World: r3wp

if is a normal function
 --> i meant "it is a typical function definition

So you want an expanding block of name value pairs to be checked 
by a function.

Something like: [title "Product X website" introduction "Product 
X is a new way to..." cost 25.90]

I want the easiest possible approach i.e. you create an html file 
and define the tags like in the example above template:

and then you collect with a function the list of tags that you've 
created and then in a function you go and define dynamically all 
those tags as variables i.e. title: func-title "xyz" ...

I want to use the power of bind and contexts to make sure this works

going to lunch. talk to u later thanks

Or wait... you want a context to keep all your template variables 
in:
	template: context [title: introduction: cost: none]
then unset all the variable in the template:
	unset bind next first template template

Your function references words in the template and receives an error 
for the first one which is unset.
	f: func [template][do bind [?? title] template]
	f template
	** Script Error: title has no value
	** Where: ??
	** Near: mold name: get name
but works fine when they have values:
	template/title: "hello"
	f template
	; ==> title: "hello"

That should give you some ideas.

how about an UNBIND function as follows:

what is the syntax of unbind ? is it costly to execute ? I would 
use it unbind the block of variables before creating the template

i imagine this could be a solution using the global context

just a moment, testing

unbind: func [
	{unbind words}
	[catch]
	words [word! block!]
	/local word result object
] [
	if word? words [words: reduce [words]]
	result: copy []
	unless parse words [
		any [set word word! (insert tail result to set-word! word)]
	] [throw make error! "unexpected data"]
	exclude result [self:]
	insert tail result none
	object: make object! result
	either find result first [self:] [first object] [
		exclude first object [self]
	] 
]

blk: [a b]
a: 1 b: 2
f: func [/local res][ 
	res: bind/copy unbind blk 'res 
	probe reduce res
]
f blk

I get script error: self has no value and I get the same error if 
I set a and b after bind in function f

show me your code, please

(the code producing the "self has no value" error)

Here my understanding would be that a and b are not
set in that context 
and the result is an error.

 - right, neither 'a nor 'b exists in the function context, therefore 
 they remained global

(or unbound, in my version of the function)

It looks, that we don't know what exactly you want to accomplish, 
and it is a bit hard to guess. Could you be more specific?

I am using linux and the clipboard doesn't worked so I typed the 
code

unbind: func [
        "unbind words"
        [catch]
        words [word! block!]
        /local word result object
][
        if word? words [words: reduce [words]]
        result: copy []
        unless parse words [

                any [set word word! (insert tail result to set-word! word)]
        ][ throw make error! "unexpected data" ]
        exclude: result [self:]
        insert tail result none
        object: make object! result
        either find result first [self:] [first object] [
                exclude first object [self]
        ]
]

blk: [a b]
a: 1 b: 2
f: func [/local res][
        res: bind/copy unbind blk 'res
        a: 1 b: 1
        probe reduce res
]
f blk

** Script Error: self has no value
** Where: f
** Near: self
>>

I get the same error when commenting a: 1 b: 1 line inside f

Ladislav, thank your help. I wanted to ask you about this via the 
mailing list and I did send the message earlier this morning but 
the mailing list is down. Please bear with me and I will provide 
the details. I think the solution you provided is exactly what I 
was looking for but I might be missing some detail. I am studying 
your code right now and will be more specific in a few minutes

Could you write the steps of the problem domain in pseudo-code for 
us? i.e. forget about the implementation for the moment.

i am preparing the pseudo-code. just a few minutes

your code contains the following bug:

    exclude: result [self:]

the original line is

    exclude result [self:]

thanks. I will post real code because I think it explains the problem 
better

template:       to block!  "<html><head><title> title </title></head><body>tag1 
tag2 tag3</body></html>"


blk:            remove-each val load copy template [tag? val]    
?? blk ; == [title tag1 tag2 tag3]

tag3:           "this is a fake value, should not appear"

eval-template: func [
        /local res
][
        res:            copy ""
        title:          "hey"
        tag1:           "this is tag1^/"
        tag2:           "i am tag 2^/"
        ;tag3:          "might not be set^/"
        bind/copy       template 'res
        repend          res template
        res
]

sorry pls look at the next example

template:       to block!  "<html><head><title> title </title></head><body>tag1 
tag2 tag3</body></html>"


blk:            remove-each val load copy template [tag? val]    
?? blk ; == [title tag1 tag2 tag3]

tag3:           "this is a fake value, should not appear"

eval-template: func [
        /local res
][
        res:            copy ""
        title:          "hey"
        tag1:           "this is tag1^/"
        tag2:           "i am tag 2^/"
        ;tag3:          "might not be set^/"
        bind/copy       unbind blk 'res
        repend          res template
        res
]

The unbind function is very close to what I want. In the previous 
example if the function body does not set the variables a or b it 
prints [none none]. What I want is that if a variable in blk is not 
set then it throws an error

IPls let me know if this example is not clear. What I want is to 
catch the case where tag3 is not set and throw an error This is useful 
where the tags are set in a large function and there are many tags

Your EVAL-TEMPLATE function does not have 'TITLE as local variable. 
Do you want 'TITLE to be local or not?

I want title to be local without defining it in the spec. Eval template 
is a function that might be used for many different templates

IMy understanding is that if you bind a word to the local context 
then it's a local even if it's not defined in the spec but I probably 
have multiple misunderstanding (though I just read the Bindology 
Bible by Ladislav)

unfortunately that is not true, at least not in this REBOL version

local contexts are somewhat "static", i.e. they cannot be enlarged

(section 15)

local without defining it in the spec
 -- I don't think that's a good way to think about modeling it.

I see. This is a major limitation. I see the rebol3 has a blog entry 
that "bind expands contexts" How could I approach this with the current 
rebol ?

No, that's right. Contexts cannot be extended with new words (at 
this time).  I would pass a context to your function with the template 
and all the words in it. This context will have to be built at the 
beginning.

It's still not clear to me what the real goal is. i.e. where values 
come from and how eval-template may be reused without giving it some 
parameters to guide it.

Sorry, I mean you eval-template function can create a context on 
the fly with the words in it.

ok Anton, I will try to implement this with your approach

It's not a terribly easy thing to do, but can be done.

Gregg, the GOAL is to be able to have a generic eval-template function 
that can evaluate multiple templates without having to include hundreds 
of locals in eval template. The values are generated dynamically 
with a code-block e.g. eval-template template code-block where code-block 
is [title: func-title "aha" tag1: emit-whatever ...

hmm, I may not understand your wish, Joe, but don't you want to have 
it this way?

Today I reread the blog, and I was not comfort with the idea that 
simple bind should auto-extend context ... maybe just my internal 
feeling, dunno .... just wanted to state opinion of less experienced 
reboller :-)