effect-domain-predicates

Domain Predicates Skill

Generate complete sets of predicates and Order instances for domain types, derived from typeclass implementations.

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
• Order source: packages/effect/src/Order.ts
• Equivalence source: packages/effect/src/Equivalence.ts
• Predicate source: packages/effect/src/Predicate.ts
• Effect source: packages/effect/src/

Pattern: Deep Structural Equality (v4)

In v4, Equal.equals performs deep structural comparison by default — no special wrapping is needed:

import { Schema, Equal, DateTime } from 'effect';

export const Task = Schema.TaggedStruct('pending', {
	id: Schema.String,
	createdAt: Schema.DateTimeUtc
});

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

declare const makeTask: (props: {
	id: string;
	createdAt: DateTime.Utc;
}) => Task;
declare const now: DateTime.Utc;

// Usage: Deep structural equality (automatic in v4)
const task1 = makeTask({ id: '123', createdAt: now });
const task2 = makeTask({ id: '123', createdAt: now });

Equal.equals(task1, task2); // true - deep structural equality

Pattern: Equivalence from Schema

When you need an Equivalence instance (for use with combinators), derive it from the schema:

import { Schema, Array } from 'effect';
import * as Equivalence from 'effect/Equivalence';

declare const Task: Schema.Schema<any, any, never>;
type Task = Schema.Schema.Type<typeof Task>;

// Derive from schema (structural equality)
export const TaskEquivalence = Schema.toEquivalence(Task);

declare const tasks: Array<Task>;

// Usage with combinators
const uniqueTasks = Array.dedupeWith(tasks, TaskEquivalence);

Pattern: Field-Based Equivalence with Equivalence.mapInput

Compare by specific fields using Equivalence.mapInput:

import { DateTime } from 'effect';
import * as Equivalence from 'effect/Equivalence';

interface Task {
	readonly _tag: string;
	readonly id: string;
	readonly createdAt: DateTime.Utc;
}

/**
 * Compare tasks by ID only.
 *
 * @category Equivalence
 * @since 0.1.0
 * @example
 * import * as Task from "@/schemas/Task"
 * import * as Array from "effect/Array"
 *
 * const uniqueById = Array.dedupeWith(tasks, Task.EquivalenceById)
 */
export const EquivalenceById = Equivalence.mapInput(
	Equivalence.String,
	(task: Task) => task.id
);

/**
 * Compare by status tag.
 *
 * @category Equivalence
 * @since 0.1.0
 */
export const EquivalenceByTag = Equivalence.mapInput(
	Equivalence.String,
	(task: Task) => task._tag
);

/**
 * Compare by creation date.
 *
 * @category Equivalence
 * @since 0.1.0
 */
export const EquivalenceByCreatedAt = Equivalence.mapInput(
	DateTime.Equivalence,
	(task: Task) => task.createdAt
);