An educational pure functional programming library in TypeScript
Add isEmpty/isNonEmpty/size predicates and isSet/isMap guards
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CHANGELOG.md
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CHANGELOG.md
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All notable changes to this project will be documented in this file.
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## [0.1.0-alpha.9] - 2026-02-07
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### Added
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- **Emptiness predicates** — `isEmpty` / `isNonEmpty` for strings, arrays, Sets, Maps, and objects
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- Uses FP naming convention (`isNonEmpty` mirrors the `NonEmpty` type from Haskell/Scala/PureScript)
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## [0.1.0-alpha.8] - 2026-02-07
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### Added
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docs-site/src/content/docs/stories/forest-election/01-the-ballot-box-problem.mdx
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docs-site/src/content/docs/stories/forest-election/01-the-ballot-box-problem.mdx
···
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matchValidation,
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match,
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isDefined,
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isEmpty,
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isNonEmpty,
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pipe,
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} from "purus-ts";
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```
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const validateVoter = (
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voter: string | undefined,
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): Validation<string, BallotError> =>
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voter && voter.trim().length > 0
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voter && isNonEmpty(voter.trim())
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? valid(voter.trim())
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: invalidOne({ _tag: "MissingVoter" });
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```
···
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const validateCandidate = (
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candidate: string | undefined,
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): Validation<Candidate, BallotError> =>
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!candidate || candidate.trim().length === 0
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!candidate || isEmpty(candidate.trim())
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? invalidOne({ _tag: "MissingCandidate" })
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: pipe(
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VALID_CANDIDATES.find((c) => c === candidate.trim()),
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examples/stories/forest-election/01-validation.ts
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examples/stories/forest-election/01-validation.ts
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matchValidation,
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match,
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isDefined,
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isEmpty,
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isNonEmpty,
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pipe,
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} from "../../../src/index"
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···
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const validateVoter = (
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voter: string | undefined,
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): Validation<string, BallotError> =>
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voter && voter.trim().length > 0
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voter && isNonEmpty(voter.trim())
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? valid(voter.trim())
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: invalidOne({ _tag: "MissingVoter" })
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···
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const validateCandidate = (
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candidate: string | undefined,
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): Validation<Candidate, BallotError> =>
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!candidate || candidate.trim().length === 0
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!candidate || isEmpty(candidate.trim())
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? invalidOne({ _tag: "MissingCandidate" })
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: pipe(
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VALID_CANDIDATES.find((c) => c === candidate.trim()),
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examples/stories/forest-election/02-result.ts
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examples/stories/forest-election/02-result.ts
···
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mapResult,
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matchResult,
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match,
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isEmpty,
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pipe,
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} from "../../../src/index"
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···
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const ensureNotEmpty = (
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ballots: readonly Ballot[],
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): Result<readonly Ballot[], CountError> =>
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ballots.length === 0 ? err({ _tag: "EmptyBallotBox" }) : ok(ballots)
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isEmpty(ballots) ? err({ _tag: "EmptyBallotBox" }) : ok(ballots)
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/**
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* Verify that the counted votes match the number of ballots.
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examples/stories/forest-election/03-effects.ts
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examples/stories/forest-election/03-effects.ts
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retry,
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all,
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match,
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isNonEmpty,
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} from "../../../src/index"
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// =============================================================================
···
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.join("\n"),
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)
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if (failed.length > 0) {
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if (isNonEmpty(failed)) {
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console.log(`\nFailed: ${failed.length}/4`)
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console.log(
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failed
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examples/user-registration/with-purus.ts
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examples/user-registration/with-purus.ts
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invalidOne,
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apValidation,
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matchValidation,
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isEmpty,
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type Validation,
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// Effect
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type Eff,
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// =============================================================================
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const validateName = (name: string): Validation<string, ValidationError> =>
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name.trim().length === 0
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isEmpty(name.trim())
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? invalidOne(ValidationError.emptyField("name"))
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: valid(name.trim())
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const validateEmail = (email: string): Validation<string, ValidationError> =>
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email.trim().length === 0
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isEmpty(email.trim())
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? invalidOne(ValidationError.emptyField("email"))
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: /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email.trim().toLowerCase())
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? valid(email.trim().toLowerCase())
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package.json
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package.json
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src/data/guards.ts
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src/data/guards.ts
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/** Type guard for array values */
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export const isArray = (x: unknown) => Array.isArray(x)
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/** Type guard for Set values */
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export const isSet = (x: unknown): x is Set<unknown> => x instanceof Set
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/** Type guard for Map values */
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export const isMap = (x: unknown): x is Map<unknown, unknown> => x instanceof Map
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// =============================================================================
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// Number Property Guards
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// =============================================================================
···
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<K extends string>(...keys: K[]) =>
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<T>(x: T): x is T & Record<K, unknown> =>
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isObject(x) && keys.every((k) => k in x)
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// =============================================================================
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// Emptiness Predicates
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// =============================================================================
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/** A value whose emptiness can be meaningfully checked */
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type Emptiable =
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| string
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| readonly unknown[]
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| ReadonlySet<unknown>
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| ReadonlyMap<unknown, unknown>
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| Record<string, unknown>
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/** Returns the number of elements in a string, array, Set, Map, or object */
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export const size = (x: Emptiable): number =>
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isString(x) || isArray(x)
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? x.length
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: isSet(x) || isMap(x)
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? x.size
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: Object.keys(x).length
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/** Checks if a string, array, Set, Map, or object is empty */
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export const isEmpty = (x: Emptiable): boolean => size(x) === 0
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/** Checks if a string, array, Set, Map, or object is non-empty */
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export const isNonEmpty = (x: Emptiable): boolean => size(x) > 0
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tests/guards.test.ts
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tests/guards.test.ts
···
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and,
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hasProperties,
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isBoolean,
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isEmpty,
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isNonEmpty,
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isDefined,
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isNotNull,
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isNotNullish,
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expect(hasIdAndName({})).toBe(false)
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expect(hasIdAndName(null)).toBe(false)
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expect(hasIdAndName("string")).toBe(false)
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})
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})
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describe("emptiness predicates", () => {
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it("isEmpty checks strings", () => {
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expect(isEmpty("")).toBe(true)
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expect(isEmpty("hello")).toBe(false)
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})
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it("isEmpty checks arrays", () => {
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expect(isEmpty([])).toBe(true)
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expect(isEmpty([1])).toBe(false)
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})
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it("isEmpty checks Sets", () => {
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expect(isEmpty(new Set())).toBe(true)
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expect(isEmpty(new Set([1]))).toBe(false)
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})
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it("isEmpty checks Maps", () => {
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expect(isEmpty(new Map())).toBe(true)
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expect(isEmpty(new Map([["a", 1]]))).toBe(false)
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})
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it("isEmpty checks objects", () => {
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expect(isEmpty({})).toBe(true)
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expect(isEmpty({ a: 1 })).toBe(false)
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})
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it("isNonEmpty is the inverse of isEmpty", () => {
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expect(isNonEmpty("")).toBe(false)
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expect(isNonEmpty("hi")).toBe(true)
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expect(isNonEmpty([])).toBe(false)
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expect(isNonEmpty([1])).toBe(true)
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})
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})
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