(author) = book["author"] ?? .null else {
> 	// bail out
> }
> guard case let .integer(age) = author["age"] ?? .null else {
> 	// bail out
> }
> 
> // now we have the age
> 
> or more succinctly
> 
> guard case let .dict(book) = data,
>       case let .dict(author) = book["author"] ?? .null,
>       case let .integer(age) = author["age"] ?? .null 
> else {
> 	// bail out
> }
> // now we have the age

This is a very nice and readable solution. IMHO much better than the proposed syntax.

-Thorsten


> 
> Because it forces me to think about the bailout case(s) and is really not that much longer than your proposed syntax
> 
> guard let book = case? .dict(inputData), 
>       let author = case? .dict(book?["author”]), 
>       let age = case? .integer(author?["age”])
> else {
> 	// bail out
> }
> // now we have the age
> 
> ~Robert Widmann
> 
>> On Sep 26, 2016, at 11:51 AM, Jérôme Duquennoy via swift-evolution <swift-evolution at swift.org <mailto: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 ?
>> 
>> Thanks
>> 
>> Jerome
>> 
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> 
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