Assignable Return

Last lesson the parent could read a child’s returned value. The full circle: let the parent assign to it. <return> accepts a valueChange handler — the same interception contract as everything in part two — and with it, the returned tag variable becomes writable from outside:

<let/count=0>
<return=count valueChange(next) { count = next }>

<button onClick() { count++ }>${count}</button>

/* the parent */
<counter/count/>

<button onClick() { count = 0 }>Reset</button>

The parent’s count = 0 routes through the child’s valueChange, which updates the child’s own state. Notice the design consistency: this is :=’s machinery — value plus change handler — applied to a tag variable. Marko keeps re-using one contract. (And here’s a mind- bender from the docs: <let> itself is conceptually just <return=value valueChange(...)> around some state.)

The game page on the right embeds a counter and has a Reset button assigning to its tag variable — currently a runtime error, because the child’s return is read-only:

1. In counter.marko, extend the return: <return=count valueChange(next) { count = next }>.
2. Play: click the counter up, hit Reset — parent writes, child obeys, both stay in sync through one state that lives where it belongs, in the child.