Maude-help mailing list

[Scott Christley <schristley AT mac.com>]

Hello,

I'm writing some rules where I want to add items to a "list" of things.  In 
the case of when the "list" is empty then I want to create the list (kind of 
like an initial condition or tail-end of recursion).  Otherwise when the list 
is already there then I just want to add to it.  So I'm writing two rules, 
one non-conditional rule for when the list is not there, and another 
conditional rule for when the list is there.  But I'm worried that one rule 
(the non-conditional rule) is too general and always matches.

For example I have these two rules, where I match a "foo" in the first part, 
then I add "bar" to a list of results for "foo".  The first non-conditional 
rule creates the first "result" while the second conditional rule adds "bar" 
to the existing list of results.


rl [test] : test( X foo, answers(Y) )
   => test( X, answers(Y result(foo, bar)) ) .


crl [test] : test( X foo, answers(Y result(Z, W)) )
    => test( X, answers(Y result(Z, add(W, bar))) )
    if Z = foo .



My concern is that it seems that the non-conditional rule will always match.  
Is this true?  Or will a conditional rule which matches always override a 
more general rule?

What is the correct way to write such rules?  I can think of one change by 
changing the non-conditional rule into a conditional rule that checks for 
non-existence, something like this:


crl [test] : test( X foo, answers(Y) )
    => test( X, answers(Y result(foo, bar)) ) .
    if not(contains(foo, Y)) .


thanks
Scott