Re: [BABEL] "unset" :var definitions for subtree

["Eric Schulte" <schulte.eric@gmail.com>]

Dan Davison <dandavison7@gmail.com> writes:

> "Eric Schulte" <schulte.eric@gmail.com> writes:
>
>> [...]
>>>
>>> I agree, the things I was talking about don't end up being simpler in
>>> terms of usage for this case. My thinking was that variable unsetting
>>> might be something that would be required fairly rarely, and so it might
>>> be worth appropriate to support it as part of a more general/powerful
>>> mechanism tied into Org properties, or even that hook.
>>>
>>
>> It does sound like if there were a way to disable inheritance for
>> certain properties for certain sub-trees of a document, then that would
>> be ideal -- although I can't imagine how such functionality would be
>> implemented.  Can we think of an Org-wide syntax for disinheriting
>> specific properties?
>
> I'm not sure whether or how this fits it, but it's worth noting that Org
> currently documents the following
>
> ,----
> | org-entry-get-with-inheritance is a Lisp function in `org.el'.
> | 
> | (org-entry-get-with-inheritance PROPERTY &optional LITERAL-NIL)
> | 
> | Get entry property, and search higher levels if not present.
> | The search will stop at the first ancestor which has the property defined.
> | If the value found is "nil", return nil to show that the property
> | should be considered as undefined (this is the meaning of nil here).
> | However, if LITERAL-NIL is set, return the string value "nil" instead.
> `----
>
> so that seems to suggest ":var nil" as a way of knocking out all :var
> assignments, but doesn't immediately suggest how to knock out on a
> per-variable basis. :var a=nil ? (Which otherwise would look for a src
> block named "nil")
>

But then (I believe) once we implement the multiple inheritance
mentioned in your other thread this would no longer work.

>
>>> Before we proceed with the variable unsetting, could someone provide a
>>> motivating example, just to convince ourselves that the extra features
>>> are justified? (The conclusion of another message was that the torque
>>> script example was more a motivating example for shebang/preamble
>>> processing than for variable unsetting.)
>>>
>>
>> Certainly.
>>
>> 1. taking Rainer's first example, lets say that you want a variable
>>    specified for all but one code block in a file (maybe that one code
>>    block is the source of the value of the variable).  In this case it
>>    would be much simpler to specify the variable file-wide, and then
>>    unset the variable for that one code block.
>
> Agreed. It's not clean, but currently in this situation one could just
> set the offending variable to some other value.
>
>> 2. say you want the same session for /nearly/ every code block in a
>>    file.
>> 3. same for :dir, :file, or :shebang...
>
> I'd just note that some of these already have natural values that can be
> used to "unset"
>
> :session none
> :dir .
> :shebang ""
>
> :file <not sure about this one, but maybe "none" should be used...>
>

So the question seems to be, do we pick some /magic/ values for :var and
:file, similar to "none" for sessions, or do we unify this unsetting
behavior into a single magic value that can be used to unset any header
argument.  The later seems cleaner to me.

Best -- Eric