0003 — :children materialises eagerly; :call pushes a stack frame

Context

A component needs two distinct ways to compose with other components:

1. Structural composition. "This component contains a left sidebar and a main panel, both of which are independent components with their own state." These children co-exist for as long as the parent does; they don't return values.
2. Sequential composition. "Ask the user a question via a child component; resume the parent with the answer." The child is temporary; it pops when it answers.

Conflating the two would force callers to express "always-on sub-component" as some weird "calls itself forever" pattern, or to wedge return values into structural children.

Decision

Two distinct primitives:

• :children on a component definition declares the structural set. :children {:slot/left (s/embed :sidebar) :slot/main (s/embed :panel)} instantiates both children eagerly at the parent's own instantiation time. They live in :conv/instances alongside the parent, addressed through :instance/children. They do not appear on :conv/stack.
• :call is an effect: (s/call :child/picker {} :on-picked). It pushes a new frame onto :conv/stack. The child's HTML replaces the parent's; on :answer, the frame pops and the parent resumes via the named key.

Consequences

• Two mental models, clearly separated. Authors don't have to decide whether a child is "structural or sequential" through some awkward shared API — the primitives are different shapes from the start.
• :children can't take return values. That's the point. If you need an answer, that's a :call site, not a child slot.
• :call-in-slot covers the hybrid case. Sometimes you want a sequential exchange that doesn't take over the page — a confirm modal inside a panel, an inline editor on one cell. :call-in-slot <slot> <embed> :on-answer temporarily swaps the child in a known slot; the answer returns to the parent without popping the page.
• Keyed children get a third primitive. Ordered, dynamic collections (a sortable list of cards) use :set-keyed-children; see keyed.clj. Same composition model — children alongside the parent, addressed by slot + key — but with append/remove/outer patch reconciliation instead of full re-render.

See also

• 0001 — how the answer routes back to the parent
• 0002 — why the whole child set is persistable