Just found myself writing these… ```fun Runnable.toFun(): () -> Unit = { this.run() } fun Functio...

# announcements

[Rob Elliot]

Just found myself writing these…

fun Runnable.toFun(): () -> Unit = { this.run() }
fun Function0<Unit>.toRunnable(): Runnable = Runnable { this() }

fun <T> Callable<T>.toFun(): () -> T = { this.call() }
fun <T> Function0<T>.toCallable(): Callable<T> = Callable { this() }

and then wondered a) do they already exist or b) is there a better way?

[Sola]

why would you do this?

[Rob Elliot]

To convert back and forward between kotlin functions and Runnable / Callable.

[Sola]

What's the use case of this? Is there any particular library requires you do this?

[Rob Elliot]

I’m writing an implementation of ExecutorService that maintains an SLF4J MDC in the asynchronous tasks. ExecutorService accepts Runnable and Callable<T>. Avoiding repetition in the implementation requires using kotlin functions. The net result is I need to convert backwards and forwards between () -> Unit and Runnable, and between () -> T and Callable<T>, and it’s more legible if I don’t keep nesting curly braces.

[Sola]

Avoiding repetition in the implementation requires using kotlin functions.

Could you explain more about this?

[Sola]

with SAM conversion you can just use Runnable interface in your kotlin code as in Java world.

[nanodeath]

btw you can say say ((T) -> Unit) instead of Function0<T> I believe

[Rob Elliot]

For me this does not compile:

class Foo {
  fun takesFunction(task: () -> Unit) = task()
  fun takesRunnable(task: Runnable) = takesFunction(task)
}

[Sola]

just use Runnable is fine I guess?
you can write the following

fun foo(fn: Runnable) = fn.run()
    fun bar() = foo { TODO() }

[Rob Elliot]

OK, there is a simpler way using method references:

override fun execute(command: Runnable) = foo(command::run)
override fun <T> submit(task: Callable<T>) = foo(task::call)
private fun foo(task: () -> T) = TODO()

[Rob Elliot]

Harder is coercing in a map call…

override fun <T> invokeAll(tasks: Collection<out Callable<T>>): List<Future<T>> {
  val wrappedTasks = tasks.map { task -> foo(task::call) }
  delegate.invokeAll(wrappedTasks) // does not compile, List<() -> T> is not a Collection<out Callable<T>>
}
private fun <T> foo(task: () -> T): () -> T = TODO()

You can trick the compiler with an identity method:

override fun <T> invokeAll(tasks: Collection<out Callable<T>>): List<Future<T>> {
  val wrappedTasks = tasks.map { task -> coerce(foo(task::call)) }
  delegate.invokeAll(wrappedTasks) // compiles!
}
private fun <T> coerce(callable: Callable<T>) = callable
private fun <T> foo(task: () -> T): () -> T = TODO()

Interestingly you cannot change coerce to make Callable<T> the receiver, it then stops compiling.

[Rob Elliot]

(And of course you can’t inline coerce either, which is the sort of thing which must bring Haskell programmers out in hives…)

[Nir]

Couldn't you define your own interface that inherits from runnable/callable

[Nir]

and makes the run/call function, interface

[Nir]

hmm maybe the conversion only goes the other way. I thought that SAMs covered this case but I must just be confused.

[Nir]

Okay, so it looks like even if you don't control Runnable/Callable, you can define your own interfaces beneath them, and make things a little nicer

[Nir]

interface Runnable {
    fun bar()
}

fun interface RunnableSam : Runnable {
    override fun bar()
}

fun accepts(x: RunnableSam) {}

fun main() { 
   accepts( { } )
}

[Rob Elliot]

I’m implementing an interface I don’t control whose methods accept Runnable and Callable

[Nir]

yeah, not much you can do then

[Rob Elliot]

FWIW it eventually occurred to me that I was better off creating a new interface KExecutorService with a single fun <T> submit(task: () -> T): Future<T> and a trivial adapter implementation that delegates to a java.util.ExecutorService. Then writing a decorator to do things around the function call became trivial without having to worry about Runnable and Callable.

[Rob Elliot]

But until that occurred to me, this is where I’d got to:
https://gist.github.com/Mahoney/a8208dcbf97e0bd0d0002ef4cee892d2