[Vm-dev] defensive error checking in a BlockContext environment
for [:n | n = 1 ifTrue: [1] ifFalse:[(factorial value: n-1) * n]]
Eliot Miranda
eliot.miranda at gmail.com
Mon Dec 16 19:08:32 UTC 2013
On Sun, Dec 15, 2013 at 8:25 AM, gettimothy <gettimothy at zoho.com> wrote:
>
> Please shoot this down, if it is incorrect. (no details necessary, just a
> yes,no is sufficient)
>
> In the discussion
> <http://www.mirandabanda.org/cogblog/category/cog/page/14/>on the
> re-entrant problem for BlockContexts regarding this code:
>
> | factorial |
> factorial := [:n| n = 1 ifTrue: [1] ifFalse: [(factorial value: n
> - 1) * n]].
> (1 to: 10) collect: factorial
>
> This statement: "The block can’t be reentered without overwriting the
> sender field, preventing return to the first caller. "
>
> Does *not* mean that the bock is reentered, the sender field is
> over-written and when we get to n=10 stage things go kablooie.
> What happens is the coders detect this condition and the primitive code
> will *not allow* a sender to be over-written.
> The error that is thrown is *purposeful and defensive*
>
> I don't know what I am looking at yet, but I think that error check
> happens here:
>
> Interpreter >> primitiveValueWithArgs
> | argumentArray blockContext blockArgumentCount arrayArgumentCount
> initialIP |
> argumentArray _ self popStack.
> blockContext _ self popStack.
> blockArgumentCount _ self argumentCountOfBlock: blockContext.
>
>
> * "If the argArray isnt actually an Array we ahve to unpop the above
> two" (self isArray: argumentArray) ifFalse: [self unPop:2. ^self
> primitiveFail].*.....
>
>
> Sound about right?
>
Well, in the BlueBook the primitive didn't detect this and one could
recurse and things *would* go kablooie when n got to 1 (its when the block
returns that things go kablooie and that happens the first time "ifTrue:
[1]" is executed).
--
best,
Eliot
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