effect-domain-modeling

Effect Domain Modeling Skill

Use this skill when creating domain models, entities, value objects, or any types that represent core business concepts. This skill covers the complete lifecycle from type definition to runtime utilities.

Effect Source Reference

The Effect v4 source is available at ~/.cache/effect-v4/. Browse and read files there directly to look up APIs, types, and implementations.

Reference this for:

• Full Schema API: packages/effect/SCHEMA.md
• Match source: packages/effect/src/Match.ts
• Migration guide: MIGRATION.md
• Effect source: packages/effect/src/

Core Pattern: Schema.TaggedStruct

The foundation of Effect domain modeling combines two key features:

1. Schema.TaggedStruct - Automatic _tag discriminator for union types
2. Schema.decodeSync - Type-safe constructors with validation

In v4, Equal.equals performs deep structural comparison by default, so no special wrapping (like the removed Schema.Data) is needed.

import { Schema, Equal } from 'effect';

// Define each variant with TaggedStruct
export const Pending = Schema.TaggedStruct('pending', {
	id: Schema.String,
	createdAt: Schema.DateTimeUtc
});

export const Active = Schema.TaggedStruct('active', {
	id: Schema.String,
	createdAt: Schema.DateTimeUtc,
	startedAt: Schema.DateTimeUtc
});

export const Completed = Schema.TaggedStruct('completed', {
	id: Schema.String,
	createdAt: Schema.DateTimeUtc,
	completedAt: Schema.DateTimeUtc
});

// Union type
export const Task = Schema.Union([Pending, Active, Completed]);

export type Task = Schema.Schema.Type<typeof Task>;

// Export member types for refinements
export type Pending = Schema.Schema.Type<typeof Pending>;
export type Active = Schema.Schema.Type<typeof Active>;
export type Completed = Schema.Schema.Type<typeof Completed>;

Use Schema.annotate(...) selectively for public or widely reused schemas when the metadata is actually consumed. Local or still-evolving domain schemas can stay unannotated.