effect-facet-effect-composition

name: effect-facet-effect-composition description: Guidance for facet effect/Effect#composition focused on APIs like map, flatMap, and mapBoth. Load after effect-skill-router when this facet is the primary owner.

Effect Facet Effect#composition

Owned scope

• Owns only effect/Effect#composition.
• Parent module: effect/Effect.

What it is for

• map/flatMap/zip/all composition APIs. The Effect module is the core of the Effect library, providing a powerful and expressive way to model and compose asynchronous, concurrent, and effectful computations.

API quick reference

• map
• flatMap
• mapBoth
• provide
• mapEager
• mapError
• flatMapEager
• mapBothEager
• mapErrorEager
• provideService
• provideServiceEffect
• provideContext
• zip
• andThen
• zipWith
• Full API list: references/api-reference.md

How to use it

• Keep work focused on the composition concern for effect/Effect.
• Prefer pipe-based composition to keep transformations explicit and testable.
• Use the reference docs to select the smallest API surface that solves your task.
• Validate behavior against existing tests before introducing new usage patterns.

Starter example

import { Console, Effect } from "effect";

// Creating a simple effect
const hello = Effect.succeed("Hello, World!");

// Composing effects
const program = Effect.gen(function* () {
  const message = yield* hello;
  yield* Console.log(message);
  return message.length;
});

// Running the effect
Effect.runPromise(program).then(console.log); // 13

Common pitfalls

• Prefer explicit, typed combinators over ad-hoc casting or unchecked assumptions.

Not covered here

• Sibling facets under the same parent are out of scope:
  • effect-facet-effect-concurrency (effect/Effect#concurrency)
  • effect-facet-effect-constructors (effect/Effect#constructors)
  • effect-facet-effect-context-di (effect/Effect#context-di)
  • effect-facet-effect-error-handling (effect/Effect#error-handling)
  • effect-facet-effect-execution (effect/Effect#execution)
  • effect-facet-effect-resource-scope (effect/Effect#resource-scope)
• Parent module ownership belongs to effect-module-effect.

Escalate to

• effect-module-effect for parent module-wide workflows.
• effect-skill-router for cross-module routing and ownership checks.

Reference anchors

• Parent tests: packages/effect/test/Effect.test.ts
• Parent tests: packages/effect/test/Channel.test.ts
• Parent tests: packages/effect/test/Deferred.test.ts
• Parent tests: packages/effect/test/Layer.test.ts
• Parent tests: packages/effect/test/Logger.test.ts
• Parent tests: packages/effect/test/LogLevel.test.ts
• API details: references/api-reference.md
• Usage notes: references/usage-reference.md
• Ownership mapping: references/owner.md