Composing functions in Kotlin

compose.kt

fun compose<T>( fa: ( T ) -> T, fb : ( T ) -> T ) : ( T ) -> T {
  return { ( a : T ) : T -> fb( fa( a ) ) }
}

fun main( args : Array<String> ) {
  val composed = compose<Int>( { ( a : Int ) : Int -> a + 10 },  // add 10
                               { ( a : Int ) : Int -> a * 2  } ) // multiply by 2
  println( composed( 2 ) )  // prints 24
}

The compose method can be written:

fun compose<T>( fa: Function1<T,T>, fb : Function1<T,T> ) : Function1<T,T> {
  return { ( a : T ) : T -> fb( fa( a ) ) }
}

Which is a bit less typing ;-)

Or you can create Extended function for Function1

infix public fun<V, T, R> Function1<T, R>.compose(before: (V) -> T): (V) -> R {
    return { v: V -> this(before(v)) }
}

Is the Function1 extension for function compose still needed or has it become part of the standard library?

It should be part of the standard library if its not already.