KUJO THE COURSESTAGE 1 / Kujo foundations · LESSON 04
Mental model
Kujo evaluates runtime values, not annotations. Its core values include null, booleans, signed 64-bit integers, floating-point numbers, strings, bytes, arrays, and dictionaries. Functions, structs, tagged values, and native handles add further runtime forms.
Truthiness contract
Falsey values are false, null, integer zero, float zero, the empty string, empty array, and empty dictionary. Everything else is truthy. In particular, the string "false" is true in a condition because it is non-empty. Parsing configuration strings is a separate operation.
&& and || short-circuit, but return booleans rather than one of their operands. A ported expression such as name || "unknown" will not produce the desired fallback string. Use explicit control flow or a null-aware operation when that is what you mean.
Numeric contract
Integer arithmetic is checked. Overflow, division by zero, and modulo by zero are runtime errors. Floating-point division by zero is also rejected. Finite float equality uses Kujo's epsilon policy; this is not an exact decimal money representation. Numeric comparison permits int/float pairs, while unrelated ordering types fail.
Arrays compare deeply in order. Dictionaries compare their effective key/value contents. JSON serialization rejects unsupported runtime values and non-finite floats, so don't assume every object that prints can be sent as JSON.
Professional pattern
Use explicit predicates for domain rules. An empty array being falsey does not tell you whether the absence of records is valid or an upstream failure. Distinguish those states in your data model. If a predicate helper returns 1/0, comparing with 1 makes its intended meaning clear at a boundary.
Runtime drill
Before running the example, write down the number of truthy inputs. Then predict the type of each logical expression. Changing one value at a time is more informative than memorizing a table without executing it.
Working example
mut hits := 0
for value in [false, null, 0, 0.0, "", [], {}, "false", [0]] {
if value { hits += 1 }
}
assert_equal(hits, 2)
assert_equal(true && 5, true)
assert_equal(false || "fallback", true)
assert_equal(1 == 1.0, true)
print(hits)
Run it
From the course repository root, use the pinned Kujo 1.3.1 runtime.
kujo check examples/04.kujo
kujo run --untrusted examples/04.kujo
Captured output
2
Break it and diagnose it
The operands are valid integers but their sum exceeds the signed 64-bit range. The runtime must reject overflow rather than wrap silently.
print(9223372036854775807 + 1)
kujo run --untrusted examples/04-break.kujo
Exit status: 4. Captured diagnostic:
[KUJOVM001] [vm] Runtime Error: Integer overflow: 9223372036854775807 + 1
--> 0:0
Exercise
Build a truthiness table for ten values. Add separate tests for null and empty input in a validator. Demonstrate short-circuiting by placing a failing operation on a branch that must not execute.
Checkpoint
- I can enumerate falsey values.
- I know logical operators return booleans.
- I expect checked integer arithmetic.
Verified 2026-09-06 · Official Kujo 1.3.1 release · Source contract · Download example