Preventing Unstable Edits

[pascallouisperez]

(Copied from README.)

Assume we have the worksheet

worksheet cyclic_edits {
	1:right bool
	2:wrong computed {
		return !right
	}
}

And we propose the edit ws.right = false. Due to the computed field, this would yield 'actual edit #1'

ws.right = false
ws.wrong = true

Now, assume that we have edit reacting code which flips these fields around

if wrong {
	right = true
} else {
	right = false
}

We would then yield the 'actual edit #2'

right = true
wrong = true

And, due to the computed field, this would yield 'actual edit #3'

right = true
wrong = false

Further modified via the event reacting code into 'actual edit #4'

right = false
wrong = false

And again, due to the computed field, this would yield 'actual edit #5'

right = false
wrong = true

Which is in fact the same as 'actual edit #1'. We have created an infinite loop in the fixed-point calculation!

To prevent such 'unstable edits', we detect cycles of edits, and error out, hence rejecting the proposed edit as being invalid. (Implementation note: this must be done by comparing worksheets', not edits, since two different edits can yield the same worksheet transformation.)