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kotlinlang

`fun Int.foo() { ... }` is an <https://kotlinlang.org/docs/reference/extensions.html|extension function>, called as `5.foo()`. `fun foo(): Int { ... }` is a method that returns a value of type `Int`. These are not independent, you can also have an extension function `fun Int.foo(): Int { ... }` that returns an `Int`, which could be, for example, `fun Int.foo(): Int { return this + 5 }` .

<@UP5818HQF> Int can be a base class, can it be a super class?

The extension method would work for `Int` and all subclasses of `Int`, although `Int` is not an open class and therefore cannot be subclassed.

<@UP5818HQF> `fun Int.foo(){ val c1=this@foo}` what this c1 is?

Try running it in <https://try.kotlinlang.org/> and see what happens.

you should probably go through the Kotlin Koans, it covers a lot of the stuff you might not be familiar with from other programming languages

there’s also a Kotlin guide for Python programmers if that’s relevant to you

also the `this@foo` question is more or less answered by one of the docs links I shared earlier — you should try experimenting with examples in the docs in the Kotlin playground or in a scratch file, or even just in a `main()` function in your existing project