Domain Predicates Skill

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

Pattern: Equality with Schema.Data

When using schemas, leverage Schema.Data for automatic structural equality:

import { Schema, Equal, DateTime } from "effect"

export const Task = Schema.TaggedStruct("pending", {
  id: Schema.String,
  createdAt: Schema.DateTimeUtcFromSelf,
}).pipe(Schema.Data) // Implements Equal.Symbol automatically

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

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

// Usage: Automatic structural equality
const task1 = makeTask({ id: "123", createdAt: now })
const task2 = makeTask({ id: "123", createdAt: now })

Equal.equals(task1, task2) // true - 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.equivalence(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
)

Key Pattern: Equivalence.mapInput

• Signature: Equivalence.mapInput(baseEquivalence, (value) => extractField)
• Compose from simpler equivalences
• Map domain type to comparable value
• Dual API: data-first and data-last

Pattern: Combining Equivalences

Use Equivalence.combine for multi-field equality:

import { DateTime } from "effect"
import * as Equivalence from "effect/Equivalence"

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

declare const EquivalenceByTag: Equivalence.Equivalence<Task>
declare const EquivalenceById: Equivalence.Equivalence<Task>
declare const EquivalenceByCreatedAt: Equivalence.Equivalence<Task>

/**
 * Compare by tag first, then by ID.
 *
 * Both must match for equivalence.
 *
 * @category Equivalence
 * @since 0.1.0
 * @example
 * import * as Task from "@/schemas/Task"
 *
 * const areSame = Task.EquivalenceByTagAndId(task1, task2)
 */
export const EquivalenceByTagAndId = Equivalence.combine(
  EquivalenceByTag,
  EquivalenceById
)

/**
 * Compare by multiple criteria for exact equality.
 *
 * @category Equivalence
 * @since 0.1.0
 */
export const EquivalenceComplete = Equivalence.combine(
  EquivalenceByTag,
  EquivalenceById,
  EquivalenceByCreatedAt
)

Key Pattern: Equivalence.combine

• Combines multiple equivalences
• All must match for equivalence (AND logic)
• Order doesn't matter (unlike Order.combine)

Pattern: Typeclass-Derived Predicates

When a domain type implements a typeclass, re-export all relevant predicates:

import { DateTime, Duration, Schema } from "effect"
import * as Order from "effect/Order"

// Inline typeclass interface declarations (instead of module augmentations)
interface SchedulableInstance<A> {
  readonly get: (self: A) => DateTime.DateTime
  readonly set: (self: A, date: DateTime.DateTime) => A
}

interface DurableInstance<A> {
  readonly get: (self: A) => Duration.Duration
  readonly set: (self: A, duration: Duration.Duration) => A
}

// Typeclass module declarations
declare const Schedulable$: {
  make: <A>(
    get: (self: A) => DateTime.DateTime,
    set: (self: A, date: DateTime.DateTime) => A
  ) => SchedulableInstance<A>
  isScheduledBefore: <A>(instance: SchedulableInstance<A>) => (date: DateTime.DateTime) => (self: A) => boolean
  isScheduledAfter: <A>(instance: SchedulableInstance<A>) => (date: DateTime.DateTime) => (self: A) => boolean
  isScheduledBetween: <A>(instance: SchedulableInstance<A>) => (start: DateTime.DateTime, end: DateTime.DateTime) => (self: A) => boolean
  isScheduledOn: <A>(instance: SchedulableInstance<A>) => (date: DateTime.DateTime) => (self: A) => boolean
  isScheduledToday: <A>(instance: SchedulableInstance<A>) => (self: A) => boolean
  isScheduledThisWeek: <A>(instance: SchedulableInstance<A>) => (self: A) => boolean
  isScheduledThisMonth: <A>(instance: SchedulableInstance<A>) => (self: A) => boolean
}

declare const Durable$: {
  make: <A>(
    get: (self: A) => Duration.Duration,
    set: (self: A, duration: Duration.Duration) => A
  ) => DurableInstance<A>
  isMoreThan: <A>(instance: DurableInstance<A>) => (min: Duration.Duration) => (self: A) => boolean
  isLessThan: <A>(instance: DurableInstance<A>) => (max: Duration.Duration) => (self: A) => boolean
  isBetween: <A>(instance: DurableInstance<A>) => (min: Duration.Duration, max: Duration.Duration) => (self: A) => boolean
  hasExactDuration: <A>(instance: DurableInstance<A>) => (duration: Duration.Duration) => (self: A) => boolean
}

interface Appointment {
  readonly date: DateTime.Utc
  readonly duration: Duration.Duration
}

declare const Appointment: {
  make: (props: Partial<Appointment>) => Appointment
}

// Create typeclass instances
export const Schedulable = Schedulable$.make<Appointment>(
  (self: Appointment) => self.date,
  (self: Appointment, date: DateTime.DateTime) => Appointment.make({ ...self, date: DateTime.toUtc(date) })
)

export const Durable = Durable$.make<Appointment>(
  (self: Appointment) => self.duration,
  (self: Appointment, duration: Duration.Duration) => Appointment.make({ ...self, duration: Duration.decode(duration) })
)

// Re-export all Schedulable predicates
export const isScheduledBefore = Schedulable$.isScheduledBefore(Schedulable)
export const isScheduledAfter = Schedulable$.isScheduledAfter(Schedulable)
export const isScheduledBetween = Schedulable$.isScheduledBetween(Schedulable)
export const isScheduledOn = Schedulable$.isScheduledOn(Schedulable)
export const isScheduledToday = Schedulable$.isScheduledToday(Schedulable)
export const isScheduledThisWeek = Schedulable$.isScheduledThisWeek(Schedulable)
export const isScheduledThisMonth = Schedulable$.isScheduledThisMonth(Schedulable)

// Re-export all Durable predicates
export const hasMinimumDuration = Durable$.isMoreThan(Durable)
export const hasMaximumDuration = Durable$.isLessThan(Durable)
export const hasDurationBetween = Durable$.isBetween(Durable)
export const hasExactDuration = Durable$.hasExactDuration(Durable)

Pattern: Order Instances with Order.mapInput

Compose orders from simpler base orders using Order.mapInput:

import { DateTime } from "effect"
import * as Order from "effect/Order"
import * as String from "effect/String"

interface Task {
  readonly _tag: "pending" | "active" | "completed"
  readonly id: string
  readonly createdAt: DateTime.Utc
}

/**
 * Order by ID using Order.mapInput.
 *
 * @category Orders
 * @since 0.1.0
 * @example
 * import * as Task from "@/schemas/Task"
 * import * as Array from "effect/Array"
 *
 * const sorted = Array.sort(tasks, Task.OrderById)
 */
export const OrderById: Order.Order<Task> =
  Order.mapInput(Order.string, (task: Task) => task.id)

/**
 * Order by creation date.
 *
 * @category Orders
 * @since 0.1.0
 */
export const OrderByCreatedAt: Order.Order<Task> =
  Order.mapInput(DateTime.Order, (task: Task) => task.createdAt)

/**
 * Order by status tag.
 *
 * @category Orders
 * @since 0.1.0
 */
export const OrderByTag: Order.Order<Task> =
  Order.mapInput(Order.string, (task: Task) => task._tag)

/**
 * Order by priority (domain-specific logic).
 *
 * @category Orders
 * @since 0.1.0
 */
export const OrderByPriority: Order.Order<Task> =
  Order.mapInput(Order.number, (task: Task) => {
    const priorities = { pending: 0, active: 1, completed: 2 }
    return priorities[task._tag]
  })

Key Pattern: Order.mapInput

• Signature: Order.mapInput(baseOrder, (value) => extractField)
• Compose from existing orders (Order.string, Order.number, DateTime.Order, etc.)
• Map domain type to comparable value
• Dual API: data-first and data-last

Pattern: Combining Orders with Order.combine

Use Order.combine for multi-criteria sorting:

import { DateTime } from "effect"
import * as Order from "effect/Order"

interface Task {
  readonly _tag: "pending" | "active" | "completed"
  readonly id: string
  readonly createdAt: DateTime.Utc
}

declare const OrderByPriority: Order.Order<Task>
declare const OrderByCreatedAt: Order.Order<Task>
declare const OrderByTag: Order.Order<Task>
declare const OrderById: Order.Order<Task>

/**
 * Sort by priority first, then by creation date.
 *
 * @category Orders
 * @since 0.1.0
 * @example
 * import * as Task from "@/schemas/Task"
 * import * as Array from "effect/Array"
 *
 * // High priority tasks first, then by oldest
 * const sorted = Array.sort(tasks, Task.OrderByPriorityThenDate)
 */
export const OrderByPriorityThenDate: Order.Order<Task> = Order.combine(
  OrderByPriority,
  OrderByCreatedAt
)

/**
 * Sort by tag, then ID, then creation date.
 *
 * @category Orders
 * @since 0.1.0
 */
export const OrderComplex: Order.Order<Task> = Order.combine(
  OrderByTag,
  OrderById,
  OrderByCreatedAt
)

Key Pattern: Order.combine

• Combines multiple orders for multi-criteria sorting
• First order takes precedence, then second, etc.
• Order matters (unlike Equivalence.combine)
• Returns combined order that can be used with Array.sort

Pattern: Comprehensive Order Instances

Provide extensive sorting capabilities:

import { DateTime, Duration } from "effect"
import * as Order from "effect/Order"
import * as String from "effect/String"

// Inline typeclass interface declarations
interface SchedulableInstance<A> {
  readonly get: (self: A) => DateTime.DateTime
  readonly set: (self: A, date: DateTime.DateTime) => A
}

interface DurableInstance<A> {
  readonly get: (self: A) => Duration.Duration
  readonly set: (self: A, duration: Duration.Duration) => A
}

// Typeclass module declarations
declare const Schedulable$: {
  OrderByScheduledTime: <A>(instance: SchedulableInstance<A>) => Order.Order<A>
  OrderByDayOfWeek: <A>(instance: SchedulableInstance<A>) => Order.Order<A>
  OrderByTimeOfDay: <A>(instance: SchedulableInstance<A>) => Order.Order<A>
  OrderByHour: <A>(instance: SchedulableInstance<A>) => Order.Order<A>
  OrderByMonth: <A>(instance: SchedulableInstance<A>) => Order.Order<A>
  OrderByYear: <A>(instance: SchedulableInstance<A>) => Order.Order<A>
  OrderByYearMonth: <A>(instance: SchedulableInstance<A>) => Order.Order<A>
  OrderByDateOnly: <A>(instance: SchedulableInstance<A>) => Order.Order<A>
  OrderByDayPeriod: <A>(instance: SchedulableInstance<A>) => Order.Order<A>
  OrderByBusinessHours: <A>(instance: SchedulableInstance<A>) => Order.Order<A>
  OrderByWeekdayFirst: <A>(instance: SchedulableInstance<A>) => Order.Order<A>
}

declare const Durable$: {
  OrderByDuration: <A>(instance: DurableInstance<A>) => Order.Order<A>
  OrderByHours: <A>(instance: DurableInstance<A>) => Order.Order<A>
  OrderByMinutes: <A>(instance: DurableInstance<A>) => Order.Order<A>
  OrderBySeconds: <A>(instance: DurableInstance<A>) => Order.Order<A>
}

type AppointmentStatus = "scheduled" | "confirmed" | "completed" | "cancelled"

interface Appointment {
  readonly date: DateTime.Utc
  readonly duration: Duration.Duration
  readonly status: AppointmentStatus
}

declare const Schedulable: SchedulableInstance<Appointment>
declare const Durable: DurableInstance<Appointment>

// Schedulable orders (temporal sorting)
export const OrderByScheduledTime = Schedulable$.OrderByScheduledTime(Schedulable)
export const OrderByDayOfWeek = Schedulable$.OrderByDayOfWeek(Schedulable)
export const OrderByTimeOfDay = Schedulable$.OrderByTimeOfDay(Schedulable)
export const OrderByHour = Schedulable$.OrderByHour(Schedulable)
export const OrderByMonth = Schedulable$.OrderByMonth(Schedulable)
export const OrderByYear = Schedulable$.OrderByYear(Schedulable)
export const OrderByYearMonth = Schedulable$.OrderByYearMonth(Schedulable)
export const OrderByDateOnly = Schedulable$.OrderByDateOnly(Schedulable)
export const OrderByDayPeriod = Schedulable$.OrderByDayPeriod(Schedulable)
export const OrderByBusinessHours = Schedulable$.OrderByBusinessHours(Schedulable)
export const OrderByWeekdayFirst = Schedulable$.OrderByWeekdayFirst(Schedulable)

// Durable orders (duration sorting)
export const OrderByDuration = Durable$.OrderByDuration(Durable)
export const OrderByHours = Durable$.OrderByHours(Durable)
export const OrderByMinutes = Durable$.OrderByMinutes(Durable)
export const OrderBySeconds = Durable$.OrderBySeconds(Durable)

// Domain-specific orders using Order.mapInput
export const OrderByStatus: Order.Order<Appointment> =
  Order.mapInput(String.Order, (appt: Appointment) => appt.status)

export const OrderByStatusPriority: Order.Order<Appointment> =
  Order.mapInput(Order.number, (appt: Appointment) => {
    const priorities: Record<AppointmentStatus, number> = {
      scheduled: 0,
      confirmed: 1,
      completed: 2,
      cancelled: 3,
    }
    return priorities[appt.status]
  })

// Combined orders for complex sorting
export const OrderByStatusThenTime: Order.Order<Appointment> = Order.combine(
  OrderByStatusPriority,
  OrderByScheduledTime
)

Usage Examples

Equality Examples

import { Equal, Array } from "effect"
import * as Equivalence from "effect/Equivalence"

declare module "@/schemas/Task" {
  export interface Task {
    readonly _tag: string
    readonly id: string
  }
  export const EquivalenceById: Equivalence.Equivalence<Task>
  export const EquivalenceByTagAndId: Equivalence.Equivalence<Task>
}

import * as Task from "@/schemas/Task"

declare const task1: Task.Task
declare const task2: Task.Task
declare const tasks: Array<Task.Task>
declare const searchTask: Task.Task

// Structural equality (automatic from Schema.Data)
const areSame = Equal.equals(task1, task2)

// Deduplicate by ID only
const uniqueById = Array.dedupeWith(tasks, Task.EquivalenceById)

// Deduplicate by tag and ID
const uniqueByTagAndId = Array.dedupeWith(tasks, Task.EquivalenceByTagAndId)

// Find if array contains equivalent task
const hasTask = Array.containsWith(tasks, Task.EquivalenceById)(searchTask)

Filtering Examples

Document how these predicates enable powerful filtering:

import { DateTime, Duration, Array } from "effect"
import { pipe } from "effect/Function"

declare module "@/schemas/Appointment" {
  export interface Appointment {
    readonly date: DateTime.Utc
  }
  export const isScheduledBefore: (date: DateTime.DateTime) => (appointment: Appointment) => boolean
}

import * as Appointment from "@/schemas/Appointment"

declare const appointments: Array<Appointment.Appointment>

/**
 * Filter appointments scheduled before a date.
 *
 * @example
 * import * as Appointment from "@/schemas/Appointment"
 * import * as DateTime from "effect/DateTime"
 * import * as Duration from "effect/Duration"
 * import * as Array from "effect/Array"
 * import { pipe } from "effect/Function"
 *
 * const tomorrow = DateTime.addDuration(
 *   DateTime.unsafeNow(),
 *   Duration.days(1)
 * )
 *
 * const beforeTomorrow = pipe(
 *   appointments,
 *   Array.filter(Appointment.isScheduledBefore(tomorrow))
 * )
 */
const tomorrow = DateTime.addDuration(
  DateTime.unsafeNow(),
  Duration.days(1)
)

const beforeTomorrow = pipe(
  appointments,
  Array.filter(Appointment.isScheduledBefore(tomorrow))
)

Sorting Examples

import { Array, DateTime } from "effect"
import * as Order from "effect/Order"
import { pipe } from "effect/Function"

declare module "@/schemas/Task" {
  export interface Task {
    readonly _tag: "pending" | "active" | "completed"
    readonly id: string
    readonly createdAt: DateTime.Utc
  }
  export const OrderById: Order.Order<Task>
  export const OrderByPriority: Order.Order<Task>
  export const OrderByCreatedAt: Order.Order<Task>
  export const isPending: (task: Task) => boolean
}

import * as Task from "@/schemas/Task"

declare const tasks: Array<Task.Task>

// Simple sort by single field
const sortedById = Array.sort(tasks, Task.OrderById)

// Multi-criteria sort
const sortedComplex = Array.sort(
  tasks,
  Order.combine(
    Task.OrderByPriority,
    Task.OrderByCreatedAt
  )
)

// Sort with filter
const sortedFiltered = pipe(
  tasks,
  Array.filter(Task.isPending),
  Array.sort(Task.OrderByCreatedAt)
)

Pattern: Complex Filtering

Combine predicates for sophisticated queries:

import { DateTime, Duration, Array } from "effect"
import { pipe } from "effect/Function"
import * as Order from "effect/Order"
import * as Equivalence from "effect/Equivalence"

declare module "@/schemas/Appointment" {
  export interface Appointment {
    readonly id: string
    readonly date: DateTime.Utc
    readonly duration: Duration.Duration
    readonly status: string
  }
  export const isScheduledThisWeek: (appointment: Appointment) => boolean
  export const hasMinimumDuration: (min: Duration.Duration) => (appointment: Appointment) => boolean
  export const isScheduledToday: (appointment: Appointment) => boolean
  export const OrderByStatusPriority: Order.Order<Appointment>
  export const OrderByScheduledTime: Order.Order<Appointment>
  export const EquivalenceById: Equivalence.Equivalence<Appointment>
  export const OrderByPriorityThenDate: Order.Order<Appointment>
}

import * as Appointment from "@/schemas/Appointment"

declare const appointments: Array<Appointment.Appointment>

// Find long appointments this week
const longThisWeek = pipe(
  appointments,
  Array.filter(Appointment.isScheduledThisWeek),
  Array.filter(Appointment.hasMinimumDuration(Duration.hours(2)))
)

// Sort by multiple criteria
const sorted = pipe(
  appointments,
  Array.filter(Appointment.isScheduledToday),
  Array.sort(
    Order.combine(
      Appointment.OrderByStatusPriority,
      Appointment.OrderByScheduledTime
    )
  )
)

// Deduplicate and sort
const uniqueSorted = pipe(
  appointments,
  Array.dedupeWith(Appointment.EquivalenceById),
  Array.sort(Appointment.OrderByPriorityThenDate)
)

Checklist for Complete Coverage

Equality

• Use Schema.Data for automatic Equal.equals()
• Export Schema.equivalence() when needed for combinators
• Export field-based equivalences using Equivalence.mapInput
• Export combined equivalences using Equivalence.combine

Orders

• Export orders for all sortable fields using Order.mapInput
• Export combined orders using Order.combine
• Document which field takes precedence in combined orders

Schedulable types

• isScheduledBefore
• isScheduledAfter
• isScheduledBetween
• isScheduledOn
• isScheduledToday
• isScheduledThisWeek
• isScheduledThisMonth
• All Order instances

Durable types

• hasMinimumDuration (isMoreThan)
• hasMaximumDuration (isLessThan)
• hasDurationBetween (isBetween)
• hasExactDuration
• All Order instances

Domain-specific fields

• Predicate for each variant (isPending, isActive, etc.)
• Order by field value using Order.mapInput
• Order by priority/importance if applicable
• Combined orders for common sorting patterns

Documentation Requirements

Every predicate, equivalence, and order MUST have:

• JSDoc description
• @category tag
• @since tag
• @example with realistic usage showing imports and pipe

Key Patterns Summary

1. Schema.Data for Equality

import { Schema, Equal } from "effect"

const TaskSchema = Schema.TaggedStruct("task", { id: Schema.String }).pipe(Schema.Data)
type Task = Schema.Schema.Type<typeof TaskSchema>

declare const t1: Task
declare const t2: Task

// Usage: Equal.equals(t1, t2)
const areSame = Equal.equals(t1, t2)

2. Schema.equivalence() for Combinators

import { Schema, Array } from "effect"

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

export const Equivalence = Schema.equivalence(Task)

declare const tasks: Array<Task>

// Usage: Array.dedupeWith(tasks, Equivalence)
const uniqueTasks = Array.dedupeWith(tasks, Equivalence)

3. Equivalence.mapInput for Field-Based

import * as Equivalence from "effect/Equivalence"

interface Task {
  readonly id: string
}

const EquivalenceById = Equivalence.mapInput(Equivalence.string, (t: Task) => t.id)

4. Equivalence.combine for Multi-Field

import * as Equivalence from "effect/Equivalence"

interface Task {
  readonly _tag: string
  readonly id: string
}

declare const EquivalenceByTag: Equivalence.Equivalence<Task>
declare const EquivalenceById: Equivalence.Equivalence<Task>

const EquivalenceCombined = Equivalence.combine(EquivalenceByTag, EquivalenceById)

5. Order.mapInput for Field-Based Sorting

import * as Order from "effect/Order"

interface Task {
  readonly id: string
}

const OrderById = Order.mapInput(Order.string, (t: Task) => t.id)

6. Order.combine for Multi-Criteria Sorting

import * as Order from "effect/Order"

interface Task {
  readonly priority: number
  readonly date: Date
}

declare const OrderByPriority: Order.Order<Task>
declare const OrderByDate: Order.Order<Task>

const OrderCombined = Order.combine(OrderByPriority, OrderByDate)

This ensures comprehensive equality checking, predicates, and sorting capabilities are discoverable and developers understand how to use them effectively with Effect's compositional patterns.