Match

The match function provides a powerful way to handle different types of input and return corresponding outputs based on the input type, like a type-safe switch statement.

Case Record API

The simplest way to define a matcher is with ArkType definition strings as keys with corresponding handlers as values:

import { match } from "arktype"

const  = match({
	"string | Array": v => v.length,
	number: v => v,
	bigint: v => v,
	default: "assert"
})

// a match definition is complete once a `default` has been specified,
// either as a case or via the .default() method

("abc") // 3
([1, 2, 3, 4]) // 4
(5n) // 5n
// ArkErrors: must be an object, a string, a number or a bigint (was boolean)
(true)

In this example, sizeOf is a matcher that takes a string, array, number, or bigint as input. It returns the length of strings and arrays, and the value of numbers and bigints.

default accepts one of 4 values:

  • "assert": accept unknown, throw if none of the cases match
  • "never": accept an input based on inferred cases, throw if none match
  • "reject": accept unknown, return ArkErrors if none of the cases match
  • (data: In) => unknown: handle data not matching other cases directly

Cases will be checked in the order they are specified, either as object literal keys or via chained methods.

Fluent API

The match function also provides a fluent API. This can be convenient for non-string-embeddable definitions:

import { match } from "arktype"

// the Case Record and Fluent APIs can be easily combined
const  = match({
	string: v => v.length,
	number: v => v,
	bigint: v => v
})
	// match any object with a numeric length property and extract it
	.case({ length: "number" }, o => o.length)
	// return 0 for all other data
	.default(() => 0)

("abc") // 3
({ name: "David", length: 5 }) // 5
(null) // 0

Narrowing input with in, property matching with at

import { match } from "arktype"

type  =
	| {
			id: 1
			oneValue: number
	  }
	| {
			id: 2
			twoValue: string
	  }

const  = match
	// .in allows you to specify the input TypeScript allows for your matcher
	.in<>()
	// .at allows you to specify a key at which your input will be matched
	.at("id")
	.match({
		1: o => `${o.oneValue}!`,
		2: o => o.twoValue.length,
		default: "assert"
	})

({ id: 1, oneValue: 1 }) // "1!"
({ id: 2, twoValue: "two" }) // 3
({ oneValue: 3 })
TypeScript:   Property 'id' is missing in type '{ oneValue: number; }' but required in type '{ id: 1; oneValue: number; }'.

this

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