Just the other day, while programming in Scala, I was thinking how nice it is that Haskell hides non-strictness (a.k.a. laziness) underneath, so no obnoxious lazy stream creation/forcing needs to be done. And then it struck me: how cool would it be to treat Promises in a language the same way that Haskell treats lazy values? Would such a language even make sense? How would you program in such a language?
Let's explore! You can find all the code included in this post in a more useful form here.
Promiseimplementation written in Scheme, with support for asynchronous resolving & error rejection, which we'll need in order to explore this weird proposition:
(define *promises* '()) (define (make-promise value state then handle thunk) (list '&promise value state then handle thunk)) ;; Field accessors omitted. (define (promise fun) (let* ((val '()) (state 'pending) (on-resolve id) (on-reject id) (resolve (lambda (v) (set! val v) (set! state 'resolved) (on-resolve val))) (reject (lambda (e) (set! val e) (set! state 'rejected) (on-reject val))) (then (lambda (t) (if (equal? state 'resolved) (t val) (set! on-resolve t)))) (handle (lambda (h) (if (equal? state 'rejected) (h val) (set! on-reject h)))) (p (make-promise (lambda () val) (lambda () state) then handle (lambda () (fun resolve reject))))) (set! *promises* (cons p *promises*)) p))