Control operations and handlers

A control operation lets a handler decide how the caller continues. Use one when a dependency must pause a program, answer a request, or end the program early.

Declare and perform an operation

// approval.effeck
export error ReviewBusy { request: string }

export control Approval {
  approve(request: string): boolean!ReviewBusy
}

export function release(version: string) {
  if (!Approval.approve(`deploy ${version} to staging`)) return "staging declined"
  if (!Approval.approve(`deploy ${version} to production`)) return "production declined"
  return `released ${version}`
}
// string!ReviewBusy!Approval

Approval.approve returns a boolean at the call. Approval stays in the program's requirements until a handler answers it. Each abstract operation states all three channels of its contract.

Control, handle, resume, resumeWith, and abandon are globals in .effeck files.

Run one program with two policies

// policies.effeck
import { Approval, release } from "./approval.effeck"

export function autoApprove(version: string) {
  try {
    return release(version)
  } handle Approval.approve(_request) {
    resume(true)
  }
}

export function scriptedReview(version: string, decisions: ReadonlyArray<boolean>) {
  let next = 0
  try {
    return release(version)
  } handle Approval.approve(_request) {
    const approved = decisions[next] ?? false
    next++
    resume(approved)
  }
}

export function comparePolicies() {
  return [autoApprove("v1"), scriptedReview("v1", [true, false])]
  // ["released v1", "production declined"]
}

The business program stays the same. A handle clause after a try block names an operation, Approval.approve, and parameters that take the operation's parameter types. Each time the try block performs the operation, the clause runs.

resume(true) continues the try block with true as the operation's result. The handler is deep: the next approval request reaches the same clause. A clause that ends with resume runs in constant stack space, however many times the block performs the operation.

A return in the try block or in a clause returns from the enclosing function, and break and continue leave its loops. A statement answers every operation of each control it names: give each operation a clause (EK2108). One statement can handle several controls.

A clause can work after resume. The try block runs first, until it finishes. Then the clause goes on, and when the clause ends, the statement ends the way the try block did: a return in the try block takes effect after the clause, as a pending return waits for a finally block.

A continuation can be consumed once. Call resume, resumeWith, and abandon in the clause's own code, not in a function nested inside it. The runtime rejects a second resume.

Stop and clean up

// stopping.effeck
import { Approval } from "./approval.effeck"

function deployment() {
  try {
    Approval.approve("deploy to production")
    return "deployed"
  } finally {
    Console.log("deployment hold released")
  }
}

export function stopDeployment() {
  try {
    return deployment()
  } handle Approval.approve(_request) {
    abandon()
    Console.log("cleanup finished")
    return "cancelled"
  }
}

abandon() runs the try block's cleanup before the clause continues. It skips the block's catch blocks and runs its finally blocks and resource releases. Cleanup failures still fail the operation.

A clause that ends without resuming also stops the try block and cleans it up first. The statement then completes, and the function continues after it. Use abandon() when the clause needs cleanup to finish before its next statement.

Resume with an expected failure

// busy.effeck
import { Approval, ReviewBusy } from "./approval.effeck"

export function busyReviewer() {
  try {
    return Approval.approve("deploy now") ? "approved" : "declined"
  } handle Approval.approve(request) {
    resumeWith(() => { throw new ReviewBusy({ request }) })
  } catch (error) {
    return `review unavailable: ${error.request}`
  }
}

resumeWith runs its program at the original operation call, inside the try block. This program can raise only errors declared by the operation, and it must need no services. Read services in the clause first, then capture their values in the program passed to resumeWith.

Catch clauses and a finally block may follow the handle clauses. They cover the try block and the clauses, so here the catch clause sees the ReviewBusy that the operation call raised.

Use handle as an expression

The statement is short for the handle function, which takes a program and returns its result:

// expression.effeck
import { Approval, release } from "./approval.effeck"

export function countReleased(versions: ReadonlyArray<string>) {
  let released = 0
  for (const version of versions) {
    const outcome = handle(() => release(version), Approval, {
      approve(_request, k) {
        return resume(k, true)
      }
    })
    if (outcome.startsWith("released")) released++
  }
  return released
}

handle(program, Control, clauses, onReturn?) takes a program, the control class, and a clause for every operation. A clause receives the operation's arguments followed by k, the continuation, which resume(k, value), resumeWith(k, program), and abandon(k) take. resume returns the resumed program's result. The optional fourth argument of handle transforms normal returns. In this preview, resume is typed with the program's original result, or never when that fourth argument is supplied; avoid inspecting the resumed result when clauses return different types. Keep k inside its clause; never save it or return it.

Keep handlers on the executing fiber

// concurrent.effeck
import { Approval } from "./approval.effeck"

export function approveAll(requests: ReadonlyArray<string>) {
  return forEach(requests, (request) => {
    try {
      return Approval.approve(request)
    } handle Approval.approve(_request) {
      resume(true)
    }
  }, { concurrency: 4 })
}

Handle the operation inside each concurrent branch. A handler on the parent fiber cannot answer a child fiber's control operations.

Stream operators run a producer on its own fiber too. Handle any control requirements inside the producer before passing it to a Stream operator. See Producers.