Now, I could do something like:

constructor(vararg cells: Cell, f: (List<T>) -> T)

but then the lambdas would go from

{ x, y -> x + y }

to something like

{ it[0] + it[1] }

which I guess would be ok, but not the most readable.

sdavids: In Kotlin 1.1 you can use destructuring in lambdas so that with

constructor(vararg cells: Cell, f: (List<T>) -> T)

you can write

{ (x, y) -> x + y}

. One caveat to this is that it will be up to the call-point to destructure the right number of params.