A continuation represents the computation that should proceed after a result becomes available. In an IO interpreter, FlatMap stores a subcomputation and a function k; once the subcomputation produces a value, k receives it and continues the program. section 13.3.1
Asynchronous APIs expose the same idea through callbacks. A Future returns immediately and accepts a continuation invoked when a value or error becomes available, allowing callback-based I/O to become compositional. section 13.5
Continuing an IO computation
In FlatMap(sub, k), the interpreter runs sub, remembers k, and applies k to the result. The IO constructor makes that continuation explicit as data rather than embedding it in immediate control flow. section 13.3.1
Listing 13.4 Creating a new IO type
enum IO[A]:
case Return(a: A) #1
case Suspend(resume: () => A) #2
case FlatMap[A, B](
sub: IO[A], k: A => IO[B]) extends IO[B] #3
def flatMap[B](f: A => IO[B]): IO[B] =
FlatMap(this, f)
def map[B](f: A => B): IO[B] =
flatMap(a => Return(f(a)))
Inspect the IO constructors and flatMap implementation to see how a continuation becomes a reusable data structure. section 13.3.1
