Implementation of ExpressJS Middleware functions for Kotlin.

MiddlewareFunction.kt

typealias Next<A, R> = (param: A) -> R

typealias Middleware<A, R> = (param: A, next: Next<A, R>) -> R

private class NextImpl<A, R>(
  private val middleware: Middleware<A, R>,
  private val next: Next<A, R>
) : Next<A, R> {
  override fun invoke(param: A): R {
    return middleware(param, next)
  }
}

fun <A, R> applyMiddlewares(next: Next<A, R>, middlewares: List<Middleware<A, R>>): Next<A, R> {
  return middlewares.foldRight(next, { middleware, acc -> NextImpl(middleware, acc) })
}

Middleware functions can perform the following tasks:

• Execute any code.
• Make changes to the input and the output objects.
• End the input-output cycle.
• Call the next middleware function in the stack.

If the current middleware function does not end the input-output cycle, it must call next() to pass control to the next middleware function. Otherwise, the request will be left hanging.