rontolisp:then

(rontolisp:then value fn)

Derives a new promise from value (usually a promise) and a callback, modeled on JavaScript's Promise.prototype.then: awaiting the derived promise awaits value and applies fn (a one-argument function) to the result. then always returns a promise, so calls chain; a callback that itself returns a promise is flattened, like JavaScript. A plain (non-promise) value is allowed and is passed to the callback as-is.

The typical use is transforming a rontolisp:fetch response:

(print (rontolisp:await
        (rontolisp:then (rontolisp:fetch "https://httpbin.org/get")
                        (lambda (r) (getf r :status)))))   ; 200

Timing

The callback runs lazily, when the derived promise is first awaited, and the result is memoized so the callback runs at most once however often the promise is awaited. This at-await timing is identical on all three backends (the WASM backend has no event loop that could run callbacks at settlement time); a chain that is never awaited never runs its callback.

Errors

A failed base promise (for example a refused connection) skips the callback: on the interpreter and JVM the failure signals from await; on WASM the failed fetch resolves to nil, which the callback receives (see the error notes on rontolisp:await). There is no error-callback (onRejected) parameter.

Backend support

Works on every backend and in every WASM mode (Preview 1 included), like rontolisp:await and rontolisp:promisep.