Bind truly-optional nullable properties about kotlin-argparser HOT 7 CLOSED

I'd also be interested in the ability to specify a nullable default. For example, in the case of this file we default to a path specified by a system property which may not be present.

val path by parser.storing("The path", ::File)
    .optional()
    .default(System.env("THE_PATH")?.let(::File))

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Does something like this do what you want?

val path by parser.storing<File?>("The path", ::File)
        .default(System.getenv("THE_PATH")?.let(::File))

It would be nice if a nullable default made the Delegate's type nullable. Actually, it would be nice if in in general setting the default could "relax" the type of the delegate (that is, make the Delegate into a Delegate where S is a super-type of T and the default value).

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Hmm I was under the impression that the internals required a non-null value but I might have been mistaken. I will give your suggestion a try today.

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If they do, that is probably a bug. I jumped through some hoops to distinguish between "not set" and "set to null".

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Ok so it works for File? when explicitly specified and which allows the use of default(null) (or some nullable thing).

It's a bit awkward for nullable string arguments though as you have to pass an identity function because the non-transformer overload doesn't have a generic parameter.

val title by parser.storing<String?>("Execution title", { this }).default(null)

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That's a good point. Commit 1bc1474 changes the way default works that fixes this. It makes default into an extension method, and so now the default value can be nullable, actually null, or even some other type. The resulting property will have the most specific superclass. In the case of starting with a Delegate<String>, you'll get a Delegate<String?> if the default is null or String?, regardless of which factory method created the original.

It's a somewhat backwards incompatible change, as Delegate is now an abstract class instead of an interface. Really, I probably should have done that from the beginning, as I never meant for people to be able to create their own Delegate implementations.

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I'm going to close this as I think 1bc1474 should solve the root issue.

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