Heyo Inspired by ArrowKt s monad comprehensions I wanted to

Question

Heyo Inspired by ArrowKt s monad comprehensions I wanted to implement one for Kotlin s nullable types The problem is that I don t really understand functional interfaces My goal Have a `bind` extension function on all nullable types that is only available inside of a `nullable` block I tried to do it like this but the `bind` function is not reachable What s the proper way to do this ```class BindNullableException IllegalAccessException fun interface NullableBlock lt A gt gt A fun lt A gt A bind = this throw BindNullableException fun lt A gt nullable block NullableBlock lt A gt A = try block catch e BindNullableException null fun main val a = getNullableA bind should not work val nullableC = nullable val a = getNullableA bind should work val b = getNullableB bind getNullableC a b fun getNullableA String = null fun getNullableB String = null fun getNullableC a String b String String = null```

tseisel · Answer

Not sure if this could help you but Arrow has an `Option lt T gt ` type that does exactly what you want maybe you can look at its source code to understand how it works internally <https arrow kt io docs apidocs arrow core arrow core option >

tseisel · Answer

I doubt that they use exceptions at all in their implementation because they explicitly advocate against them <https arrow kt io docs patterns error handling the issues with exceptions>

rednifre · Answer

Yeah they do it exactly like this functional interface and throwing a ShortCircuit exception Instead of `nullable` being a function that takes a block in their implementation it s an invokable object though I m also not sure why their monad comprehensions are suspend functions that seems unnecessary

rednifre · Answer

Figured it out this works ```class BindNullableException IllegalAccessException class NullableBlock lt A gt val func NullableBlock lt A gt gt A gt A fun lt B gt B bind B = this throw BindNullableException override fun invoke A = func throw BindNullableException Suppress ClassName object nullable operator fun lt A gt invoke func NullableBlock lt A gt gt A A = try NullableBlock func catch e BindNullableException null fun main val cantWork = getNullableA bind should not work val nullableC = nullable val a = getNullableA bind should work val b = getNullableB a bind getNullableC a b println Got $nullableC fun getNullableA String = null fun getNullableB a String String = null fun getNullableC a String b String String = null```