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# What kind of bug is this?

### [1/4] from: robbo1mark:aol at: 31-Jul-2001 5:35

What kind of bug is this? Ladislav, myself & others have been having a discussion concerning evaluation and self modifying code, and some errors? / very strange behaviour that REBOL displays. Consider the following code in which the block 'a modifies itself at each call by appending + 1 to it's latter series of values. Do'ing this block results in an incrementing sum at each call except when you get to iterations 6, 14 & 30 when some very wierd things start happening. By printing the SUM part of 'a after each call I thought I might be able to deduce the behaviour / evaluation model and predict the result of the next call BUT NO! I'm stumped. Anybody got any explanations? Mark Dickson Heres' the code.............................................
>> a: [ append a [+ 1] 1]
== [append a [+ 1] 1]
>> loop 50 [ print [do a newline] print [remove/part b: copy
a 3 newline]] 1 1 + 1 2 1 + 1 + 1 3 1 + 1 + 1 + 1 4 1 + 1 + 1 + 1 + 1 5 1 + 1 + 1 + 1 + 1 + 1 6 1 + 1 + 1 + 1 + 1 + 1 + 1 8 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 9 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 10 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 11 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 12 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 13 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 14 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 14 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 16 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 17 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 18 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 19 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 20 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 21 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 22 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 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### [2/4] from: carl:cybercraft at: 31-Jul-2001 22:32

On 31-Jul-01, [Robbo1Mark--aol--com] wrote:
> What kind of bug is this? > Ladislav, myself & others have been having a discussion concerning
<<quoted lines omitted: 9>>
> the result of the next call BUT NO! I'm stumped. > Anybody got any explanations?
I think I remember it being said (by Holger?) that self-modifying code is a no-no with REBOL.
> Mark Dickson > Heres' the code.............................................
<<quoted lines omitted: 4>>
> 1 > 1 + 1
-- Carl Read [carl--cybercraft--co--nz]

### [3/4] from: rotenca::telvia::it at: 31-Jul-2001 12:53

> What kind of bug is this?
Look at this:
>> a: [ 1 insert head a [1 +]]
== [1 insert head a [1 +]]
>> reduce a
== [1 [1 insert head a [1 +]]]
>> reduce a
== [2 [1 + 1 insert head a [1 +]]]
>> reduce a
== [3 [1 + 1 + 1 insert head a [1 +]]]
>> reduce a
== [4 [1 + 1 + 1 + 1 insert head a [1 +]]]
>> reduce a
== [5 [1 + 1 + 1 + 1 + 1 insert head a [1 +]]]
>> reduce a
== [6 [1 + 1 + 1 + 1 + 1 + 1 insert head a [1 +]]]
>> reduce a
== [7 [1 + 1 + 1 + 1 + 1 + 1 + 1 insert head a [1 +]] [1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 insert head a [1 +]] [1 +]] So Rebol can become confused if evaluation changes the block it is evaluating. I tried many iterations with many variations and Rebol often crashed. --- Ciao (my english is bad) Romano

### [4/4] from: brian:hawley at: 31-Jul-2001 15:57

Hi Mark, At 05:35 AM 7/31/01 -0400, you wrote:
>What kind of bug is this? >Ladislav, myself & others have been having a discussion concerning
<<quoted lines omitted: 15>>
> >> loop 50 [ print [do a newline] print [remove/part b: copy >a 3 newline]]
[results skipped] REBOL blocks are preallocated in chunks. When you increase the size of the block (ie. by inserting data) you use up this space until it runs out. At that point, REBOL copies the contents of the block into a new, bigger preallocated chunk of RAM. This process happens automatically, at least as far as REBOL code is concerned. Your code demonstrates that the DO native does not take this into account. DO apparently continues to operate on the old copy of the block it has been passed when this reallocation occurs. Whoops! Possibly a speed optimization, but just as possibly this kind of behavior wasn't anticipated. There's no reason to assume that this technique would work if REBOL wasn't specifically designed to handle this behavior. You should be thankful that it doesn't crash :) As it is, I would suggest one of four techniques: - Specify the maximum size of the block ahead of time. - Perform modifications that don't change the length of the block you are modifying. - Perform your modifications in a different block of code than the one you are modifying, at a time that you are not DOing the modified block. DO/NEXT can help here. - Precalculate your modifications. COMPOSE can help here. I hope this helps. Brian Hawley

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