Hi Jon,
> <http://www.software-lab.de/doc/ref.html#symbol>
> ...
> Can you please explain which of the boxes represent the "three
> properties"? I guess the two at the bottom are two of them ...
Correct. The third one is the single 'KEY' in between.
The explanation in "doc/ref.html" is indeed not very clear, but the text
below the diagram tries to specify it a little. Just now I also extended
it a little, in the hope that it improved.
The property list of a symbol (a possibly empty sequence of cells
between the symbol's main cell and the name (which is physically a
number)) holds either single symbols (like that 'KEY' above), or cells
with key-value pairs.
If it is only a single symbol, it is meant as a boolean property, i.e.
the value is 'T'. The property list functions will never produce a
property list cell like
|
V
+-----+-----+
| | | ---+---> ...
+--+--+-----+
|
V
+-----+-----+
| T | key |
+-----+-----+
but a simple
|
V
+-----+-----+
| key | ---+---> ...
+-----+-----+
instead. The only reason for that is to save space.
Let's look at an example:
: (put 'A 'a 1) # A "normal" property
-> 1
: (put 'A 'f T) # A boolean property
-> T
: (getl 'A) # Get the property list
-> (f (1 . a))
: (show 'A)
A NIL
f
a 1
-> A
: (put 'A 'f 2) # Change the value of 'f'
-> 2
: (getl 'A)
-> ((2 . f) (1 . a))
: (put 'A 'a T) # Change the value of 'a' to boolean
-> T
: (getl 'A)
-> (a (2 . f))
Cheers,
- Alex
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