I've just joined the Slack channel. Hello to you all. I've been working for a couples of days tryin...

# announcements

[Erik Colban]

I've just joined the Slack channel. Hello to you all.

I've been working for a couples of days trying to implement something similar to Python decorators in Kotlin. I've created a Github repository [here](https://github.com/ecolban/FunctionDecorators). The README file provides details of what I've achieved so far, and what I'd like to achieve. I'm open to suggestions.

[bbaldino]

i've wanted something like this so much in java/kotlin. will take a look!

[bbaldino]

what sort of syntax are you thinking for applying them? like what would a method you wanted to add TimeIt to look like?

[Erik Colban]

I've used extension functions. I've been playing around with top level functions and classes, but I think that extension functions work best. E.g., ::myFun.timeIt()(arg1, arg2, ...)

[bbaldino]

that would mean that a caller must know/change to call it this way, right? unless you hide the real function in the class (as _myFun or something) and then have myFun invoke _myFun via the extension?

[Erik Colban]

For an extension such as timeIt, the function provider would just provide the function any form she prefers, as a fun or a lambda. The caller would add the decorator as an extension function. For decorators such as memoize, the function provider would have to write the function to be decorated in a special form, which is what I don't like with my approach. This is discussed in detail in the README.

[Mike]

What's the advantage of this over casting the function timeit and having it take a lambda? For this specific case, kotlin has a function like this.

https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.system/measure-time-millis.html

But I'm not familiar with python decorators so guessing based on quick read on timeit.

[bbaldino]

the thing i think is nicest about them is how easy and unobtrusive they are to add. both the caller and even the method itself can stay just as they are...it's just a transparent wrapper

[Erik Colban]

measureTimeMillis is different from timeIt
1. timeIt prints the time, measureTimeMillis doesn't print anything
2. measureTimeMillis is a function that takes a code block as argument, whereas timeIt is a decorator, i.e. it takes an object or class of some type T and augments or modifies the behavior in some way and returns an object or class of the same type T.

[Erik Colban]

Both measureTimeMillis and timeIt have their usages, and comparing them is not the issue of this post. timeIt is just an example of a function decorator. Decorators are well-known and even have their own pattern; the decorator pattern. Kotlin's by delegate makes this pattern even easier to adopt. The point I wanted to address is how do you write a decorator on a recursive function in Kotlin that is applied recursively.

[bbaldino]

can you use a delegate on a function though?

[Erik Colban]

You could, but since there is only one method to override (i.e., invoke()) there is no value in doing so.

[bbaldino]

but what is the syntax, it's not by like properties, right? are you talking about doing something like

class Foo {
    fun something() = delegate {
    }
}

?

[Erik Colban]

class TimeIt<X, Y>(delegate: (X)->Y): (X)->Y by delegate {
    override fun invoke(x: X): Y {
        val start = System.currentTimeMillis()
        val result = delegate.invoke(x)
        println( System.currentTimeMillis() - start)
        return result
    }
}

[bbaldino]

oh, i thought you mean a way to 'delegate' a method to a decorator (which would invoke the method)

[Erik Colban]

The above code snippet is the standard way of specifying a decorator. You would use it:
TimeIt(::myFun)(x) or, if you were to use a lambda,
TimeIt({....})(x)
Using an extension function is an alternative, and it's a matter of taste which to prefer.

[bbaldino]

i see. i'd love to be able to do it in a way like python: at the method's definition itself and without neither the method nor the caller having to be aware of it at all. i guess this would be doable at the method definition site (wrapping it in some way) but not as neat as python