Why does the compiler (IntelliJ) not accept this

fold

lambda? error is

Kotlin: Type mismatch: inferred type is Unit but Int was expected

fun <T, Int> Iterable<T>.sum(tx: (T) -> Int): Int {
    return map { tx(it) }
        .also { it -> println(it) }
        .fold(0 as Int) { acc, next -> acc + next }
}

typo

<T, Int>

Thanks

By declaring

<T, Int>

you have actually made a new generic parameter called

Int

which is not the same as (and is hiding) the real

Int

type. If you just drop that generic parameter from the signature, it should work.