14. Capture Model

capture is a common KAL binding syntax used at operator boundaries.

capture is not a general KDL operator.

It binds values from an operator's capture domain into writable caller destinations.

The universal execution outcome is distinct from the operator's capture domain. capture reads capture-domain source names, not status or message.

Within capture, _ means the whole capture-domain object for that operator invocation.

Named sources should be used when only a specific value is needed.

14.1 Capture Mapping

Current mapping form:

source -> destination

Rules:

• the left side must be either a named capture source or _
• the right side must be an explicit writable destination reference
• writable destinations include locals.*, outputs.*, and declared writable globals.*, subject to normal scope rules
• loop.result.* is also a writable capture destination inside a core.each do body
• inputs.* is not writable
• capture does not create a new reference namespace
• uncaptured results are discarded unless operator semantics say otherwise

Example:

capture {
  result -> locals.result
  _ -> locals.full_result
}

14.2 Local Declaration Shorthand

The current draft allows capture-only local declaration shorthand:

local name

Rules:

• local foo introduces a new locals.foo slot
• after introduction, later references use locals.foo
• if locals.foo already exists, local foo fails validation
• if the destination already exists, use locals.foo
• body-level local introduction is limited to capture destinations
• local foo is not a general write syntax
• if a local needs header-style type or default semantics, it must be declared in the component header

Example:

capture {
  result -> local normalized_name
}

14.3 Failure Behavior

Capture writes occur only when the enclosing operator invocation completes with status=ok.

If an operator invocation is skipped or failed:

• its capture block performs no destination writes
• partial implementation-local values are not exposed
• a failed child component does not expose partial outputs to its caller

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