[augeas-devel] Fixing ambiguous iteration errors

[David Lutterkort]

On Fri, 2011-10-14 at 20:19 +0200, Raphaël Pinson wrote:
> Just a note. If the file is really like lisp and can take any depth of
> recursion, then you need a recursive lens, probably very close to json.aug,
> so you might want to study how json.aug works.

For a little more background: regular, i.e. nonrecursive, lenses can
only deal with regular expressions, which means it is impossible to
process matching pairs of parentheses with arbitrarily deep nesting.

Recursive lenses can process context-free languages, and therefore are
able to handle parens. Unfortunately, it is not possible to detect
ambiguity for them by just looking at lenses; ambiguity isn't detected
until you actually try and process an ambiguous word with a recursive
lens.

To get some level of confidence that the recursive json lens (Json.lns)
isn't horribly broken, json.aug builds a regular lens 'value1' which can
only handle one level of braces, and 'value2' which can handle two
levels of braces.

As it was until a few minutes ago, the json lens actually was ambiguous,
as was shown by the value2 lens - I just fixed that ambiguity in git.
The problem was that whitespace was handled haphazardly: some lenses ate
the whitespace after the actual text they were processing, others at the
beginning (or both), which causes problems when you concatenate lenses
and have an overall lens that's structured like '(l1 . ws) . (ws . l2)'
- it's now not clear whether whitespace should be processed by the first
occurence of ws, or by the second, or some combination of the two. To
fix this problem, I went through and made sure that lenses only gobble
up whitespace at the end.

David