EventCallback<Float> { if (it > 0) printl...

# announcements

[Ifvwm]

EventCallback<Float> { if (it > 0) println("ouch!") else println("1") } I don't understand the syntax, put a type before a lambda, is ?

[Shawn]

How familiar are you with generics

[Shawn]

you’re not putting a type before a lambda, you’re passing in a type parameter to a genericized function

[Ifvwm]

I know generics, but I don't know how generics work with lambda

[Shawn]

have you looked at this particular function definition?

[Shawn]

if I had to guess it probably looks something like this

fun <T> EventCallback<T>(block: (T) -> Unit)

[Ifvwm]

"""

[Ifvwm]

object : EventCallback<Float> {
override fun onEvent(arg0: Float): Unit = if (arg0 > 0) println("ouch!")
else println("1")

[Shawn]

that looks more accurate, actually, EventCallback<T> would be an interface

[Shawn]

in this case a SAM interface

[Shawn]

you also want to use triple backticks ` ``` ` to wrap code, and shift-enter to insert newlines without sending messages

[Ifvwm]

object : EventCallback<Float> {
            override fun onEvent(arg0: Float): Unit = if (arg0 > 0) println("ouch!")
             else println("1")

[Ifvwm]

I don't know this stuff turn to

EventCallback<Float> { if (it > 0) println("ouch!") else println("1") }

[Shawn]

so if you ctrl/cmd+b into EventCallback you’ll see the definition of the interface

[Shawn]

probably looks like this if I had to guess

[Shawn]

public interface EventCallback<T> {
  void onEvent(T arg0);
}

[Shawn]

with a single method defined, making it a Single Abstract Method interface, which can be converted to a lambda

[Ifvwm]

aha,

[Shawn]

Kotlin lets you implement SAMs with a single lambda because it can substitute in for that single method

[Shawn]

you need to provide a parameter for T if it can’t be inferred

[Ifvwm]

ok

[Shawn]

you could probably do something like this if you wanted

EventCallback { param: Float ->
  if (param > 0) println("ouch!") else println("1")
}

[Ifvwm]

what this `it' here means?

[Ifvwm]

should it be param?

[Shawn]

https://kotlinlang.org/docs/reference/lambdas.html#it-implicit-name-of-a-single-parameter

[Shawn]

you really should read through the docs if you haven’t already

[Ifvwm]

fine

[Ifvwm]

but your code is
```

[Ifvwm]

param: Float ->
  if (it > 0)

[Shawn]

That might not work actually, depends on if type inference is smart enough in this case

[Ifvwm]

I thought it should be

it: Float ->
if (it>0)

[Shawn]

you can explicitly type it or you can pass an explicit parameter to the interface

[Shawn]

ah I see

[Shawn]

I typo’d, my bad

[Ifvwm]

this lambda stuff in kotlin seems wonderful!

[Shawn]

They’re pretty great, I’d consider their use to be a defining feature of the language

[Ifvwm]

aha "That's because Kotlin has SAM ("single abstract method") only for Java interfaces."