[swift-evolution] Remove Failable Initializers

[James Campbell]

True but in my example above for parsing a JSON into a model I can do this:

class Model {

let property: String
let property1: String
let property2: String
let property3: String

init?(json: [String: AnyObject]) {

guard property = json["property"] as? String else {
 return nil
}

guard property = json["property1"] as? String else {
 return nil
}

guard property = json["property2"] as? String else {
 return nil
}

guard property = json["property3"] as? String else {
 return nil
}

}

}

There is a lot of boilerplate for very little information and I'm worried
by letting people initialize without returning an error we will get complex
objects like this being constructed and failing for no reason.

Compare this to error handling:

class Model {

let property: String
let property1: String
let property2: String
let property3: String

init?(json: [String: AnyObject]) throws {

property = try json.require("property")
property1 = try json.require("property1")
property2 = try json.require("property2")
property3 = try json.require("property3")

}

}

Require reads the key from the dict and tries to cast it to the same type
as the variable its being assigned to and throws an error if this fails
(like it doesn't exist or it can't be casted). This has much less
boilerplate, its easier to read and also we can throw an error saying ("We
couldn't parse property1").

Some frameworks like Freddy JSON so something similar to this but with a
much more complex model using Enums and it involves setting a bunch of
parameters to tell the library when to throw a runtime error or not if it
can't find something which if not handled correctly can cause a crash.

We could make this even simpler yet if we could somehow treat casting as an
error:

class Model {

let property: String
let property1: String
let property2: String
let property3: String

init?(json: [String: AnyObject]) throws {

property = try json["property"] as String
property1 = try json["property1"] as String
property2 = try json["property2"] as String
property3 = try json["property3"] as String

}

}

If casting fails it throws an error, unfortunately we lose error
information about the key so probably having a casting method like the one
above is probable better. But in someways it explains what is happening.




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On Thu, Mar 3, 2016 at 8:59 AM, Brent Royal-Gordon via swift-evolution <
swift-evolution at swift.org> wrote:

> > Error handling forces you to do something about the possibility of a
> failure.
>
> So does using a failable initializer: the value is in an Optional, so you
> can't access it without either testing and unwrapping or just
> force-unwrapping. The type system, rather than the `try` requirement, is
> what forces you to handle the error, but it's the same idea.
>
> --
> Brent Royal-Gordon
> Architechies
>
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
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