[REBOL] word Re:(4)
From: fantam:mailandnews at: 16-Aug-2000 18:26
Marvelous. I'll try to remember all this.
Thanks again.
> Hi Fantam
> You wrote:
>> What a wonderful explanation. Thank you for the insight, Larry.
>>
> You are welcome! I realized after my post that there is another important
> point about 'bind and contexts which I failed to mention.
> I showed you how to bind the words in the code block to the local variable
> 'member
>>> block: ["a" "b" "c"]
>>> code: [print member]
>>> foreach member block [do bind code 'member]
> a
> b ;member in code bound to local var member
> c
> What I failed to mention is that the use of bind is permanent (until some
> other 'bind occurs) so that the after the foreach loop the global block
> referenced by 'code remains bound to the local context of foreach.
>>> get second code
> == "c"
> This is a side effect of using 'bind as above, and may not be the desired
> effect. It can also lead to a crash when accessing the value of the code
> block if there has been a 'recycle or garbage collection which destroys the
> local context of the 'foreach function. The easy way to leave the original
> code block unchanged is to copy before the bind.
>>> code: [print member]
>>> get second code
> ** Script Error: member has no value.
> ** Where: get second code
> So 'member is bound to the global context but has no value.
>>> foreach member block [do bind copy code 'member]
> a
> b ;member in COPY of code bound to local var 'member
> c
>>> get second code
> ** Script Error: member has no value.
> ** Where: get second code
> The global code block remains unchanged.
> In general, it is probably better, when possible, to define code blocks in
> the context in which they will be used, as there are further complications
> with nested sub-blocks as well as in figuring out how to do the necessary
> binding when the functions, function calls, and code blocks are complex. In
> addition, when modules are added to REBOL some of our current code relying
> on bind may be broken.
> Cheers
> -Larry
--
Fantam
