Effect vs Promise

In this guide, we will explore the differences between Promise and Effect, two approaches to handling asynchronous operations in TypeScript. We’ll discuss their type safety, creation, chaining, and concurrency, providing examples to help you understand their usage.

Comparing Effects and Promises: Key Distinctions

Evaluation Strategy: Promises are eagerly evaluated, whereas effects are lazily evaluated.
Execution Mode: Promises are one-shot, executing once, while effects are multi-shot, repeatable.
Interruption Handling and Automatic Propagation: Promises lack built-in interruption handling, posing challenges in managing interruptions, and don’t automatically propagate interruptions, requiring manual abort controller management. In contrast, effects come with interruption handling capabilities and automatically compose interruption, simplifying management locally on smaller computations without the need for high-level orchestration.
Structured Concurrency: Effects offer structured concurrency built-in, which is challenging to achieve with Promises.
Error Reporting (Type Safety): Promises don’t inherently provide detailed error reporting at the type level, whereas effects do, offering type-safe insight into error cases.
Runtime Behavior: The Effect runtime aims to remain synchronous as long as possible, transitioning into asynchronous mode only when necessary due to computation requirements or main thread starvation.

Type safety

Let’s start by comparing the types of Promise and Effect. The type parameter A represents the resolved value of the operation:

Promise

Promise<A>

Effect

Effect<A, Error, Context>

Here’s what sets Effect apart:

It allows you to track the types of errors statically through the type parameter Error. For more information about error management in Effect, see Expected Errors.
It allows you to track the types of required dependencies statically through the type parameter Context. For more information about context management in Effect, see Managing Services.

Creating

Success

Let’s compare creating a successful operation using Promise and Effect:

Promise

1
const success = Promise.resolve(2)

Effect

1
import { Effect } from "effect"
2

3
const success = Effect.succeed(2)

Failure

Now, let’s see how to handle failures with Promise and Effect:

Promise

1
const failure = Promise.reject("Uh oh!")

Effect

1
import { Effect } from "effect"
2

3
const failure = Effect.fail("Uh oh!")

Constructor

Creating operations with custom logic:

Promise

1
const task = new Promise<number>((resolve, reject) => {
2
  setTimeout(() => {
3
    Math.random() > 0.5 ? resolve(2) : reject("Uh oh!")
4
  }, 300)
5
})

Effect

1
import { Effect } from "effect"
2

3
const task = Effect.gen(function* () {
4
  yield* Effect.sleep("300 millis")
5
  return Math.random() > 0.5 ? 2 : yield* Effect.fail("Uh oh!")
6
})

Thenable

Mapping the result of an operation:

map

Promise

1
const mapped = Promise.resolve("Hello").then((s) => s.length)

Effect

1
import { Effect } from "effect"
2

3
const mapped = Effect.succeed("Hello").pipe(
4
  Effect.map((s) => s.length),
5
  // or Effect.andThen((s) => s.length)
6
)

flatMap

Chaining multiple operations:

Promise

1
const flatMapped = Promise.resolve("Hello").then((s) =>
2
  Promise.resolve(s.length),
3
)

Effect

1
import { Effect } from "effect"
2

3
const flatMapped = Effect.succeed("Hello").pipe(
4
  Effect.flatMap((s) => Effect.succeed(s.length)),
5
  // or Effect.andThen((s) => Effect.succeed(s.length))
6
)

Comparing Effect.gen with async/await

If you are familiar with async/await, you may notice that the flow of writing code is similar.

Let’s compare the two approaches:

Promise

1
const increment = (x: number) => x + 1
2

3
const divide = (a: number, b: number): Promise<number> =>
4
  b === 0
5
    ? Promise.reject(new Error("Cannot divide by zero"))
6
    : Promise.resolve(a / b)
7

8
const task1 = Promise.resolve(10)
9

10
const task2 = Promise.resolve(2)
11

12
const program = async function () {
13
  const a = await task1
14
  const b = await task2
15
  const n1 = await divide(a, b)
16
  const n2 = increment(n1)
17
  return `Result is: ${n2}`
18
}
19

20
program().then(console.log) // Output: "Result is: 6"

Effect

1
import { Effect } from "effect"
2

3
const increment = (x: number) => x + 1
4

5
const divide = (a: number, b: number): Effect.Effect<number, Error> =>
6
  b === 0
7
    ? Effect.fail(new Error("Cannot divide by zero"))
8
    : Effect.succeed(a / b)
9

10
const task1 = Effect.promise(() => Promise.resolve(10))
11

12
const task2 = Effect.promise(() => Promise.resolve(2))
13

14
const program = Effect.gen(function* () {
15
  const a = yield* task1
16
  const b = yield* task2
17
  const n1 = yield* divide(a, b)
18
  const n2 = increment(n1)
19
  return `Result is: ${n2}`
20
})
21

22
Effect.runPromise(program).then(console.log)
23
// Output: "Result is: 6"

It’s important to note that although the code appears similar, the two programs are not identical. The purpose of comparing them side by side is just to highlight the resemblance in how they are written.

Concurrency

Promise.all()

Promise

1
const task1 = new Promise<number>((resolve, reject) => {
2
  console.log("Executing task1...")
3
  setTimeout(() => {
4
    console.log("task1 done")
5
    resolve(1)
6
  }, 100)
7
})
8

9
const task2 = new Promise<number>((resolve, reject) => {
10
  console.log("Executing task2...")
11
  setTimeout(() => {
12
    console.log("task2 done")
13
    reject("Uh oh!")
14
  }, 200)
15
})
16

17
const task3 = new Promise<number>((resolve, reject) => {
18
  console.log("Executing task3...")
19
  setTimeout(() => {
20
    console.log("task3 done")
21
    resolve(3)
22
  }, 300)
23
})
24

25
const program = Promise.all([task1, task2, task3])
26

27
program.then(console.log, console.error)
28
/*
29
Output:
30
Executing task1...
31
Executing task2...
32
Executing task3...
33
task1 done
34
task2 done
35
Uh oh!
36
task3 done
37
*/

Effect

1
import { Effect } from "effect"
2

3
const task1 = Effect.gen(function* () {
4
  console.log("Executing task1...")
5
  yield* Effect.sleep("100 millis")
6
  console.log("task1 done")
7
  return 1
8
})
9

10
const task2 = Effect.gen(function* () {
11
  console.log("Executing task2...")
12
  yield* Effect.sleep("200 millis")
13
  console.log("task2 done")
14
  return yield* Effect.fail("Uh oh!")
15
})
16

17
const task3 = Effect.gen(function* () {
18
  console.log("Executing task3...")
19
  yield* Effect.sleep("300 millis")
20
  console.log("task3 done")
21
  return 3
22
})
23

24
const program = Effect.all([task1, task2, task3], {
25
  concurrency: "unbounded",
26
})
27

28
Effect.runPromise(program).then(console.log, console.error)
29
/*
30
Output:
31
Executing task1...
32
Executing task2...
33
Executing task3...
34
task1 done
35
task2 done
36
(FiberFailure) Error: Uh oh!
37
*/

Promise.allSettled()

Promise

1
const task1 = new Promise<number>((resolve, reject) => {
2
  console.log("Executing task1...")
3
  setTimeout(() => {
4
    console.log("task1 done")
5
    resolve(1)
6
  }, 100)
7
})
8

9
const task2 = new Promise<number>((resolve, reject) => {
10
  console.log("Executing task2...")
11
  setTimeout(() => {
12
    console.log("task2 done")
13
    reject("Uh oh!")
14
  }, 200)
15
})
16

17
const task3 = new Promise<number>((resolve, reject) => {
18
  console.log("Executing task3...")
19
  setTimeout(() => {
20
    console.log("task3 done")
21
    resolve(3)
22
  }, 300)
23
})
24

25
const program = Promise.allSettled([task1, task2, task3])
26

27
program.then(console.log, console.error)
28
/*
29
Output:
30
Executing task1...
31
Executing task2...
32
Executing task3...
33
task1 done
34
task2 done
35
task3 done
36
[
37
  { status: 'fulfilled', value: 1 },
38
  { status: 'rejected', reason: 'Uh oh!' },
39
  { status: 'fulfilled', value: 3 }
40
]
41
*/

Effect

1
import { Effect } from "effect"
2

3
const task1 = Effect.gen(function* () {
4
  console.log("Executing task1...")
5
  yield* Effect.sleep("100 millis")
6
  console.log("task1 done")
7
  return 1
8
})
9

10
const task2 = Effect.gen(function* () {
11
  console.log("Executing task2...")
12
  yield* Effect.sleep("200 millis")
13
  console.log("task2 done")
14
  return yield* Effect.fail("Uh oh!")
15
})
16

17
const task3 = Effect.gen(function* () {
18
  console.log("Executing task3...")
19
  yield* Effect.sleep("300 millis")
20
  console.log("task3 done")
21
  return 3
22
})
23

24
const program = Effect.forEach(
25
  [task1, task2, task3],
26
  (task) => Effect.either(task), // or Effect.exit
27
  {
28
    concurrency: "unbounded",
29
  },
30
)
31

32
Effect.runPromise(program).then(console.log, console.error)
33
/*
34
Output:
35
Executing task1...
36
Executing task2...
37
Executing task3...
38
task1 done
39
task2 done
40
task3 done
41
[
42
  {
43
    _id: "Either",
44
    _tag: "Right",
45
    right: 1
46
  }, {
47
    _id: "Either",
48
    _tag: "Left",
49
    left: "Uh oh!"
50
  }, {
51
    _id: "Either",
52
    _tag: "Right",
53
    right: 3
54
  }
55
]
56
*/

Promise.any()

Promise

1
const task1 = new Promise<number>((resolve, reject) => {
2
  console.log("Executing task1...")
3
  setTimeout(() => {
4
    console.log("task1 done")
5
    reject("Something went wrong!")
6
  }, 100)
7
})
8

9
const task2 = new Promise<number>((resolve, reject) => {
10
  console.log("Executing task2...")
11
  setTimeout(() => {
12
    console.log("task2 done")
13
    resolve(2)
14
  }, 200)
15
})
16

17
const task3 = new Promise<number>((resolve, reject) => {
18
  console.log("Executing task3...")
19
  setTimeout(() => {
20
    console.log("task3 done")
21
    reject("Uh oh!")
22
  }, 300)
23
})
24

25
const program = Promise.any([task1, task2, task3])
26

27
program.then(console.log, console.error)
28
/*
29
Output:
30
Executing task1...
31
Executing task2...
32
Executing task3...
33
task1 done
34
task2 done
35
2
36
task3 done
37
*/

Effect

1
import { Effect } from "effect"
2

3
const task1 = Effect.gen(function* () {
4
  console.log("Executing task1...")
5
  yield* Effect.sleep("100 millis")
6
  console.log("task1 done")
7
  return yield* Effect.fail("Something went wrong!")
8
})
9

10
const task2 = Effect.gen(function* () {
11
  console.log("Executing task2...")
12
  yield* Effect.sleep("200 millis")
13
  console.log("task2 done")
14
  return 2
15
})
16

17
const task3 = Effect.gen(function* () {
18
  console.log("Executing task3...")
19
  yield* Effect.sleep("300 millis")
20
  console.log("task3 done")
21
  return yield* Effect.fail("Uh oh!")
22
})
23

24
const program = Effect.raceAll([task1, task2, task3])
25

26
Effect.runPromise(program).then(console.log, console.error)
27
/*
28
Output:
29
Executing task1...
30
Executing task2...
31
Executing task3...
32
task1 done
33
task2 done
34
2
35
*/

Promise.race()

Promise

1
const task1 = new Promise<number>((resolve, reject) => {
2
  console.log("Executing task1...")
3
  setTimeout(() => {
4
    console.log("task1 done")
5
    reject("Something went wrong!")
6
  }, 100)
7
})
8

9
const task2 = new Promise<number>((resolve, reject) => {
10
  console.log("Executing task2...")
11
  setTimeout(() => {
12
    console.log("task2 done")
13
    reject("Uh oh!")
14
  }, 200)
15
})
16

17
const task3 = new Promise<number>((resolve, reject) => {
18
  console.log("Executing task3...")
19
  setTimeout(() => {
20
    console.log("task3 done")
21
    resolve(3)
22
  }, 300)
23
})
24

25
const program = Promise.race([task1, task2, task3])
26

27
program.then(console.log, console.error)
28
/*
29
Output:
30
Executing task1...
31
Executing task2...
32
Executing task3...
33
task1 done
34
Something went wrong!
35
task2 done
36
task3 done
37
*/

Effect

1
import { Effect } from "effect"
2

3
const task1 = Effect.gen(function* () {
4
  console.log("Executing task1...")
5
  yield* Effect.sleep("100 millis")
6
  console.log("task1 done")
7
  return yield* Effect.fail("Something went wrong!")
8
})
9

10
const task2 = Effect.gen(function* () {
11
  console.log("Executing task2...")
12
  yield* Effect.sleep("200 millis")
13
  console.log("task2 done")
14
  return yield* Effect.fail("Uh oh!")
15
})
16

17
const task3 = Effect.gen(function* () {
18
  console.log("Executing task3...")
19
  yield* Effect.sleep("300 millis")
20
  console.log("task3 done")
21
  return 3
22
})
23

24
const program = Effect.raceAll([task1, task2, task3].map(Effect.either)) // or Effect.exit
25

26
Effect.runPromise(program).then(console.log, console.error)
27
/*
28
Output:
29
Executing task1...
30
Executing task2...
31
Executing task3...
32
task1 done
33
{
34
  _id: "Either",
35
  _tag: "Left",
36
  left: "Something went wrong!"
37
}
38
*/

FAQ

Question. What is the equivalent of starting a promise without immediately waiting for it in Effects?

1
const task = (delay: number, name: string) =>
2
  new Promise((resolve) =>
3
    setTimeout(() => {
4
      console.log(`${name} done`)
5
      return resolve(name)
6
    }, delay),
7
  )
8

9
export async function program() {
10
  const r0 = task(2_000, "long running task")
11
  const r1 = await task(200, "task 2")
12
  const r2 = await task(100, "task 3")
13
  return {
14
    r1,
15
    r2,
16
    r0: await r0,
17
  }
18
}
19

20
program().then(console.log)
21
/*
22
Output:
23
task 2 done
24
task 3 done
25
long running task done
26
{ r1: 'task 2', r2: 'task 3', r0: 'long running promise' }
27
*/

Answer: You can achieve this by utilizing Effect.fork and Fiber.join.

1
import { Effect, Fiber } from "effect"
2

3
const task = (delay: number, name: string) =>
4
  Effect.gen(function* () {
5
    yield* Effect.sleep(delay)
6
    console.log(`${name} done`)
7
    return name
8
  })
9

10
const program = Effect.gen(function* () {
11
  const r0 = yield* Effect.fork(task(2_000, "long running task"))
12
  const r1 = yield* task(200, "task 2")
13
  const r2 = yield* task(100, "task 3")
14
  return {
15
    r1,
16
    r2,
17
    r0: yield* Fiber.join(r0),
18
  }
19
})
20

21
Effect.runPromise(program).then(console.log)
22
/*
23
Output:
24
task 2 done
25
task 3 done
26
long running task done
27
{ r1: 'task 2', r2: 'task 3', r0: 'long running promise' }
28
*/