Cake pattern?

gistfile1.rkt

#lang racket

;; ref: http://jonasboner.com/2008/10/06/real-world-scala-dependency-injection-di/

(define-signature on-off-device^
  (on
   off))

(define-signature sensor-device^
  (is-coffee-present?))

(define-unit heater@
  (import)
  (export on-off-device^)
  
  (define (on) (displayln "heater.on"))
  (define (off) (displayln "heater.off")))

(define-unit pot-sensor@
  (import)
  (export sensor-device^)
  
  (define (is-coffee-present?) #t))

(define-signature warmer^ (trigger))

(define-unit warmer@
  (import on-off-device^ sensor-device^)
  (export warmer^)
  
  (define (trigger)
    (if (is-coffee-present?)
        (on)
        (off))))

(define-values/invoke-unit/infer
  (link warmer@ pot-sensor@ heater@))