Forms: form-actions don't close over variables in original form-handler call

[MarcKaufmann]

We use (form a-form an-action a-renderer) to create forms. The action should be taken upon a valid form submission. Suppose we have the following:

(define (handler some-argument)
  (define n (random 10))
  (define a-form ...) ; asks to guess the number
  (define (an-action guessed-number)
    (if (= n guessed-number) (put 'success? #true) (put 'success? #false)))
  (define (a-renderer rw) ...)
  (form a-form an-action a-renderer))

Then after submission of the form with a guessed-number, the handler is called again, which generates a new n. If a valid guess was provided, it then compares the guessed-number against this new value of n, not the original one -- which is of course not the intended behavior.

How can we fix this?

[Bogdanp]

For next session: require that handlers return pages. Those pages can then capture the current renderer for each handler, which forms can then re-use w/o re-running the other code in the handler. Effectively, page is when the "response" part of a handler starts and it should be re-runnable.