Context variables

Every rule is evaluated against a context: a set of variables describing the candidate step. This page is the authoritative list. For a gentler introduction see the guide.

Base variables

Supplied directly for every evaluation.

Variable	Type	Definition
prev_sum	number	The running total from all previous steps, before the candidate is applied.
current_value	number	The signed value of the candidate step. Positive is addition, negative is subtraction. Never 0.
step_index	number	Zero-based position of the current step. 0 is the first step.
last_step_index	number	Index of the final step of the exercise — that is, total steps minus one.

Derived variables

Computed from the base variables. You never set these; they are always consistent with the values above.

Variable	Type	Definition
current_sum	number	prev_sum + current_value — the total that results if the candidate is accepted.
isLastStep	boolean	true when step_index = last_step_index.
place	number | null	The single decimal place current_value affects (1, 10, 100, …), or null if it affects more than one.
formula	number | null	abs(current_value) / place, a digit 1–9. null when place is null.
digitBefore	number | null	floor(abs(prev_sum) / place) % 10 — the digit of the running total at the affected place. null when place is null.

How place is determined

A value is atomic when exactly one of its decimal digits is non-zero. place is the positional value of that digit:

current_value	place	formula
7	1	7
-7	1	7
40	10	4
300	100	3
47	null	null
-89	null	null
105	null	null

Sign is ignored throughout: place, formula, and digitBefore are all computed from absolute values. Use current_value > 0 / current_value < 0 to test direction.

The null contract

WARNING

When place, formula, or digitBefore is null, comparisons against them still evaluate. They do not skip the rule, and they do not uniformly fail.

Null behaves as 0 in ordering comparisons and is unequal to every number in equality comparisons. The practical result, for a composite candidate such as +47:

Form	Example	Result
Equality	formula = 8	fails
Membership	place IN (1, 10)	fails
Lower bound	formula >= 1	fails
Strictly greater	formula > 0	fails
Inequality	formula != 8	passes
Negative membership	place NOT IN (1, 10)	passes
Upper bound	formula <= 9	passes
Strictly less	formula < 1	passes

So inclusive tests reject composite steps and exclusive tests admit them. This is the single most common source of surprising rule behavior — see the null trap for what it means in practice.

The upside of the inclusive half: a standard pattern rule of the form current_value > 0 AND formula = N AND digitBefore IN (…) automatically fails on composite steps with no special handling. Pattern rules are about atomic techniques, and they self-exclude correctly.

Scope and which variables apply

A rule declares a scope, which decides when it runs:

Scope	Runs
step	Against every candidate step.
last-step	Against the final step only.
sequence	Once, after a complete candidate sequence has been assembled.

All the variables above are available in step and last-step scope. A sequence-scope rule is evaluated once with the final step's values in scope, plus whole-sequence values describing the assembled exercise.

INFO

Whole-sequence variables are supplied by the generator rather than by the base context, and the editor's linter does not currently recognize them as known identifiers — expect an "unknown variable" diagnostic even where they work. Prefer step-scope rules unless you specifically need to reason about a finished sequence.

Evaluation outcomes

Every rule evaluates to one of three outcomes:

Outcome	Meaning
Passed	The condition held.
Failed	The condition did not hold. For a constraint, this rejects the candidate.
Skipped	The rule's IF guard was false, so the rule did not apply. A skipped rule never blocks a candidate, and never counts as a pattern match.

The distinction between failed and skipped matters for pattern rules: only a passed rule counts as a match toward a recipe's count quotas.

See also

• Built-in functions
• Grammar and precedence
• Decimal places — the practical guide to place / formula / digitBefore