fix promise bugs on corner cases (first one)

[stibear]

Ref: #339

[KeenS]

It seems that all the call of make-promise% followed by box so why don't you define the combination as an procedure?

[nyuichi]

@stibear

Can you split the patch into two things, one for the first bug and the other for the second one? I can merge the second one immediately, while the first needs more discussion, I think.

[nyuichi]

Can we implement delay-force just like the following instead of using boxes?:

  (define-syntax delay-force
    (syntax-rules ()
      ((_ expr)
       (letrec ((p (make-promise% #f (lambda ()
                                       (let ((v (begin expr)))
                                         (set-promise-done! p #t)
                                         p)))))
         p))))

I'm not sure if this works. And there are maybe more elegant solutions anyway.

[nyuichi]

@stibear

I made an alternative implementation using tri-state rather than boxes: https://github.com/picrin-scheme/picrin/tree/stibear-lazy

[stibear]

@nyuichi It looks no problem. I tested it and it passed all tests.