feat parse → derive → emit · in the browser

The toolchain, live. Not a demo of it.

These are the real corpus exemplars and the real compiler — the same parse → derive → emit pipeline the CLI runs, bundled into this page. Edit any file and every output re-derives: the IR, the test cases, the generated Vitest suite. Byte-for-byte what feat generate writes.

Corpus

Break it

SPEC-USR-001 CRUD-only — one acid service, absent + literal matchers

feat 1.0

# Corpus exemplar #2 — CRUD-only, no event sourcing: one acid service, no
# convergence timeouts. Proves the language carries zero infrastructure vocabulary.

spec SPEC-USR-001 "CreateUser"
context Users
aggregate User
type command
status agreed

construct:
  handler at handlers/create-user.ts
  touches handlers/**
  imports UserRepository from repositories/user-repository
  Uses Prisma ORM for database access.

enforce:
  validate input against CreateUserInput
  check email uniqueness via UserRepository.findByEmail
  rejects DUPLICATE_EMAIL when a user with this email already exists
  insert new user record
  return created user

contract:
  input CreateUserInput
  response UserResponse
  record UserRow
  error ErrorResponse

scenario "successful user creation":
  given:
    No user with this email exists.
  when: CreateUser { email: "alice@example.com", name: "Alice" }
  predict success:
    response 201 UserResponse { id: any uuid, email: @when.email, name: @when.name }
    database has [ INSERT with UserRow {
      email: @when.email
      name: @when.name
      id: any uuid
      status: "active"
      deletedAt: absent
    } ]

scenario "rejects duplicate email":
  given:
    execute CreateUser { email: "alice@example.com", name: "Alice" }
  when: CreateUser { email: "alice@example.com", name: "Bob" }
  predict rejection DUPLICATE_EMAIL:
    response 409 ErrorResponse { code: "DUPLICATE_EMAIL" }
    database has []

feat 1.0

# Corpus exemplar #2 — CRUD-only, no event sourcing: one acid service, no
# convergence timeouts. Proves the language carries zero infrastructure vocabulary.

spec SPEC-USR-001 "CreateUser"
context Users
aggregate User
type command
status agreed

construct:
  handler at handlers/create-user.ts
  touches handlers/**
  imports UserRepository from repositories/user-repository
  Uses Prisma ORM for database access.

enforce:
  validate input against CreateUserInput
  check email uniqueness via UserRepository.findByEmail
  rejects DUPLICATE_EMAIL when a user with this email already exists
  insert new user record
  return created user

contract:
  input CreateUserInput
  response UserResponse
  record UserRow
  error ErrorResponse

scenario "successful user creation":
  given:
    No user with this email exists.
  when: CreateUser { email: "alice@example.com", name: "Alice" }
  predict success:
    response 201 UserResponse { id: any uuid, email: @when.email, name: @when.name }
    database has [ INSERT with UserRow {
      email: @when.email
      name: @when.name
      id: any uuid
      status: "active"
      deletedAt: absent
    } ]

scenario "rejects duplicate email":
  given:
    execute CreateUser { email: "alice@example.com", name: "Alice" }
  when: CreateUser { email: "alice@example.com", name: "Bob" }
  predict rejection DUPLICATE_EMAIL:
    response 409 ErrorResponse { code: "DUPLICATE_EMAIL" }
    database has []

Closed reference spaces come from the files on the left — rename a service in feat.config.json and the spec's predictions become parse errors. Derived cases are rendered, not executed: point feat run at your implementation for that.