Status: implemented

const person = open({ name: string, age: number })
const alice = { name: 'Alice', age: 30, admin: true }
parse(person)(alice)     // ['ok', { name: 'Alice', age: 30 }]
validate(person)(alice)  // ['ok', alice] — the same object, `admin` included

Both readers walk a schema against a value, both accept exactly the same values, and both report the same error on failure. They differ in what a success carries:

parse is the reader for a value coming in — from JSON, from a protocol frame — where a value built to the schema is what the caller wants next, and where dropping what the schema does not name is a feature.

validate is the reader for a value the caller already holds and must keep. A content-addressed document is the clear case: its bytes are its identity, so a reconstruction is a different document under a different hash, and "the payer did not report this figure" (absent) is not "the payer reported nothing here" (present, undefined). Answering "is this a T?" must not edit the thing being asked about. Against an open schema, validate also lets a consumer check the part it understands without discarding the part it does not.

Neither is more fundamental than the other: validate is not a parse whose result is thrown away. A third reader, the data form's validate, reads the same values against toData(s), the schema as plain data, and accepts exactly the DataJS values the other two do (a function or a symbol it may admit, see data-validate-admits-non-djs-values); validate/proof.f.mjs runs one acceptance table through all three. How the two schema-form readers share their code is in common's README.

A reader over a value that is already built cannot see how it was written. For reading straight from JSON text against a schema — one pass, no intermediate value, and a refusal of 1.00000000000000001 against a bigint — see rtti-parse.

The demo on this page runs the Built or handed back lesson: open({ name: string, age: number }) against {"name":"Alice","age":30,"admin":true}. Both readers accept it; parse's answer has no admin, and validate's is the value as typed. The lesson is defined in sample.f.mjs, and the module page's demo lists it among all the others.

• TypeScript: a type is erased at run time, so a check like these is what narrowing needs at a boundary; validate's result is typed Ts<s>.
• JSON Schema: validation answers yes or no, as validate does; nothing in it builds a value, as parse does.

parse, validate and the data form's validate in data, over the shared kernel in common.