How to check a value is defined in an object?

Hi, I would like show how to check a value is defined in an object or not. I decided to post this to show the usages of found?, value?, in and get functions.

Here is the situation, O is an object, there a two words defined in O.

O: context [A: 3 B: none]

B is defined and its value is none. C is not defined in O. And that is what I want to show.
You can try using get and found? functions:

found? get in o 'A
>> true

found? get in o 'B
>> false

found? get in o 'C
>> false

This is because found? simply tests the value is none or not. (Note that false and none are different in Rebol. "found? false" will return true)

So we could not recognize the word B is defined or its value is none. Same question is for C.

You can get value of a word inside O using get:

get in o 'A
>> 3

get in o 'B
>> none

get in o 'C
>> none

This is why we could not use found? and get together. So you may thing that we can use value? function. "value? returns TRUE if the word has been set." the document says.

value? get in o 'A
>> true

value? get in o 'B
>> true

value? get in o 'C
>> true

Hmm.. this is because "get in o 'C" returns none. And none IS a value in Rebol.

get in o 'C
>> none

value? none
>> true

So lets look closer,

in o 'A
>> A

in o 'B
>> B

in o 'C
>> none

in returns the word itself if it is defind in the object, not its value.
This is why we cannot use value? and in together. So our mistake is to use get function. The solution is simple:

found? in o 'A
>> true

found? in o 'B
>> true

found? in o 'C
>> false

This is what we want. So we can write this small function:
defined?: func [w [word!] o [object!]] [found? in o w]

defined? 'A O
>> true

defined? 'B O
>> true

defined? 'C O
>> false

Well, this is how I learnt the meanings of in, get, found? and value? functions.
"get a value of a word in an object, if it is none than it is not defined." INCORRECT! And I made this mistake several times when learning Rebol.
I hope this will be helpful for beginners.

thanks Endo for taking the time to share your experiments, and your findings. I am sure it will help other beginners.

Thanks Yuem,
The title should be "How to check a WORD is defined in an object?" not VALUE I guess.

Hi,

I just wanted to share with you some interesting view I found on the Net about REBOL and its particular way of binding words with both values and their respective contexts.

It's been posted by Gregory Higley and he shows a deep understanding of the way REBOL works ...

Here it is on the Loudly Recursive blog : http://blog.revolucent.net/2009/07/deep-rebol-bindology.html

Yes I already read this blog post, and it is a very deep and nice explanation of binding in rebol. Thanks for sharing it here.