Shadow Test Blocks

Functions may carry inline test blocks that document and verify expected behaviour at ilo check time. Test blocks extend ilo’s verify-before-run philosophy from type-checking to behaviour-checking.

Syntax

ilo

fn add a:n b:n>n;+a b
test add { ok add 2 3 5; ok add 0 0 0; ok add -1 1 0 }

fn div a:n b:n>R n t;=b 0 ^"divide by zero";~/a b
test div { ok div 10 2 5; err div 10 0 "divide by zero" }

Element	Form
Block header	test <fn-name> {
Ok assertion	ok <fn-name> <arg>... <expected> ;
Err assertion	err <fn-name> <arg>... <expected-err> ;
Block end	}

How it works

• ok calls fn-name with the given literal arguments and asserts the return value equals expected. For R-returning functions, this checks the ~v (Ok) path.
• err calls fn-name and asserts it returns ^expected-err. The expected error text is a string literal.
• Literal args only. Test assertions accept number, text, boolean, and nil literals, not arbitrary expressions.
• Last literal is the expected value. All preceding literals are call arguments. So ok add 2 3 5 means: call add(2, 3), assert result is 5.
• ;-separated. Assertions are separated by ; inside the braces. Trailing ; is optional.

Error codes

Code	Severity	Trigger
ILO-T050	Error	A test assertion failed (actual != expected)
ILO-W020	Warning	Function has no test block (under --strict)

Relationship to ilo test

Shadow test blocks live inside the language (in the source AST). They complement the existing -- run: / -- out: / -- err: annotation format used by the ilo test command, which lives above the language (in comments the test harness reads).

Feature	Shadow test blocks	-- run: / -- out:
Where	In-source AST	In comments
When	ilo check time	ilo test command
Args	Literal values only	CLI args (strings, numbers, lists)
Engine	Verify pass	All engines (VM, JIT)
Assertion shape	ok/err	stdout/stderr/exit code

Example

ilo

-- A function with its shadow test
fn safe-div a:n b:n>R n t
;=b 0 ^"divide by zero"
;~/a b

test safe-div {
  ok safe-div 10 2 5;
  ok safe-div 0 1 0;
  err safe-div 10 0 "divide by zero"
}

Run ilo check to verify. If safe-div returns the wrong value for any assertion, ILO-T050 fires with the actual vs expected values. If safe-div has no test block and you run ilo check --strict, ILO-W020 warns that the function lacks self-verification.