On 21/09/08 2:27, Luke Kanies wrote:
> On Sep 20, 2008, at 1:39 PM, Brice Figureau wrote:
>
>> Hi,
>>
>> The more I think about that patch, the more I think I should expand it
>> to a more generic expression parser/evaluator by adding arithmetic
>> operators and such, so that we could write these stupid things:
>>
>> class superfeaturethatneedsplentyofram {
>> $min_ram = 2048 * 1024 * 1024
>> if ($ram / (1024*1024) < $min_ram) {
>> notice("not enough ram on node ${fqdn}")
>> }
>> }
>>
>> I already started coding this in fact and I'm wondering if that's
>> something that:
>> * is needed (I never myself got the need)
>> * would be welcome
>>
>> Anyway, that's an interesting exercise (mainly due to the lack or racc
>> documentation or examples). Right now I'm struggling with shift/reduce
>> conflicts I don't really know how to solve (anyone familiar with LALR
>> parsers lurking here, please I welcome any help :-)).
>
> Note that Puppet's language doesn't have numbers -- they're all
> treated as strings (only 'true' and 'false', when unquoted, are
> anything but strings) -- so you'll have to do a touch of magic to make
> math work.
Yes, I know, the code has some "to_i" to convert strings matching
numbers to integer. But I'm not satisfied with this, as it doesn't
handle float numbers.
I'm willing to do something along the line (pseudocode) of:
safeevaluate operand
if !string or !number
raise error
if string matches /^\d+$/
call to_i
else if string matches /^\d+.\d+$/
call to_f
then do the computation...
And also, is it ok to raise ArgumentErrors during evaluate() when I
encounter something that can't be safely converted in a number?
Can I move this part to the scope class, as there is a true? method
doing the same kind of things?
Also how does ruby manage coercion (or implicit cast) when doing
integer/float computation?
I couldn't find at first glance anything online discussing this (I guess
I'll have to purchase a ruby reference book sometimes).
> Generally, I'm not opposed to this but I don't think it's all that
> important, either. I expect others will stauchly disagree with me,
> though, and would love this functionality.
I understand, it's just that's the low hanging fruits for me while
coding in Puppet: I know this part of the code quite well now, and it's
only a minor modification above my previous if expression patch.
And I know some people would welcome arithmetic computations, I already
saw some threads on the user list mentioning this.
> The lack of racc documentation is quite fun -- I can only ever find my
> own threads and code when looking for it. I can't imagine I'm the only
> person to ever use it, but I might be the only one to ever publish
> anything using it.
It seems people are now using Coco/R where I could find more examples.
In fact Racc mimic bison/yacc, so the bison documentation and samples
are enough (but I found that after I sent my previous e-mail).
> In terms of shift/reduce conflicts, they are the bane of LALR parser
> generators. I'm willing to do what I can to help, but they're pretty
> unfun. What's your code look like right now?
I left my Dragon book at the office, so from home that was not easy to
remember what I learnt 15 years ago about parsers :-)
But I managed to rewrite the grammar to not be ambiguous, and there is
no s/r conflicts anymore (for the moment), it even works fine for
integers :-)
> BTW, one thing that might be useful is to create a stand-alone parser
> that just does the arithmetic, and when you're happy with it, import
> it into Puppet. Then again, that might be 10x harder. :)
Yes, puppet defines most of the thing (ie AST tree, the lexer and so
on). I don't want to rewrite all of this :-)
Thanks for your help,
--
Brice Figureau
Days of Wonder
http://www.daysofwonder.com
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