data-exit.mdx

title	Modeling Effect Results with Exit
id	data-exit
skillLevel	intermediate
applicationPatternId	core-concepts
summary	Use Exit<E, A> to represent the result of running an Effect, capturing both success and failure (including defects) in a type-safe way.
tags	Exit effect result error-handling concurrency data-type
rule	description Use Exit to capture the outcome of an Effect, including success, failure, and defects, for robust error handling and coordination.
related	data-cause data-either
author	PaulJPhilp
lessonOrder	8

Modeling Effect Results with Exit

Guideline

Use the Exit<E, A> data type to represent the result of running an Effect, capturing both success and failure (including defects) in a type-safe way.
Exit is especially useful for coordinating concurrent workflows and robust error handling.

Rationale

When running or supervising effects, you often need to know not just if they succeeded or failed, but how they failed (e.g., error vs. defect).
Exit provides a complete, type-safe summary of an effect's outcome.

Good Example

import { Effect, Exit } from "effect";

// Run an Effect and capture its Exit value
const program = Effect.succeed(42);

const runAndCapture = Effect.runPromiseExit(program); // Promise<Exit<never, number>>

// Pattern match on Exit
runAndCapture.then((exit) => {
  if (Exit.isSuccess(exit)) {
    console.log("Success:", exit.value);
  } else if (Exit.isFailure(exit)) {
    console.error("Failure:", exit.cause);
  }
});

Explanation:

• Exit captures both success (Exit.success(value)) and failure (Exit.failure(cause)).
• Use Exit for robust error handling, supervision, and coordination of concurrent effects.
• Pattern matching on Exit lets you handle all possible outcomes.

Anti-Pattern

Ignoring the outcome of an effect, or only handling success/failure without distinguishing between error types or defects, which can lead to missed errors and less robust code.