(define-module (data optional)
  #:use-module (oop goops)
  #:use-module (ice-9 match)
  #:use-module (control monad)
  #:export (nothing just
                    nothing? just?
                    return-optional)
  ;; TODO is this reexport needed?
  #:re-export (>>=))

(define-class <optional> ()
  (slot #:init-value #f
        #:init-keyword #:slot)
  (just  #:init-value #t
        #:init-keyword #:just))

(define (nothing) (make <optional> #:just #f))

(define (just obj) (make <optional>
                     #:just #t
                     #:slot obj))

(define (nothing? this)
  (not (slot-ref this 'just)))

(define (just? this)
  (not (nothing? this)))


(define-method (write (this <optional>) port)
  (if (just? this)
      (format port "[Just ~s]" (slot-ref this 'slot))
      (format port "[Nothing]")))

(define return-optional just)

(define-method (>>= (this <optional>)
                    (proc <procedure>))
  (cond ((nothing? this) (nothing))
        ((just? this) 
         (match this
           (($ <optional> slot) (proc slot))))))