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VBL Predicates: Named Logical Expressions

Defining Predicates

Section titled “Defining Predicates”

A predicate binds a name to a logical expression:

predicate: sufficient_funds: account_balance >= transaction_amount
predicate: risk_acceptable: risk_score <= 75
predicate: amount_within_limit: transaction_amount <= 50000

Each predicate has:

  • Name: a unique identifier (lowercase, alphanumeric + underscore)
  • Expression: a comparison that evaluates to true or false

Predicate Names

Section titled “Predicate Names”

Predicate names follow evidence class naming rules:

  • Start with a lowercase letter
  • Contain only lowercase alphanumeric characters and underscores
  • Must be unique within the boundary
  • Cannot be a reserved keyword

Comparison Operators

Section titled “Comparison Operators”

Predicates support all comparison operators:

Operator Meaning
== Equal to
!= Not equal to
> Greater than
< Less than
>= Greater than or equal to
<= Less than or equal to

Integer Comparisons

Section titled “Integer Comparisons”
predicate: creditworthy: credit_score >= 600
predicate: within_authority: loan_amount_cents <= 500000

String Comparisons

Section titled “String Comparisons”
predicate: read_only: access_level == "read_only"

Boolean Comparisons

Section titled “Boolean Comparisons”
predicate: sanctions_ok: sanctions_cleared == true
predicate: not_break_glass: is_break_glass == false

Field-to-Field Comparisons

Section titled “Field-to-Field Comparisons”
predicate: dosage_safe: prescribed_mg <= max_daily_mg
predicate: refund_ok: refund_amount_cents <= order_total_cents

Set Membership

Section titled “Set Membership”
predicate: not_blocked: destination_country NOT IN @blocked_jurisdictions
predicate: severity_actionable: severity IN ("HIGH", "CRITICAL")
predicate: modality_cleared: modality IN @cleared_modalities

List Containment

Section titled “List Containment”
predicate: has_required_approver: approver_list CONTAINS "compliance_officer"

Failure Messages

Section titled “Failure Messages”

Attach a human-readable failure message to a predicate:

predicate: sanctions_ok: sanctions_cleared == true | "Sanctions screening must clear"
predicate: refund_ok: refund_amount_cents <= order_total_cents | "Refund cannot exceed order total"

Messages are metadata-only: they appear in failure reasons but do not affect evaluation outcome.

Conjunctive Evaluation

Section titled “Conjunctive Evaluation”

All predicates are evaluated conjunctively: every predicate must pass for the evaluation to produce ASSERT. If any predicate fails, the result is DEFER.

predicate: creditworthy: credit_score >= 600        # must pass
predicate: affordable: debt_to_income_pct <= 40      # must pass
predicate: established: account_age_months >= 6      # must pass
predicate: identity_confirmed: identity_verified == 1  # must pass

All four must evaluate to true. If affordable fails, the result is DEFER even if all others pass.

No Short-Circuit Within Stage 3

Section titled “No Short-Circuit Within Stage 3”

All predicates are evaluated even if an earlier predicate fails. Both branches of AND/OR expressions are always evaluated. This ensures deterministic instruction counting: the execution path does not depend on predicate ordering.

All failing predicates appear in the failure_reasons response, giving the caller complete visibility into what needs to change.

Predicates vs. BOUNDARY Expression

Section titled “Predicates vs. BOUNDARY Expression”

Predicates define individual conditions evaluated conjunctively (all AND). The BOUNDARY expression uses inline field comparisons with full logical operators (AND, OR, NOT, grouping).

If you define predicates without a BOUNDARY expression, the predicates alone determine the verdict. If you define a BOUNDARY expression, it is evaluated in addition to the predicates: both must pass.

Use predicates alone when all conditions must pass (conjunctive). Use a BOUNDARY block when you need OR, NOT, or complex grouping.

Next Steps

Section titled “Next Steps”