Exit

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An Exit<A, E> describes the result of executing an Effect workflow.

There are two possible values for an Exit<A, E>:

  • Exit.Success contains a success value of type A.
  • Exit.Failure contains a failure Cause of type E.

Matching

To handle the different outcomes of an Exit, we can use the Exit.match function:

ts
import { Effect, Exit } from "effect"
 
const simulatedSuccess = Effect.runSyncExit(Effect.succeed(1))
 
Exit.match(simulatedSuccess, {
onFailure: (cause) =>
console.error(`Exited with failure state: ${cause._tag}`),
onSuccess: (value) => console.log(`Exited with success value: ${value}`)
})
// Output: "Exited with success value: 1"
 
const simulatedFailure = Effect.runSyncExit(Effect.fail("error"))
 
Exit.match(simulatedFailure, {
onFailure: (cause) =>
console.error(`Exited with failure state: ${cause._tag}`),
onSuccess: (value) => console.log(`Exited with success value: ${value}`)
})
// Output: "Exited with failure state: Fail"
ts
import { Effect, Exit } from "effect"
 
const simulatedSuccess = Effect.runSyncExit(Effect.succeed(1))
 
Exit.match(simulatedSuccess, {
onFailure: (cause) =>
console.error(`Exited with failure state: ${cause._tag}`),
onSuccess: (value) => console.log(`Exited with success value: ${value}`)
})
// Output: "Exited with success value: 1"
 
const simulatedFailure = Effect.runSyncExit(Effect.fail("error"))
 
Exit.match(simulatedFailure, {
onFailure: (cause) =>
console.error(`Exited with failure state: ${cause._tag}`),
onSuccess: (value) => console.log(`Exited with success value: ${value}`)
})
// Output: "Exited with failure state: Fail"

In this example, we first simulate a successful Effect execution using Effect.runSyncExit and Effect.succeed. We then handle the Exit using Exit.match, where the onSuccess callback prints the success value.

Next, we simulate a failure using Effect.runSyncExit and Effect.fail, and handle the Exit again using Exit.match, where the onFailure callback prints the failure state.

Exit vs Either

An Exit<A, E> is conceptually an Either<Cause<E>, A>. However, it's important to note that Cause encompasses more states than just the expected error type E. It also includes other states such as interruption and defects (unexpected errors), as well as the possibility of combining multiple Cause values together.

Exit vs Effect

The Exit data type is a subtype of the Effect type, which means that an Exit is itself an Effect. The reason for this is that a result can be considered as a constant computation. Technically, Effect.succeed is an alias for Exit.succeed, and Effect.fail is an alias for Exit.fail (avoiding conversions between Exit and Effect is important for performance, as boxing and unboxing have a cost).