Arktype Discriminated Unions

Patterns for composing discriminated unions with arktype’s .merge() and .or() methods.

When to Apply This Skill

• Defining a discriminated union schema (e.g., commands, events, actions)
• Composing a base type with per-variant fields
• Working with defineTable() schemas that use union types

base.merge(type.or(...)) Pattern (Recommended)

Use when you have shared base fields and per-variant payloads discriminated on a literal key. .merge() distributes over unions — it merges the base into each branch of the union automatically.

import { type } from 'arktype';

const commandBase = type({
	id: 'string',
	deviceId: DeviceId,
	createdAt: 'number',
	_v: '1',
});

const Command = commandBase.merge(
	type.or(
		{
			action: "'closeTabs'",
			tabIds: 'string[]',
			'result?': type({ closedCount: 'number' }).or('undefined'),
		},
		{
			action: "'openTab'",
			url: 'string',
			'windowId?': 'string',
			'result?': type({ tabId: 'string' }).or('undefined'),
		},
		{
			action: "'activateTab'",
			tabId: 'string',
			'result?': type({ activated: 'boolean' }).or('undefined'),
		},
	),
);

How it works

1. type.or(...) creates a union of plain object definitions — each is a variant with its own fields.
2. commandBase.merge(union) distributes the merge across each branch of the union. Internally, arktype calls rNode.distribute() to apply the merge to each branch individually (source).
3. The result is a union where each branch has all commandBase fields plus its variant-specific fields.
4. Arktype auto-detects the action key as a discriminant because each branch has a distinct literal value.
5. switch (cmd.action) in TypeScript narrows the full union — payload fields and result types are type-safe per branch.

Why this pattern

.merge().or() Chaining Pattern (Good for 2-3 variants)

Use when you have a small number of variants where chaining reads naturally.

const Command = commandBase
	.merge({
		action: "'closeTabs'",
		tabIds: 'string[]',
		'result?': type({ closedCount: 'number' }).or('undefined'),
	})
	.or(
		commandBase.merge({
			action: "'openTab'",
			url: 'string',
			'result?': type({ tabId: 'string' }).or('undefined'),
		}),
	);

For 4+ variants, prefer base.merge(type.or(...)) to avoid repeating commandBase.merge(...) per branch.

The "..." Spread Key Pattern (Alternative)

Use when defining inline without a pre-declared base variable, or when you prefer a more compact syntax.

const User = type({ isAdmin: 'false', name: 'string' });

const Admin = type({
	'...': User,
	isAdmin: 'true',
	permissions: 'string[]',
});

The "..." key spreads all properties from the referenced type into the new object definition. Conflicting keys in the outer object override the spread type (same as .merge()).

Spread key in unions

const Command = type({
	'...': commandBase,
	action: "'closeTabs'",
	tabIds: 'string[]',
}).or({
	'...': commandBase,
	action: "'openTab'",
	url: 'string',
});

Functionally equivalent to .merge().or(). Choose based on readability preference.

.or() Chaining vs type.or() Static

Chaining (preferred for 2-3 variants)

const Command = variantA.or(variantB).or(variantC);

Static type.or() (preferred for 4+ variants)

const Command = type.or(variantA, variantB, variantC, variantD, variantE);

The static form avoids deeply nested chaining and creates the union in a single call.

.merge() Distribution Over Unions

.merge() distributes over unions on both sides. If you merge a union into an object type (or vice versa), the operation is applied to each branch individually:

// base.merge(union) — distributes merge across each branch
const Result = baseType.merge(type.or({ a: 'string' }, { b: 'number' }));
// Equivalent to: type.or(baseType.merge({ a: 'string' }), baseType.merge({ b: 'number' }))

Constraint: Each branch of the union must be an object type. If any branch is non-object (e.g., 'string'), arktype will throw a ParseError:

// ❌ WRONG: 'string' is not an object type
commandBase.merge(type.or({ a: 'string' }, 'string'));

// ✅ CORRECT: all branches are object types
commandBase.merge(type.or({ a: 'string' }, { b: 'number' }));

Optional Properties in Unions

Use arktype’s 'key?' syntax for optional properties. Never use | undefined for optionals — it breaks JSON Schema conversion.

// Good: optional property syntax
commandBase.merge({
	action: "'openTab'",
	url: 'string',
	'windowId?': 'string',
	'result?': type({ tabId: 'string' }).or('undefined'),
});

// Bad: explicit undefined union on a required key
commandBase.merge({
	action: "'openTab'",
	url: 'string',
	windowId: 'string | undefined', // Breaks JSON Schema
});

The 'result?': type({...}).or('undefined') pattern is correct — the ? makes the key optional, and .or('undefined') allows the value to be explicitly undefined when present. This is the standard pattern for “pending = absent, done = has value” semantics.

Merge Behavior

• Override: When both the base and merge argument define the same key, the merge argument wins
• Optional preservation: If a key is optional ('key?') in the base and required in the merge, the merge argument’s optionality wins
• No deep merge: .merge() is shallow — it replaces top-level keys, not nested objects
• Distributes over unions: Both the base and the argument can be unions — merge is applied per-branch

Discriminant Detection

Arktype auto-detects discriminants when union branches have distinct literal values on the same key:

const AorB = type({ kind: "'A'", value: 'number' }).or({
	kind: "'B'",
	label: 'string',
});

// Arktype internally uses `kind` as the discriminant
// Validation checks `kind` first, then validates only the matching branch

This works with any literal type — string literals, number literals, or boolean literals.

Always Wrap Extracted Types with type()

When extracting reusable arktype types into named constants, always wrap them with type() — even for simple string literal unions. This ensures the value is a proper arktype Type with .infer, .or(), .merge(), etc.

// GOOD: wrapped with type() — composable, has .infer, works with .or()/.merge()
const tabGroupColor = type(
	"'grey' | 'blue' | 'red' | 'yellow' | 'green' | 'pink' | 'purple' | 'cyan' | 'orange'",
);

const commandBase = type({
	id: CommandId,
	deviceId: DeviceId,
	createdAt: 'number',
	_v: '1',
});

// BAD: plain string — not a Type, can't compose, no .infer
const tabGroupColor =
	"'grey' | 'blue' | 'red' | 'yellow' | 'green' | 'pink' | 'purple' | 'cyan' | 'orange'";

Both work when used as a value inside type({...}) object literals (arktype coerces strings). But only the type()-wrapped version is a first-class Type that works in all positions.

Anti-Patterns

JS object spread (loses Type composition)

// Bad: base is a plain object, not a Type
const baseFields = { id: 'string', deviceId: DeviceId, createdAt: 'number' };
const Command = type({ ...baseFields, action: "'closeTabs'" }).or({
	...baseFields,
	action: "'openTab'",
});

This works but baseFields is not an arktype Type — you can’t call .merge(), .or(), or inspect it at runtime. Prefer .merge() when the base should be a proper type.

Repeating base.merge(...) per variant

// Bad: repetitive — base.merge repeated for every variant
type.or(
	commandBase.merge({ action: "'closeTabs'", tabIds: 'string[]' }),
	commandBase.merge({ action: "'openTab'", url: 'string' }),
	commandBase.merge({ action: "'activateTab'", tabId: 'string' }),
);

// Good: merge once, union the variants
commandBase.merge(
	type.or(
		{ action: "'closeTabs'", tabIds: 'string[]' },
		{ action: "'openTab'", url: 'string' },
		{ action: "'activateTab'", tabId: 'string' },
	),
);

Forgetting 'key?' syntax for optionals

// Bad: makes windowId required but accepting undefined
commandBase.merge({ windowId: 'string | undefined' });

// Good: makes windowId truly optional
commandBase.merge({ 'windowId?': 'string' });

References

• apps/tab-manager/src/lib/workspace.ts — Commands table using commandBase.merge(type.or(...))
• .agents/skills/typescript/SKILL.md — Arktype optional properties section
• .agents/skills/workspace-api/SKILL.md — defineTable() accepts union types
• arktype source: merge distributes — rNode.distribute() in merge implementation