[rules-users] Catch 22 ANTLR DSL matching issue

[Wolfgang Laun]

See inline.

On 27 May 2011 20:19, jstroup <jstroup at regenstrief.org> wrote:

> Hi Droolers,
>
> Regarding this blurb from the 5.2 DSL reference:
>
>
> [snip]

>
> Given this DSL (test_expander.dsl)
>
>  [when](C|c)heese is "{type}"=Cheese(type=="{type}")
>  [when](is|hails|comes) from {country}=Cheese(country=="{country}")
>  [then]Add the message {message}=System.out.println({message});
>
> And this DSLR
>
>  package com.sample
>  import com.sample.DroolsTest.Cheese;
>  expander test_expander.dsl
>
>  rule "rule_1"
>        when
>                   Cheese is "cheddar" and is from Italy
>        then
>            Add the message "Cheddar IS from Italy"
>  end
>
>
This will not do what the text implies because the expansion is
   Cheese(type=="cheddar") Cheese(country==="Italy")
and there is nothing that links these Cheese facts, which could
be one and the same but also any other pair matching the
individual constraints.

>
>
> I know that rule_2 fails because I removed the “distinctive characters” so
> ANTLR is confused on what to capture.


ANTLR has nothing to do with DSL expansion, which is entirely based on
regular expressions.


> It just seems so unnatural for a rule
> author to have to somehow magically know to add quotes in order to merge
> different DSL expressions to generate a composite DRL pattern.


The DSL designer has a range of options (characters, buzz words,...) but at
the
end of the day (s)he will have to document it for the rule authors. There's
no
way they'll know what to write, out of the blue.


  2) We can no longer just write “Cheese is cheddar” by itself but are
> forced to write “Cheese is cheddar and” if we want to match only the first
> expression. You might as well just combined the two expressions into one
> since there is no real advantage now to having two expressions!
>

There is a feature for adding arbitrary constraints into a preceding
pattern.


>
> One nice thing would be for the Guvnor DSL editor to
>
>  1)try the capture greedily first,
>  2)get the annoying “no viable alternative error”,
>  3)CATCH the error instead of just giving up,
>  4)capture non-greedily and match only the first word,
>  5)then search for matching DSL expressions beyond that.
>
> Or something like that.


Exactly. There is the fundamental problem of different levels of grammars
for DRL and natural languages, and this can't be overcome easily.



> The only time you should have to add quotes is if
> the text you are matching actually contains multiple words. Anything else
> is
> counter-intuitive.


The DSL designer can avoid this, at the cost of slightly more verbose
phrases.



> Please tell me the planned replacement for DSL addresses
> this!
>

Feel free to contribute proposals (but not "something like that", please).
Wolfgang


>
> Jeff
>
>
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