[swift-evolution] Generic Subscripts

[Rien]

> On 14 Jan 2017, at 18:45, Anton Zhilin via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I’m not sure, but I think that in this case the specific type of these values is determined at runtime.
> Then a safe approach would be separate string: String?, bool: Bool?, int: Int? computed properties, as it’s done in JSON parsers.

Yup. This is what I do:

var json = VJson()
var name: String?

json[“books”][1][“title”] &= “THHGTTG” 

and the reverse

name &= json[“books”][1][“title"]

The overloaded operator ‘&=‘ takes care of the types. (‘=‘ cannot be overloaded)



> 
> if let bookCount = row.value(named: "bookCount").int {
>     ...
> }
> if let bookCount = row["bookCount"].int {
>     ...
> }
> let bookCount = 
> row.int
> ("bookCount")!   // crash if database is corrupt
> 
> Additionally, this is an overall bad example of generics. Fields of database tables can only map to a limited set of static types in Swift, which are supported by database adapter.
> 
> 2017-01-14 16:50 GMT+03:00 Gwendal Roué via swift-evolution <swift-evolution at swift.org>:
> 
> 
> 
> This is a consequence of your vision of subscript. If interesting, it is also limiting for no real purpose.
> 
> As the developer of a Swift database library, I'd like to offer a better API than the following:
> 
>     // Current state of affairs
>     let name: String = row.value(named: "name")
>     let bookCount: Int = row.value(named: "bookCount")
>     let hasBooks: Bool = row.value(named: "bookCount")
> 
> Instead, I wish I could offer GRDB.swift would let its users write:
> 
>     // With improved subscripts
>     let name: String = row["name"]
>     let bookCount: Int = row["bookCount"]
>     let hasBooks: Bool = row["bookCount"]
> 
> And this requires genericity on return type.
> 
> Gwendal
> 
> 
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> 
> 
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Regards,
Rien

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