[cells-devel] c-formula and setf

[Kenneth Tilton]

Bastian Müller wrote:
> Hi,
> 
> I don't know if that has been discussed yet, but I've several use cases 
> where I want to do sth. like that: A cell should be calculated by a 
> formula if it isn't set to a value. I thought that's possible with
> (c-formula (:inputp t) ... ), which does calculate the slot by a formula 
> and can be setf, but how is it possible to unset the setf value and let 
> cells calculate it again?
> 
> Example: A textbox is by default calculated by a formula. If the user 
> enters sth. into it, that value is used. If the user clears the field of 
> his input, the value should be calculated again.

Could you just have?:

    (or (user-entered-value self)
        (calculated-value self))

One thing not clear to me is how you know they first indicate they want 
to enter a value and then communicate that they have decided against, 
and those things really come into play.

Lemme know and I'll tell you how I would handle it.

Pretty much I would avoid SETF as much as possible and end up with 
something still formulaic:

    (if (user-owns-value self)
        (editor-result self (key-input self))
      (application-decides-value self))


> 
> Maybe the c-formula ... approach is completely the wrong way or that's 
> not possible at all. I've read the documentation and primer but couldn't 
> find anything helpful. Sorry, I'm new to cells.

No problem. This is a common case where one really wants to think 
procedurally and it gets to be a little fun thinking declaratively. But 
really the interesting part is this change from user-owning to 
application-owning mode, so lemme know your protocol for that and I will 
do what I can.

btw, as Jakub suggested often it is easier to divide and conquer:


   (or (user-value self)
       (application-decided-value self))

...but I think you will end up with the same issue at some point of 
needing to know what state the field is in, so tell me more and I'll 
give it a shot.

cheers, ken

ps. might make a good use case. k