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title Expressions (CEL)
description Canonical CEL-based expression language for formulas, predicates, conditions, and dynamic seed values

Expressions

ObjectStack uses a single canonical expression language for every place a piece of metadata needs to compute a value or evaluate a condition:

  • Formula fields (type: 'formula')
  • Predicates — validation condition, sharing condition, field conditional rules (visibleWhen, readonlyWhen, requiredWhen), view section/column visibility (visibleOn), action disabled, view filter criteria, hook condition, flow decisions
  • Dynamic seed values — fixtures whose value depends on the install-time clock or identity context

The language is CEL (Google's Common Expression Language, Apache-2.0), evaluated through @objectstack/formula (thin wrapper over @marcbachmann/cel-js) plus the ObjectStack standard library.

Why CEL? Formal grammar, abundant public training corpus (so AI authors emit it natively), AST-first persistence, and sandboxed execution with bounded cost. ObjectStack does not ship a Salesforce-flavor DSL — the previous custom 22-function engine was deleted in M9. See north-star §8 "No private expression DSL".

The Expression envelope

Every expression in metadata is persisted as the same envelope:

type Expression = {
  dialect: 'cel' | 'js' | 'cron' | 'template';
  source?: string;          // surface syntax
  ast?: unknown;            // parsed AST (filled by `objectstack compile`)
  meta?: { rationale?: string; generatedBy?: string };
};

ObjectStack ships four registered dialects (M9.9):

Dialect Use for Helper
cel computed values, predicates, defaults, formulas cel`...` / F`...` / P`...`
cron recurring schedules (Job, ETL, sync, exports, jobs) cron`...`
template text interpolation ({{path}}) — notif/prompt/title tmpl`...`
js sandboxed L2 hook bodies (TypeScript source) n/a

All dialects share the same variable scope (record, previous, os.user, os.org, os.env, vars for flow steps), so you author once and never have to relearn the syntax across surfaces.

**Predicates are bare CEL — never wrap field references in `{…}` braces.** The most common mistake (and the root cause of issue #1491) is a condition like `{record.rating} >= 4`: in CEL, `{…}` is a **map literal**, so it is a parse error. Write bare CEL: `record.rating >= 4`. Braces belong only in `{{ … }}` text templates. As of 7.6 a malformed expression no longer silently does nothing — it fails `objectstack build` and throws at runtime.

Template formatting (7.6). A template hole is a field path with an optional whitelisted formatter — {{ path | formatter[:arg] }} — with defined value→string semantics (no arbitrary logic; put logic in a CEL field):

tmpl`Deal {{record.name}} — {{record.amount | currency}} closes {{record.close_date | date:long}}`

Formatters: currency[:CODE], number[:decimals], percent[:decimals], date[:short|long|iso], datetime[:…], upper, lower, trim, truncate:N, default:'…', json. Single-brace {x} is not a valid template hole.

At input time you may write a bare string for shorthand — the spec transforms it into the right envelope based on the field type. The compiled artifact always contains the full envelope (and, after M9.2, the AST).

import { F, P, cel, cron, tmpl } from '@objectstack/spec';
import { ObjectSchema, Field } from '@objectstack/spec/data';

export const Invoice = ObjectSchema.create({
  name: 'invoice',
  titleFormat: tmpl`Invoice {{record.invoice_no}} – {{record.customer.name}}`,
  fields: {
    total: Field.formula({
      returnType: 'number',
      expression: F`record.subtotal + record.subtotal * record.tax_rate`,
    }),
    po_number: Field.text({
      requiredWhen: P`record.amount > 10000`,
    }),
    // M9.9b — declarative default, evaluated at insert time
    issued_date: Field.date({
      defaultValue: cel`today()`,
    }),
  },
});
**A formula field's `type` is always `formula`** — that is the key the runtime evaluates (`objectql` computes a formula virtual field from its `expression` only). Do **not** pass `type: 'currency' | 'number' | …` to `Field.formula` — it is rejected by the typed `FieldInput` and, untyped, would override `type:'formula'` so the field silently never computes. To declare what the formula returns, set **`returnType`** (`'number' | 'text' | 'boolean' | 'date'`); it is inferred and stamped automatically by the AI build path, and consumers (dashboard measures, formatting) read it instead of re-parsing the expression. The CEL source is the **`expression`** key (the `formula` alias is normalized to it on save).

The three CEL tagged-template helpers — cel, F (formula alias), P (predicate alias) — are interchangeable; pick whichever reads best at the call site. cron and tmpl produce their respective envelopes.

CEL primer

CEL is documented at cel-spec. Cheat-sheet of what you'll write daily:

Concept Syntax
Field on current record record.first_name
Previous (in update hooks) previous.status
Input to a hook input.amount
Equality / inequality == / !=
Logical && / || / !
Ternary cond ? then : else
String literal 'single quotes'
Membership record.region in ['us', 'eu']
Field present has(record.foo) (returns true even when value is null)

has() gotcha: has(record.x) is true whenever the key exists, even when its value is null. To check for "not blank" use isBlank(...) or record.x != null — see stdlib below.

ObjectStack CEL standard library

Registered automatically into every Environment by @objectstack/formula. All functions are pure given a pinned now, which is what makes objectstack build artifacts byte-stable across runs.

Function Returns Description
now() timestamp Pinned wall-clock at evaluation start
today() timestamp now() truncated to UTC start-of-day
daysFromNow(n) timestamp now() + n days (preserves wall-clock time, not start-of-day)
daysAgo(n) timestamp now() - n days
isBlank(v) bool True for null, undefined, '', []
coalesce(v, fallback) dyn v when non-null, else fallback
trim(v) string v with leading/trailing whitespace removed
joinNonEmpty(list, sep) string list joined by sep, skipping blank entries

Add new helpers in packages/formula/src/stdlib.ts. Keep them pure, dependency-free, and AI-readable.

Variable scope

@objectstack/formula builds the scope per evaluation:

Binding Source Available in
record the row being evaluated formulas, validation, sharing, visibility
previous row before update hooks, validation on update
input hook payload hooks
os.user install / request user seed, predicates with identity
os.org active organization seed, predicates
os.env install env (prod, dev, test) seed, predicates

Cookbook

Computed full name (handle nullable parts)

{
  name: 'full_name',
  type: 'formula',
  expression: F`joinNonEmpty([record.salutation, record.first_name, record.last_name], ' ')`,
}

joinNonEmpty skips blank parts, so you avoid the verbose triple-coalesce. Building the string by hand with + would also work, but CEL throws on null + string, so each nullable operand must be wrapped in coalesce(..., '').

Conditional formula

{
  name: 'roi',
  type: 'formula',
  expression: F`coalesce(record.actual_cost, 0) > 0
             ? ((coalesce(record.actual_revenue, 0) - record.actual_cost) * 100.0) / record.actual_cost
             : 0.0`,
}

Predicate (visibility, read-only, conditional required)

{
  name: 'rating',
  type: 'picklist',
  visibleWhen: P`record.status == 'qualified'`,
}

Field.text({
  visibleWhen: P`record.type == 'business'`,
  readonlyWhen: P`record.status == 'approved'`,
  requiredWhen: P`record.amount > 10000`,
})

Dynamic seed dates

Use CEL inside seed records to defer evaluation to install time. The SeedLoader pins a single now per load run, so the same dataset produces identical SHA-1 across builds while still being "fresh" relative to whoever installs the package.

import { defineDataset, cel } from '@objectstack/spec';

export const opportunityDataset = defineDataset({
  object: 'opportunity',
  records: [
    {
      id: 'opp_acme',
      name: 'Acme Q3 Renewal',
      close_date: cel`daysFromNow(45)`,
      created_at: cel`now()`,
    },
  ],
});

Without CEL, new Date() would be evaluated at compile time — your package would ship with the timestamp from your laptop and every customer would see "stale" demo data. With CEL, the date resolves on the customer's clock at install time.

Validation rule

{
  name: 'amount_positive',
  type: 'script',
  condition: P`record.amount <= 0`,
  message: 'Amount must be positive.',
}

The generic CEL rule is type: 'script'. For script rules, the condition is the failure predicate — when it evaluates TRUE the validation fails, so invert the test (here amount <= 0 rejects non-positive amounts).

Hook condition

{
  name: 'notify_on_escalation',
  object: 'case',
  events: ['afterUpdate'],
  condition: P`previous.status != 'escalated' && record.status == 'escalated'`,
  body: { language: 'js', source: 'await ctx.notifyOpsTeam(record)' },
}

Determinism contract

Two consecutive objectstack build runs with no source changes must produce byte-identical dist/objectstack.json. CEL plus pinned now is what makes this possible — there are zero compile-time-evaluated Date.now() calls in seed metadata. CI enforces this via SHA-1 comparison; if you add a new dialect or stdlib helper, ensure it preserves determinism.

Migration from the legacy formula DSL

The previous custom recursive-descent engine (packages/objectql/src/formula.ts, deleted in M9.5) had a Salesforce-style function set. Mechanical translation:

Legacy CEL equivalent
bare_field record.bare_field
= / <> == / !=
AND / OR / NOT && / || / !
"text" 'text' (single quotes preferred)
IF(c, a, b) c ? a : b
ISBLANK(x) isBlank(record.x)
CONCAT(a, b, …) a + b + … (with coalesce(...,''))
TODAY() / NOW() today() / now()
IN (a, b) record.x in [a, b]
ISCHANGED(x) (in update hook) previous.x != record.x
MONTH_DIFF(a, b) not yet — add to stdlib

Salesforce compatibility is not a project goal. Authors targeting Salesforce semantics rewrite their formulas in CEL.

Programmatic API

import { ExpressionEngine } from '@objectstack/formula';

// Compile + cache
const compiled = ExpressionEngine.compile({ dialect: 'cel', source: 'record.x * 2' });

// Evaluate
const result = ExpressionEngine.evaluate(
  { dialect: 'cel', source: 'record.x * 2' },
  { record: { x: 21 } },
);
// => { ok: true, value: 42 }

The low-level engine never throws — evaluate() returns { ok: false, error }. But call sites must not silently swallow that (ADR-0032): objectstack build fails on an invalid expression (with a located, schema-aware message), and at runtime the flow/rule engines throw a loud, attributed error instead of treating a bad expression as false/null. The same validateExpression validator backs objectstack build and metadata registration (and a planned validate_expression agent tool), so an expression is checked before it ships.

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