fun <A> sequence(fs: FList<Rand<A>>): Rand<FList<A>> = { rng ->
    when (fs) {
        is FList.Nil -> unit(FList.empty<A>())(rng)
        is FList.Cons -> {
            val (a, nrng) = fs.head(rng)
            val (xa, frng) = sequence(fs.tail)(nrng)
            FList.Cons(a, xa) to frng
        }
    }
}

This 1st answer was said to be not stack-safe, and the correct solution uses foldRight. But, I’m trying to understand the syntax. They have the ‘rng’ or the ‘nrng’ var enclosed in parantheses following the two return values:

unit(FList.empty<A>())(rng)

sequence(fs.tail)(nrng)

What does this mean? Is invoke being called via the parantheses? Or is the RNG type getting passed thru?

sequence

returns a lambda. So

sequence(...)(...)

calls

sequence

and executes the lambda returned by

sequence

. @Randall Perry

Ok thanks. Abstraction twice removed 😬

It's natural in FP, because unlike OO, it's quite common for functions to produce other functions (i.e. lambdas). You get used to it after a while, and appreciate its power.