Combining Function Type in Kotlin with 'or'

Question

I'm pretty new to Kotlin and wondered if you can combine a list of function type into a single function.

I have an interface presents a filter, and some predefined filters:

interface Filter : (Int) -> Boolean {
    val a: Int
}

class FilterA(override val a: Int) : Filter {

    override fun invoke(numberToFilter: Int): Boolean {
        ....
    }
}

class FilterB(override val a: Int) : Filter {

    override fun invoke(numberToFilter: Int): Boolean {
        ....
    }
}

I'm now have a list of Filter object, which I want to combine using inclusive disjunction OR , so that I can passed that to a filter a list of Int. For example:

val combinedFilter = listOf(FilterA(a), FilterB(b)). { `do something here` }

val filteredInt = listToBeFiltered.filter { combinedFilter }

However, I'm somehow stuck in defining the do something here part. I believe you can use reduce method to do it, but it keeps telling me error somehow. Below is how I tried it

val combinedFilter = listOfFilters.reduce { filterA, filterB -> { filterA or filterB } }

infix fun <T> ((T) -> Boolean).or(that: (T) -> Boolean): (T) -> Boolean = { this(it) || that(it) }

But got

Type mismatch.
Required:
Filter
Found:
() → (Int) → Boolean

I believe I did something wrong with the syntax. Hope that you guys can help. Thanks.

Answer 1

That's because you wrapped the expression in an extra lambda, also the type of the accumulator in reduce wiil be Filter for your listOfFilters but or returns (T) -> Boolean . You can fix this by specifying the type of accumulator and getting rid of extra lambda:

val combinedFilter = listOfFilters.reduce<(Int) -> Boolean, Filter> { filterA, filterB -> filterA or filterB }

But there is a better way to do it without allocating a lot of lambdas:

val combinedFilter = { x: Int -> listOfFilters.any { filter -> filter(x) } }
val filteredInt = listToBeFiltered.filter(combinedFilter)

or

fun <T> List<(T) -> Boolean>.combineAny() = { x: T -> this.any { filter -> filter(x) } }
val combinedFilter = listOfFilters.combineAny()
val filteredInt = listToBeFiltered.filter(combinedFilter)