15. Guard Model

when is a universal execution guard.

It is a structured guard, not a full arbitrary boolean expression language.

Current direction:

• a single-condition guard may use inline form
• multi-condition guards use block form with all or any
• check is the predicate clause used inside block-form guards
• inline when and block-form check use the same predicate set

15.1 Inline Form

Inline form:

when <left-operand as required by predicate> <predicate> [right-operand as required by predicate]

Rules:

• inline when begins with the predicate's required left operand
• the left operand must be an explicit KAL reference for exists, empty, not_empty, is, is_not, contains, >, >=, <, and <=
• the left operand for in may be a scalar literal, a string literal, or a readable KAL reference
• inline when contains exactly one predicate clause
• operand presence is determined by predicate arity
• inline when does not support composition
• inline when does not support nesting
• inline when does not support wrapper negation
• &&, ||, and ! are not part of the current when surface

Examples:

when inputs.enabled is #true
when inputs.count >= 1
when inputs.name not_empty

15.2 Block Form

A when block contains one root condition group.

A condition group is either all or any.

An all or any group may contain:

• check predicate clauses
• nested all groups
• nested any groups

Block form uses:

when
  all
  any
  check

Example:

when {
  all {
    check inputs.enabled is #true

    any {
      check inputs.env is "prod"
      check inputs.env is "staging"
    }
  }
}

Group-level not is not part of the current draft.

15.3 Predicate Set

Current predicate set:

is
is_not
exists
empty
not_empty
contains
in
>
>=
<
<=

Predicate forms:

Predicate	Form
exists	<reference> exists
empty	<reference> empty
not_empty	<reference> not_empty
is	<reference> is <value>
is_not	<reference> is_not <value>
contains	<container> contains <item>
in	<item> in <container>
> / >= / < / <=	<reference> <predicate> <value>

Rules:

• exists, empty, and not_empty do not take a value operand
• is, is_not, contains, in, >, >=, <, and <= require a value operand
• a predicate value operand may be a scalar literal, a string literal, or a readable KAL reference
• predicate value operands must not be function calls, general expressions, operator invocations, or inline object/array literals
• forms such as when inputs.name exists #true fail validation

Direction:

• is and is_not are equality predicates
• == and != are not part of the current when surface
• contains is read as <container> contains <item>
• in is read as <item> in <container>

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