Values and undecided

A number in an engineering model is never only a number. How well it is known travels with it, and the kernel refuses to lose that on the way through a calculation.

Every value carries a status

from fang.lang import V, kOhm

Resistor(resistance=10 * kOhm)   # explicit: someone wrote it

Status	Means
explicit	Someone wrote it
inferred	The kernel computed it from other facts
assumed	A working claim with no evidence behind it
unknown	Not known, which is itself a value rather than a gap

null never means “unknown.” Value.unknown() is a distinct status, so a field that is absent and a field that is known to be unavailable are different facts and stay different.

A value whose sources disagree becomes a ConflictingValue holding the candidates. It is not resolved by picking the first, the newest or the most precise. It stays unresolved and visible until something decides it.

Quantities have dimensions

A Quantity is a decimal magnitude over a dimension vector on the seven SI bases. It can be a scalar, a range or a nominal with a tolerance:

from fang.lang import V, between, tolerance

between(3.0 * V, 3.6 * V)          # a range
tolerance(10 * kOhm, "1%")         # nominal and tolerance

Dimensional errors are rejected where they are written. Arithmetic checks dimensions in __post_init__, so 3 * V + 10 * kOhm raises UNIT-0001 at construction. A dimensionally invalid expression cannot be stored, let alone evaluated. That is why nothing downstream re-checks units.

Undecided is a truth value

Constraints evaluate to one of three values, not two:

Truth	When
true	Satisfied
false	Violated
undecided	An operand is unknown

Undecided propagates through Kleene three-valued logic, which is stronger than saying unknown poisons everything. false and undecided is false, because no value of the unknown operand could make the conjunction true. true or undecided is true. The kernel decides what it can and stops exactly where it must.

Check results carry the same idea, plus one more case:

CheckStatus	Means
PASS	Decided and satisfied
FAIL	Decided and violated
UNKNOWN	Not decidable on what is known
NOT_APPLICABLE	The check does not cover this

UNKNOWN and NOT_APPLICABLE are separate because “we could not tell” and “there was nothing to tell” are different engineering situations. Collapsing them is how a checklist comes to report coverage it does not have.

Undecided is never silently a pass

An undecided result is never quietly a pass and never quietly a failure. Whether it blocks is a policy decision made by the commit gate in KernelGraph.propose, not by the evaluator. The evaluator’s job is to be honest about what it knows. The gate’s job is to decide what to do about it.

That separation is what lets an early-stage design proceed with unknowns while a release gate refuses them, with no change to any constraint.