Maude-help mailing list

[Steven Eker <eker AT csl.sri.com>]

WIthout the signature, it's hard to know what you are trying to do or what a 
given pattern will match. Always post complete modules.

If you are just manipulating data in a functional way (instead of changing 
state with alternative possibilities that might have to be explored using 
search or model checking) you are much better off using equations - they are 
_much_ faster and allow common subexpression evaluation (with rules, common 
subexpressions might evaluate differently in each occurrence).

Steven

On Monday 07 February 2011 10:32:03 am Scott Christley wrote:
> 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
> 
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