[news.eclipse.modeling.m2m] [ATL] Re: What is the meaning of "mapsTo"?

[Dennis Wagelaar <dennis.wagelaar@xxxxxxxxx>]

Matthias Bohlen schreef:
> Hi Dennis,
>
> in some of your transformations, I see the keyword "mapsTo":
>
> rule DurationInterval {
>    from s : UML2!uml::DurationInterval (thisModule.inElements->includes(s))
>    to t : UML2!uml::DurationInterval mapsTo s ( ... )
> }
>
> What does this keyword do?
>
> Cheers
> Matthias

Short answer:
The keyword currently does nothing. You may ignore it.

Long answer:
The "mapsTo" keyword is meant to specify that "s" is replaced by "t" for 
every reference to "s". The current behaviour of ATL is to always 
replace the source element by the first target element. Consider the 
following example:

rule A {
  from s : UML!Class
  to t : RDB!Table (
    columns <- s.ownedAttribute)
}

rule B {
  from s : UML!Attribute
  to t : RDB!Column (
    name <- s.name),
   index : RDB!Index (
    name <- s.name)
}

RDB!Table cannot contain UML!Attribute elements as columns (rule A). 
That's why s.ownedAttribute is replaced with what it "mapsTo". This is 
defined in rule B: each UML!Attribute is mapped to a RDB!Column. So, 
what happens when I change the order of "t" and "index" in rule B? 
Remember that only the first target element in each rule gets this 
special "mapsTo" treatment: if each UML!Attribute maps to an RDB!Index, 
rule A becomes invalid: it tries to assign a Sequence of RDB!Index to 
columns.

I found this special treatment of the first target element too 
significant to remain implicit and I've added the "mapsTo" keyword to 
make it explicit.

I'm not sure if this was a wise decision, since it likely makes you 
believe that you can use "mapsTo" on other target elements as well. This 
is not true, since "mapsTo" does nothing at the moment. It also provokes 
questions like "What does this keyword do?" ;-). I suppose Frédéric 
should just take it out of the parser and force me to update my 
transformations, so I can stop shooting myself in the foot with this ;-).
--
Regards,
Dennis