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[REBOL] Re: Contexts of contrived blocks

From: joel:neely:fedex at: 9-May-2001 13:51

Hi, John, [john--schuhr--com] wrote:
> Anyone know how to explain the following? > Whenever I create a block using 'to-block > on a string, the words in the block have their > own context.. what gives? >
Well, as the old gentleman said, "I don't know the answer, but I surely do admire the question!" This seems like it might be related to another small mystery I've been pondering on recently. Given the code: 8<------------------------------------------------------------ #!/usr/local/bin/rebol rebol [] make object! [ scoper: func [ argvar [integer!] recur? [logic!] /local locvar ][ locvar: argvar + 1 use [usevar] [ usevar: argvar + 2 print [argvar recur? locvar usevar] if recur? [scoper argvar + 10 false] print [argvar recur? locvar usevar] ] ] scoper 1 true ] 8<------------------------------------------------------------ we get the following behavior: 8<------------------------------------------------------------
>> do %scoper.r
1 true 2 3 11 false 12 13 11 false 12 13 1 true 2 13
8<------------------------------------------------------------ Now, it's clear (in some sense) *WHAT* is happening. The USE establishes a separate context in which USEVAR is known. The single instance of that context persists across the recursive call, therefore the value to which USEVAR is set in the inner call is still acessible upon return to the outer call. What is not clear to me is *HOW* and *WHEN* that takes place. Does the USE establish that context... 1) when the source file is loaded? 2) when SCOPER is first invoked? 3) at some other time I haven't imagined? Option (1) seems inconsistent with what I think I understand about how FUNC and MAKE OBJECT! do (create a new context when executed). Option (2) leaves me wondering, "How does it know that the recursively nested call should re-use the context created in the outer call (instead of creating another one) and how does it find that existing context?" It still appears that contexts have much deep magic associated with them! -jn-