How to read this? val filter = {predicate: (Int) -> Boolean -> {collection: List<Int&...

# getting-started

[Sam]

How to read this? val filter = {predicate: (Int) -> Boolean -> {collection: List<Int> ->
collection.filter(predicate)
}}

[Sam]

The part i get is predicate being a function type with one Int parameter and returning boolean

[diesieben07]

It defines a lambda function that takes another lambda (predicate). It then returns a third lambda, which will take a collection as it's parameter and then return that collection filtered by the predicate.

[diesieben07]

Replacing the top level lambda with a function makes it more readable:

fun createFilterFunction(predicate: (Int) -> Boolean): (List<Int>) -> List<Int> {
    return { collection: List<Int> ->
        collection.filter(predicate)
    } 
}

[robin]

@Sam Please enclose code blocks with three backticks ``` to give the code proper formatting and a monospace font.

[Sam]

@diesieben07 Thanks, that helps. @robin sure

[Sam]

val testNestedLambda = { input : () -> () -> Unit -> {
        println("executing returned lambda")
        input()() } 
    }
    
    val returnedLambda = testNestedLambda{ { println( "passed lambda") } }
    
    returnedLambda()

[Sam]

Higher order functions, nostalgic functions pointers from C :-)

[Sam]

I guess, the key is to read from left to right

[Sam]

val filter : ( (Int) -> Boolean ) -> ( List<Int> ) -> List<Int> = { predicate -> { collection ->
             collection.filter( predicate )        
       } 
    }

[Sam]

Moving type declaration to the variable also helps in reading it