---

6.5 Let-On

The (goblins actor-lib let-on) module provides convenient syntax to facilitate promise pipelining.

Syntax: let-on ((var exp) …) body ¶

Syntax sugar around on, all-of, lambda, and match-lambda to allow using promise pipelining with a let-like syntax. Each exp (an asynchronous invocation of an actor resulting in a promise) is evaluted concurrently; the resulting promises are bound to their corresponding vars; and finally body is evaluated in an environment where all promises are resolved.

For example, this use of let-on…

(define (^doubler bcom)
  (lambda (x)
    (* x 2)))
(define doubler (spawn ^doubler))
(let-on ((four (<- doubler 2))
         (eight (<- doubler 4)))
  (+ four eight))

…is equivalent to…

(use-modules (goblins actor-lib joiners))
(on (all-of (<- doubler 2)
            (<- doubler 4))
  (match-lambda*
    ((four eight)
     (+ four eight))))

Syntax: let*-on ((var exp) …) body ¶

Like let-on, except paralleling let*; each exp is resolved sequentially rather than concurrently, and each var is available in the body of the succeeding exps. This is equivalent to nested on and lambda calls, without using match-lambda or all-of.

So this use of let*-on…

(let*-on ((four (<- doubler 2))
          (eight (<- doubler four)))
  (+ four eight))

…is equivalent to…

(on (<- doubler 2)
    (lambda (four)
      (on (<- doubler four)
          (lambda (eight)
            (+ four eight))
          #:promise? #t))
    #:promise? #t)