well I think that the best example of extension functions done right is C# with LINQ

@kmruiz: The core of why you would start with extension functions is as an alternative solution to the SRP other than inheritance.

C#'s LINQ is a great example,

IEnumerable

has a tiny surface area. It is 2 functions I believe.

@kmruiz: myself, it's not that I don't see the point, it's that I don't know how to do it

You can fully test

IEnumerable

very easily. It has a very clear single responsibility. Etc.

well I think that it wasn't the point

Ahh, sorry.

The argument is that the definition of deprecation says "without prohibiting its use"

Deprecation is a process IMO.

I guess Kotlin just has a different definition

Not a single state.

@ribesg: it helps migration more than just “don’t use it” flag.

@kmruiz: headers of CSV doesn't have to do anything with

Person.toCSVRow()

. all this extension function does is return a CSV-Row of the Person. headers should be handled somewhere else

@ribesg: e.g. if you want new code not use it, but old code still work (binary compatibility) you make it error or hidden.

and yeah, if you take the extension functions-way, i'd create 2 extension functions for XML

@orangy: Hmm, how does that work?

Oh, if you upgrade it as a transitive dependency?

@zoltu: yep

A depends on B, B depends on C. A depends on newer version of C. B doesn't break.

Clever.

as a side effect, deprecate-error can be used to generate library-defined errors.

it’s a little bit of misuse, but it’s very useful

What do you mean?

@Andreas Sinz what about csv format?

There are 3 standards, one function for each?

Also, extension functions only use public API

serialization means state inspection so you need to publish your data outside the class scope

If the underlying class exposes at least one serialization mechanism then the others could build on top of that.

Or if the underlying class has public accessors for all data members.

@zoltu: consider this example:

class Foo<T>
fun foo(): Foo<Unit> {}
fun <R> Foo<Unit>.bind(value: R): Foo<R> {}

@JvmName("bind_error")
@Deprecated("Cannot bind already bound Foo.", ReplaceWith(""), DeprecationLevel.ERROR)
fun <T,R> Foo<T>.bind(value: R): Foo<R> = TODO()

fun user() {
    val unbound = foo()
    val bound = foo().bind("")
    val doubleBound = foo().bind("").bind(1)
}