Async and futures

A future represents a result that may not be ready yet. runAsync() starts work and returns a future; get() waits for its result.

Starting work

slow = runAsync( () => {
    sleep( 30 );
    return "slow";
} );
fast = runAsync( () => "fast" );

println( fast.get() & " then " & slow.get() );
println( runAsync( () => 4 ).then( value => value * 2 ).get() );

fast then slow
8

Use then() to transform a result when it arrives. asyncAllApply( items, callback ) applies a function to a collection and returns the results.

Handling errors

Errors from asynchronous work surface when you collect the result. Use try/catch, or provide a recovery callback:

result = runAsync( () => {
    throw( message = "unavailable" );
} ).exceptionally( error => "fallback" );

println( result.get() );

fallback

See error handling for normal catch and cleanup patterns.

Concurrency

BoxLang futures take turns on the VM's thread. Native I/O can run independently, but CPU-heavy BoxLang callbacks do not spread across multiple cores. sleep() yields to other tasks rather than making computation parallel.

The browser playground uses the same cooperative model and stops runs after five seconds. For a guided introduction, see the async lesson.