KUJO THE COURSESTAGE 1 / Kujo foundations · LESSON 05
Why this exists
Automation spends much of its time filtering records and handling unusual input. Clear control flow makes those decisions reviewable. A loop also creates an obligation: define how it terminates before adding an external effect inside it.
Language contract
if and else select branches using runtime truthiness. for item in items iterates a collection. while checks a condition; loop repeats until you break or otherwise exit. break and continue belong inside a loop.
Function bodies and block bodies create lexical boundaries. The for-loop variable does not leak after the loop. A local declaration inside a branch should not be used later as though it were a global assignment. Declare mutable accumulated state outside the loop when its result must survive the iteration.
Read the example
The program accepts positive records from a fixed input. This separates the selection rule from the stopping rule. Neither depends on elapsed time or a provider response, so the result is repeatable.
Do not confuse the number of visited items with the number accepted. In a production import, those are different metrics and often need separate counters. A machine-readable receipt should name each one precisely.
Professional pattern
Prefer a for-loop over known bounded input when possible. For a while-loop, identify the variable that moves toward termination and test the boundary cases: zero work, one item, and the maximum allowed work. For retry loops, later lessons add a finite attempt budget and an independent decision about whether retrying is useful.
Common mistakes
A continue placed before a counter update can make a while-loop run forever. A shadowed counter may leave the condition unchanged. Nested loops need clear break behavior; break does not express a general escape from all enclosing work.
The failure drill uses a name that was never declared. The release-specific loop-scope discrepancy is documented below.
Working example
func select(values) {
mut accepted := []
for value in values {
if value > 0 { accepted = push(accepted, value) }
}
return accepted
}
assert_equal(select([-1, 2, 0, 4]), [2, 4])
print(to_json(select([-1, 2, 0, 4])))
Run it
From the course repository root, use the pinned Kujo 1.3.1 runtime.
kujo check examples/05.kujo
kujo run --untrusted examples/05.kujo
Captured output
[2,4]
Break it and diagnose it
A never-declared name is a runtime error. The separate scope probe records the release discrepancy.
print(undefined_course_name)
kujo run --untrusted examples/05-break.kujo
Exit status: 4. Captured diagnostic:
[KUJOVM001] [vm] Runtime Error: Undefined variable: undefined_course_name
--> 0:0
= help: Define this name before using it, check the spelling, or import the module that provides it.
Exercise
Rewrite the selection as a while-loop with an explicit index bound. Test empty input, all rejected values, and fewer accepted values than the limit. State why each version terminates.
Checkpoint
- I can distinguish skip and stop behavior.
- I keep accumulators in an intentional scope.
- I can state a finite loop bound.
Contract versus release behavior
The written contract says a for-loop variable does not leak. In the official 1.3.1 VM, a top-level loop variable remained visible afterward; interpreter mode rejected that reference. A continue/break collection loop also exceeded the external two-second probe deadline. The successful lesson uses a finite for-loop without those control transfers. Do not infer that the release meets the documented scope contract in every case. See the evidence ledger for exact probes and runtime results.
Verified 2026-09-06 · Official Kujo 1.3.1 release · Source contract · Download example