Testing ​

Mesh includes a first-class testing framework accessible via meshc test. Test files use the .test.mpl extension and can contain individual tests, grouped tests with shared setup/teardown, mock actors, and receive assertions.

Production backend proof: This page covers the testing primitives. For the real backend proof targets — including reference-backend/ package tests, ignored deploy/runtime proofs, and the documentation verifier — start with Production Backend Proof and reference-backend/README.md.

Running Tests ​

meshc test reference-backend
meshc test reference-backend/tests
meshc test reference-backend/tests/config.test.mpl

meshc test discovers all *.test.mpl files under the requested project root or directory target, compiles and runs each independently, and prints a summary:

test arithmetic is correct ... ok
test string operations/length ... ok

2 tests, 0 failures

On failure, the output includes the failing assertion, the expected and actual values, and the file/test name. The exit code is non-zero if any test fails.

Writing Tests ​

Test files are standalone .test.mpl programs. Each test block defines a named test:

test("arithmetic is correct") do
  assert(1 + 1 == 2)
  assert_eq(10, 5 + 5)
  assert_ne(3, 4)
end

test("string operations") do
  assert(String.length("hello") == 5)
  assert_eq("hello", String.downcase("HELLO"))
end

Assertions ​

test("assertions") do
  assert(true)
  assert_eq(42, 40 + 2)
  assert_ne("hello", "world")
  assert_raises fn() do
    panic("intentional")
  end
end

Grouping with describe ​

Use describe to group related tests. The group name appears in failure output:

describe("string operations") do
  test("length") do
    assert(String.length("hello") == 5)
  end

  test("concat") do
    assert_eq("ab", "a" <> "b")
  end
end

Failed test output shows: string operations/length ... FAIL

Setup and Teardown ​

setup and teardown blocks run before and after each test in a describe group:

describe("counter") do
  setup do
    assert(true)   # runs before each test in this describe
  end

  teardown do
    assert(true)   # runs after each test in this describe
  end

  test("increments") do
    assert_eq(1, 0 + 1)
  end
end

setup and teardown are scoped to the describe block — they do not affect tests outside of it.

Mock Actors ​

Use Test.mock_actor to spawn a lightweight actor in a test. The actor runs a callback for each message it receives:

test("mock actor messaging") do
  let me = self()
  let mock = Test.mock_actor(fn msg do
    send(me, msg)
    "ok"
  end)
  send(mock, "hello")
  assert_receive "hello", 500
end

Test.mock_actor(fn msg -> ... end) returns a Pid you can send messages to. The mock callback must return a string — "ok" to continue, "stop" to terminate the mock.

assert_receive ​

assert_receive waits for the current test actor to receive a message matching a pattern:

test("receive a message") do
  let me = self()
  send(me, 42)
  assert_receive 42, 500   # pattern, timeout_ms
end

If the message is not received within the timeout, the test fails with the pattern and elapsed time. The default timeout is 100ms when omitted:

assert_receive "done", 1000   # explicit timeout

Coverage ​

Coverage requests are intentionally honest today:

meshc test --coverage reference-backend

--coverage currently exits non-zero with an explicit unsupported message instead of returning a stub report.

What's Next? ​

• Standard Library — Crypto, Encoding, DateTime modules
• Developer Tools — meshc, meshpkg, formatter, REPL, LSP
• Concurrency — actors and supervision for testing async code