What does it mean to have a function that is invok...

# codereview

[Kev]

What does it mean to have a function that is invoked like .map(::MyDto) or .map(::println) ?

[Joffrey]

::println is a function reference. map is a higher order function, meaning that it takes a function as a parameter. A value of type function can be in different forms. You can pass a lambda expression like { println(it) }, or you can pass a variable of a function type, or you can pass a reference to a function with this ::funName syntax.

[Joffrey]

It's important to note that you're not directly calling println or the MyDto constructor and passing the result to map here, you're passing the function itself. This is what higher-order functions are all about.

[Kev]

Thank you for the explanation. Is it idiomatic kotlin to prefer high order functions over inline lambda expressions?

[Kev]

Also, how would you construct aligning functions that conform to higher order functions? Could you please provide a simple example?

[Joffrey]

Sorry I think there is some confusion here. The higher-order function here is map. The :: syntax denotes a function reference. Now whether that is idiomatic is debatable. Lambdas are extremely common, and make no perf difference when the higher-order function is inline. Function references are less common I would say, because you can't use them if the signature of the function doesn't match with what the higher order function expects. I wouldn't say one is more idiomatic than the other, though.

[Joffrey]

Also, how would you construct aligning functions that conform to higher order functions? Could you please provide a simple example?

Sorry I don't get the question. Could you please elaborate?

[Joffrey]

By the way, I would suggest you get familiar with this topic from the docs:
https://kotlinlang.org/docs/lambdas.html