Values and undecided

Every value carries how well it is known alongside the number itself, and that status survives arithmetic.

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

null never means “unknown.” Value.unknown() is a distinct status, so an absent field and a field known to be unavailable are different facts and the model keeps them apart.

A value whose sources disagree becomes a ConflictingValue holding the candidates. The kernel does not resolve it by picking one; the candidates stay visible until something decides between them.

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, so 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 rather than poisoning every expression it touches. false and undecided is false, because no value of the unknown operand could make the conjunction true. true or undecided is true.

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.

Undecided is never silently a pass

An undecided result is never quietly a pass or a failure. Whether it blocks is a policy decision, made by the commit gate in KernelGraph.propose rather than by the evaluator. That separation lets an early-stage design proceed with unknowns while a release gate refuses them, with no change to any constraint.