rontolisp:then

(rontolisp:then future function)

Attaches a transform to a future as a value: returns a fresh future that, on the input's successful settlement, invokes function with the settled value and settles to the function's return value. If function returns a future, await on the returned future flattens it (users never observe future<future<T>>). On upstream error the callback is skipped and the condition propagates through the returned future unchanged.

Use it to compose asynchronous work when the future crosses a boundary as a value -- the plain caller does not have to be an rontolisp:async-defun just because its callee is:

A non-future first argument is a type-error -- there is no JavaScript-style auto-coercion to a resolved promise.

Backend support

Supported on the interpreter, the JVM backend and WASM --component. Preview 1 WASM supports the success-only shape (the degenerate synchronous semantics of the async surface there: an errored body signals at the call rather than at await, so the error-propagation contract falls back to the enclosing handler-case). --no-gc rejects the whole async surface at compile time.