On Fri, 2011-10-14 at 08:39 -0700, Tom Limoncelli wrote:
> I'm trying to write a lens to parse Xen configuration files
> (xend-config.sxp) which is a very lisp-like format "(key value) (key2
> (key value))" and I'm getting nowhere. I've learned a lot from
> working through the tutorials. However I've also realized that I
> don't remember enough from my parsing and compilers class back in
> college :-). (it was more than 20 years ago)
>
> If you don't mind some noob questions....
>
> 1. The Schema Tutorial (http://augeas.net/docs/writing-schemas.html)
> doesn't explain what |=| means. I assume it is just a "pointer" to
> where the ambiguity starts. Clarification, please?
Yes, that's exactly what it is. The '|=|' was just my attempt to mark
the position where stuff went wrong with something that's unlikely to be
in the original string. And take 'position where stuff went wrong' with
a grain of salt ... the position is generally not bad, but not always
perfect.
> 2. I keep getting "ambiguous iteration" errors. Unsurprisingly, I
> remember this being difficult for me to grasp when I was in college
> doing homework assignments in Yacc. Is there a technique for
> diagnosing and fixing these? A rule of thumb for avoiding them?
A very common reason to have ambiguous iteration errors is to star a
lens twice, i.e. write something like (l*)*, usually well hidden behind
a couple of let's.
Formally, ambiguous iteration means that you have a lens l* that matches
a word uv such that both u and v already match l; a very simple example:
let l = [ key /a+/ ]
let s = l*
The word 'aa' is ambiguous; applying s to it can yield the trees { 'a' }
{ 'a' } and { 'aa' }.
> The tutorials are great in that they go through the debugging process
> but they don't explain the thinking that leads to the changes made to
> fix the bugs as they are found. I'd be glad to work with someone to
> improve them.
I'd love any suggestions in that direction; it's always bothered me that
ambiguity is such an important concept when writing lenses, and doesn't
have a very good intuitive explanation in the docs.
> P.S. As an example, the error I'm dealing with right now is:
If you want to post the lens, I can try and see if I can find a
reasonable explanation for how to avoid the type check error.
> $ augparse -I. tests/test_xendconfsxp.aug Syntax error in lens
> definition./xendconfsxp.aug:25.0-.44:Failed to compile
> lns./xendconfsxp.aug:25.10-.44:exception: ambiguous iteration
> Iterated regexp: /\\([\t\n ]*[0-9A-Z\\a-z-]+[\t\n ]+[^\t\n ]+[\t\n
> ]*\\)|[\t ]*#[\t ]*([^\t\n ].*[^\t\n ]|[^\t\n ])[\t ]*\n|[\t ]*\n/
> '(A\nA)#)\n' can be split into '(A\nA)|=|#)\n'
> and '(A\nA)#)|=|\n'
> Iterated lens: ./xendconfsxp.aug:25.12-.41:
In that error message, the '|=|' indicates how the input string would be
split: the first would apply lns to '(A\nA)' and again to '#)\n', the
second would do it to '(A\nA)#)' and then to '\n', yielding two
different trees.
David
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