is there a way to impl a custom interface for an f...

# getting-started

[King River Lee]

is there a way to impl a custom interface for an final class say "String"?

[Joffrey]

You mean make the existing String class implement your custom interface? This is not possible at the moment

[King River Lee]

OK. I just want to pass String to a fun that accept a custom Interface as param

[Sam]

Value classes will get you close, but it's not exactly what you want

[Joffrey]

Then maybe you can wrap that string into a custom class (possibly value class as Sam mentioned) that implements that interface. What does the interface look like?

[King River Lee]

and I found a discuss 'https://discuss.kotlinlang.org/t/implementing-an-interface-for-an-existing-type/20967/13' here

[King River Lee]

The interface now just look like a placeholder interface Selectable

[King River Lee]

the way @Joffrey I just noticed. It is a way to go. But It maybe annoying than just a String

[Joffrey]

I don't quite understand your point. If you're defining the parameter type to be this interface (and not string), isn't it the whole point to forbid other types like String?

[King River Lee]

Main idea is I want accept Selectable interface as param. Assume Class Column impl Selectable , so I can pass Column to the fun. Like SQL SELECT Selectable . so I want to let String Impl Selectable for some more common way to select.

[Joffrey]

For those cases you could probably create your own value class to wrap a string. It could even use an init block to perform validation on the string, because surely not any arbitrary string should be considered a Selectable, right?

[King River Lee]

Yes. @Joffrey you're right. I will try value class. and do more validation. correct is more important than simple.

[King River Lee]

Thanks

[Joffrey]

It will still be pretty simple to call, btw: yourFunction(Sql("SELECT Selectable")) (if we define value class Sql(val text: String) : Selectable)

[Klitos Kyriacou]

I would have thought Selectable is anything that can be a parameter to SELECT - such as a column name or, indeed, any String literal. After all, both SELECT mycolumn FROM mytable and SELECT 'any arbitrary string' are both valid statements.

[Joffrey]

My bad, when King River Lee said "Like SQL `SELECT Selectable`" I didn't think twice and just took the whole thing at face value. But you're right, most likely only the part after SELECT is the string that is passed to the function. Nevertheless, I still believe it's worth using a type-safe column/selectable type for this, and maybe a simple string wrapper for a column name: yourFunction(ColumnName("Selectable"))

[King River Lee]

@Klitos Kyriacou is my main idea. that any thing impl Selectable can be param of myFun(selectable: Selectable)