Worked rule examples

Reading the operators reference tells you what each piece does in isolation. This page is the other half: three complete, real rules, each shown exactly as the rule editor renders it and explained one piece at a time — what it reads, what it computes, and why it is written that particular way rather than some other way that merely looks equivalent.

Example 1 — age from a date of birth

A programme intake form asks for a date of birth and needs an age in whole years, displayed back to the respondent but never hand-typed by them — the two must never disagree.

Rule 1age_yearsyearsBetween(date_of_birth, today())

Do this

The respondent cannot edit a calculated field — the formula owns its value.

Set it to

today()

Reads the instant the form is being evaluated against — the browser's clock while the respondent is typing, and the actual submission timestamp when the server re-checks it at submit. The two can disagree by the few seconds it took to fill in the rest of the form, which is never enough to change a whole year.

yearsBetween(date_of_birth, …)

Counts whole birthdays that have passed between date_of_birth and that instant — 17 years and 11 months returns 17, not 17.9.

Example 2 — a follow-up that is only sometimes required

The same intake form collects household size. Above six people, the programme's rules require a second form of identification for the extra household members — but asking every applicant for it, most of whom have small households, would be pure friction.

Rule 1additional_idgt(household_size, 6)

Do this

The question becomes mandatory whenever this is true.

Only when

gt(household_size, 6)

True once household_size holds a number greater than 6.

→ REQUIRE additional_id

While the condition is false, additional_id is optional and can be left blank at submission — it only becomes mandatory once household_size crosses the threshold.

Example 3 — rejecting a contradiction across two fields

A monitoring visit form records a visit's start and end time. Neither field alone can be wrong, but the combination can be — an end time before the start time is a data-entry slip that should never reach the database.

Rule 1end_timenot(gt(end_time, start_time))

Do this

Write the condition that is WRONG — the message shows next to the target question.

Message

End time must be after the start time.

Reject when

gt(end_time, start_time)

The good case: true when the visit ended after it started.

not( … )

Validate rules describe the BAD case, so the good condition is inverted. The submission is rejected exactly when this is true — i.e. exactly when the good case is false.

Starting templates

Every rule above is common enough that you rarely need to build it node by node. The rule editor offers these as ready-made starting points — like picking a form letter instead of writing one from a blank page — so the common shapes are one click rather than four nested pickers. Each produces a tree that compiles as-is; you then swap a field or a constant to match your form.

Template	Kind	What it builds
Age from date of birth	Calculate	`yearsBetween(date_field, today())`. Swap the date field to match your question.
Add two answers together	Calculate	Adds two numeric questions. Replace the second operand with another field or a constant.
Show when an answer matches	Show	Reveals a question when another has a specific value. Fill in the target value.
Show once something is answered	Show	Reveals a follow-up the moment an earlier question is filled in.
Require a follow-up	Require	Makes a question mandatory once another has any answer.
Reject a value outside a range	Validate	`not(between(field, 0, 100))` — blocks submission when a number falls outside the allowed band.

Worked rule examples

Example 1 — age from a date of birth#

Example 2 — a follow-up that is only sometimes required#

Example 3 — rejecting a contradiction across two fields#

Starting templates#

Example 1 — age from a date of birth

Example 2 — a follow-up that is only sometimes required

Example 3 — rejecting a contradiction across two fields

Starting templates