[REBOL] Re: Newbie query on array indexing
From: brett:codeconscious at: 23-Nov-2001 22:14
> I can tell you _what_ got set, but the _why_ is beyond me ...
...
> >> one
> == two
> >>
Hello, my turn :)
a: context [b: 'one]
Here an object is created with a single instance variable "b" whose value is
literal word "one". This object becomes
the value of the word "a".
b: 'b
Then a new word "b" is given a value. The value of this new word is the
literal word "b".
set a/:b 'two
Now Ammon is getting tricky. Let's break it down. Firstly it is pretty
obvious we are setting a word to a value, this value being the literal word
two
. Volker has shown that the word that is being set is the word "one".
So the mystery comes down to the evaluation of the path. Here are two quotes
from the Core user guide:
<quote>
Paths are a collection of words and values delineated with forward slashes
(/). Paths
are used to navigate to or find something. The words and values of a path
are called
refinements, and they are combined to provide a means of navigating through
a
value or function."
</quote>
<quote>
The words supplied as paths are symbolic and therefore unevaluated. This is
necessary to allow the most intuitive form for object referencing. To use a
word's
reference, an explicit word value reference is required:
city: 'Ukiah
probe USA/CA/:city
</quote>
BTW, The word "reference" is used twice in that last sentence in two
different ways. First to refer to the value of a word, second to talk about
the Rebol expression (using a get-word syntax) that will retrieve the word's
value.
So back to the path Ammon has created Here it again:
a/:b
Imagine how this path is evaluated. First "a" is encountered it evaluates to
an object. Then the
next element of the path is evaluated. This is the ":b". Guess what :b will
evaluate to.
>> :b
== b
So it evaluates to the word "b".
But the evalution of the path is not finished. This word b is used now as
the selector (of an instance variable) into the object that was returned
from the first element in the path. Thus the evalution now is equivalent
too:
>> a/b
== one
So the path evaluation is complete and has resulted in a value of the word
one
. This value now becomes a parameter
to the set function and so now we have something equivalent to:
set 'one 'two
HTH
Brett.
