The one implementation behind @prelude/assert (throwing), @prelude/predicate (type guards) and @prelude/refute (result values). A Validator<T> checks an unknown value and returns { ok: true, value } or a structured Failure — the path to the offending value, what was expected (as data, not text) and the value itself — which each host package renders in its own words. Use it directly when you want that structure, or when you write a validator that should plug into all three packages.
npm i -E @prelude/validation
import * as V from '@prelude/validation'
const message = V.object({
method: V.string,
params: V.tuple(V.number, V.nullOr(V.string))
})
message({ method: 'add', params: [ 1, 2 ] })
// { ok: false,
// path: [ { kind: 'key', key: 'params' }, { kind: 'index', index: 1 }, { kind: 'or', alternative: 'null' } ],
// expected: { kind: 'type', name: 'string' },
// received: 2 }
Validators: string, number, boolean, bigint, symbol, undefined_, null_, true_, false_, unknown, defined, nullish, finite, safeInteger, positive, nonBlankString, gt/gte/lt/lte, between, eq, is, oneOf, regexp, instance, strftime, calendarDate; containers array, tuple, object, partial, exact, exactPartial, record, unique; combinators or, and, nullOr, undefinedOr, nullishOr, lift (a primitive becomes an equality check, a RegExp a match, null the null check).
Semantics shared by every host: exact/exactPartial look at own properties only (a declared __proto__ key works), partial skips undefined properties, tuple checks every declared position and rejects extra elements, unique keys on the validated element (or on f(element)), regexp resets lastIndex of global/sticky patterns, or collects every alternative's failure, and returns the first.
Failure.path segments: key, index, keyOf (a failing property name in record) and or (a nullOr/undefinedOr/nullishOr wrapper the failure passed through). Failure.expected is a tagged union (type, literal, oneOf, regexp, extraKeys, union, strftime, ... see Expected in prelude.ts); Failure.received is the innermost failing value.
Host packages attach their core validator to every function they export under the well-known symbol validatorOf (unwrap(fn)), so assert.object({ a: assert.string }) composes structurally rather than by catching errors — and any function you wrap with wrapped(fn, validator) composes the same way.
This package is dedicated to the public domain under CC0 1.0.