There is a web application, and two ‘pages’ - ‘View’ and ‘Editor’. And
when i press ‘Edit’ button on the ‘View’ it should:
Freeze current execution (callcc {bla-bla})
Give control to ‘Editor’
User fill data in ‘Editor’
‘Editor’ return entered data and control back to ‘List’
(continuation.call data)
Friezed execution resumes
THE PROBLEM:
I implement this the same way as in Seaside, and it seems to should
works, but it throws:
“Caught RuntimeError: continuation called across threads”
Probably, because WebServer runs each request in a new thread. Do you
hawe any idea how this can be solved?
Thanks!
DETAILS:
In pseudo code:
class Editor < Form
def initialize
data = TextField.new self, ‘’
FormButton.new(self, ‘Save’) {answer data.text}
end
end
class View < Panel
def initialize editor
view= Label.new self, ‘Some initial value …’
Button.new(self, ‘Edit’) {view.text = call(editor)}
end
end
class MySuperApp < Panel
def initialize
View.new(self, Editor.new)
end
end
Done!
I founded an ugly solution (closures instead of continuation). It works,
looks like:
All other components are the same as earlier, i just a little changed
the ‘View’.
To complete edition user should 3 times enter something in Wizzard-like
dialogs.
WITH CONTINUATION IT SHOULD LOOKS LIKE:
class View < Panel
def initialize editor
view= Label.new self, ‘Value’
Button.new(self, ‘Edit’) {
first = call(editor)
second = call(editor)
third = call(editor)
view.text = first+second+third
}
end
end
WITH MY UGLY SOLUTION IT LOOKS:
class View < Panel
def initialize editor
view= Label.new self, ‘Value’
Button.new(self, ‘Edit’) {
call(editor) { |v|
first = v
call(editor){ |v|
second = v
call(editor){ |v|
third = v
view.text = first+second+third
}
}
}
}
end
end
However, continuations in Ruby tend to leak RAM, and tend to be slow.
You
can get 80% of the love with none of the pain by using techniques not
based on continuations.
Yes i goggled it about 3 months ago but sadly don’t found any
ready-to-show samples or sites running on these frameworks and go
further.
Yesterday i investigated it in more details, looks cool, and my solution
looks very close to it, but now i almost finished my prototype
Main difference is in HTML builder(renderer?), i use Swing(Java) like
API (i.e. no HTML/HTTP at all, just Labels, Panels, Buttons,
LayoutManagers and so on).
It’s better not to use continuation, especially in multi threads.
There is a way to imitate it, like code sample below:
…
call MessageBox.new(“Delete?”){|response|
if response
call MessageBox.new(“Are you sure?”){|response2|
delete if response2
}
end
}
…
Correct. Wee uses one thread per session (i.e. user) and keeps this
thread alive. This is required at least when using continuations,
because you can’t continue a continuation from another thread.
With continuations, you could write it as:
delete if call(MessageBox.new(“Delete?”)) and
call(MessageBox.new(“Are you sure?”)
That is a lot clearer!
But, continuations have some downsides (that applies to
Matz’ Ruby interpreter):
