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Representable Continuations

(Remind me to explain this more clearly at some point). Why representable continuations are important for Links:
fun do_something()
  creds = validate_user();     # if the user isn't signed in, throw a login page, continue from here when finished
                                              # if user never validates, this never returns
  if (user_can_do_something(creds) then
  else display_error()

fun validate_user()
    user_cookie = get_cookie('user')
    (username, password) = regex.match("(.*):(.*)", user_cookie);
    if not valid_creds(username, password) then
        (username, password)

fun login_screen(handler) {
This can't be done if the l:action must always be a complete expression. "validate_user" needs to be like a subroutine that can be invoked from anywhere, even though it has its own handler (its own l:action). That means the serialized form action must include the complete continuation, not just an expression.

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