Replace Section 8.1.13 with the following text to be
placed before the current 8.1.12.
8.1.12 Mapping Overloading
Invocation of a mapping selects a disjunction of
one or more candidate mappings at compile
time. At run-time, the first matching candidate
mapping is selected and invoked. The disjunction may
be specified explicitly using the disjuncts
keyword or implicitly by an overloaded mapping.
8.1.12.1 Explicit Disjuncts
In the following example, the explicit disjunction
defines convertFeature as a disjuncting
mapping name that may be invoked on a UML::Feature
with a Boolean argument. convertAttribute,
convertConstructor and convertOperation
are candidate mapping names.
mapping UML::Feature::convertFeature(asUpper: Boolean) : JAVA::Element
disjuncts convertAttribute, convertOperation, convertConstructor {}
mapping UML::Attribute::convertAttribute(asUpper: Boolean) : JAVA::Field {
name := if asUpper then name.toUpper() else name endif;
}
mapping UML::Operation::convertConstructor(asUpper: Boolean) : JAVA::Constructor
when {self.name = self.namespace.name;} {
name := if asUpper then name.toUpper() else name endif;
}
mapping UML::Operation::convertOperation(asUpper: Boolean) : JAVA::Constructor
when {self.name <> self.namespace.name;} {
name := if asUpper then name.toUpper() else name endif;
}
The explicit disjuncts causes the mapping
invocation to successively assess the implicit and
explicit predicates of convertAttribute, convertConstructor
and convertOperation to identify the first
match. If no match is found the mapping invocation
returns null.
The explicit predicates are provided by arbitrary
constraints specified in when clauses.
Implicit predicates are provided by the type
signatures; each source and argument must conform to
the type of the disjuncting mapping. An
abstract mapping has a false implicit
predicate; an abstract mapping cannot be executed.
The candidate return type must be
covariant, that is the same as, or derived from that
of, the disjuncting return type to ensure
that no result incompatibility arises.
Since the argument types contribute to implicit
predicates, the candidate argument types
may be supertypes or subtypes of the disjuncting
mapping. The number of candidate and disjuncting
argument types must be the same.
An explicit candidate mapping is
identified by its mapping identifier which
identifier may contribute to more than one
disjunction.
8.1.12.2 Implicit Disjuncts
An implicit disjunction groups overloaded mappings.
One mapping overloads another when the overloading
source type extends the overloaded source type and
when the overloading and overloaded mappings have
same name and argument count.
When UML::Attrbute and UML::Operation
extend UML::Feature, the previous example
may be simplified to use an implicit disjunction.
mapping UML::Feature::convertFeature(asUpper: Boolean) : JAVA::Element {}
mapping UML::Attribute::convertFeature(asUpper: Boolean) : JAVA::Field {
name := if asUpper then name.toUpper() else name endif;
}
mapping UML::Operation::convertFeature(asUpper: Boolean) : JAVA::Constructor
when {self.name = self.namespace.name;} {
name := if asUpper then name.toUpper() else name endif;
}
mapping UML::Operation::convertFeature(asUpper: Boolean) : JAVA::Constructor
when {self.name <> self.namespace.name;} {
name := if asUpper then name.toUpper() else name endif;
}
he explicit disjuncts provides distinct names and
so facilitates explicit calls direct to the candidate
mappings. The implicit disjuncts requires no disjuncting
declaration and so faciltates extension by addition
of further contributions.
8.1.12.3 Disjunct candidates
All mappings with the required name, argument and
matching or derived source type are candidate
mappings for the invocation of a disjuncting
mapping. This includes mappings inherited from
extended transformations. The candidate
mappings referenced in a disjuncting
mapping may introduce new names and consequently a
further disjunction of candidate mappings;
the explicit disjunct is transitive.
For instance invocation of convertFeature
for a Property in the explicit disjuncts
example should consider a Property::convertOperation(Boolean)
inherited from an extended transformation since the
explicit disjunct adds convertOperation to
the transitive candidates. Conversely, the implicit
disjunct example considers only candidates whose
signature is convertFeature(Boolean).
For non-strict evaluation, a deterministic
evaluation order for evaluation of the predicates of
the candidates as guards is established by sorting
using the following proritized criteria. A
distinction by an earlier criteria overrules all
later criteria.
- directly invoked explicitly
disjuncted candidate mappings are evaluated in
declaration order
- mappings in the current
transformation are evaluated before those in an
extended transformation, then mappings in an
extended transformation before those in an
extended extended transformation, and so forth
- mappings for a more derived type are
executed before those for a less derived type
- mappings are prioritized by
alphabetical mapping name order
- mappings are prioritized by
alphabetical context type name order
- mappings are prioritized by
alphabetical context type containing package name
order, then by containing package containing
package name order, and so forth
The ordering above ensures that an extending
transformation can occlude a mapping in an extended
transformation and that a mapping for a derived type
occludes that for a base type. An implementation may
use static analysis of the predicates to eliminate
occluded candidates completely and to provide
reduced candidate lists according to the source type
of the mapping invocation.
For strict evaluation, the same ordering applies
but the first candidate for which the source type
conforms is selected without evaluating the
predicate as a guard. The predicate is instead
evaluated as a pre-condition giving a null
return when not satisfied.
In the QVTo model and Fig 8.3. Add
MappingOperation ::isAbstract : Boolean[1] default
false.
In 8.2.15 MappingOperation Attributes Add
isAbstract : Boolean[1]
Indicates whether the mapping is abstract, requiring
an overload for all derived context types. Default
is false.
Hi
+1 for choosing the most derived in case of a family of overloaded mappings.
I also like the approach of defining an overload as an implicit disjuncts. In particular, this could simplify the traceability recording rules.
When calling a.doIt() dispatches to B::doIt(), I expect a traceability record for a.resolveIn(A::doIt), even if A::doIt didn't really execute.
For disjuncts, Eclipse QVTo already records traceability links also for the disjuncting mapping. Therefore the above requirement would be fulfilled.
It is more interesting when we have b:B and call b.doIt(). Does it still consult A::doIt() and produce the traceability link for b.resolveIn(A::doIt) ?
Regards
Christopher
-----Ursprüngliche Nachricht-----
Von: qvto-dev-bounces@xxxxxxxxxxx [mailto:qvto-dev-bounces@xxxxxxxxxxx] Im Auftrag von Ed Willink
Gesendet: Sonntag, 4. Oktober 2015 13:30
An: QVTOML developer mailing list <qvto-dev@xxxxxxxxxxx>
Betreff: [qvto-dev] What are the mapping refinement rules
Hi
When preparing my "QVT Traceability : What does it really mean?"
presentation to AMT 2015, I was forced to think hard about mapping signatures and their relationships.
https://www.eclipse.org/mmt/qvt/docs/ICMT2014/QVTtraceability.pdf
http://www.slideshare.net/EdWillink/qvt-traceability-what-does-it-really-mean
Declaratively, independent mappings are all invoked independently, dependent mappings such as refinements are arbitrated by their predicates to select the best of a group of related mappings.
Imperatively, it is much simpler, mappings are explicitly invoked, so exactly one compatible mapping is invoked per source object.
The simplest case is the invoked name corresponds to a declared mapping.
More interesting, the invoked name corresponds to a disjuncted mapping allowing the best match of the explicit disjuncts to be invoked, else null.
I am unclear about what happens when the invoked name corresponds to a family of 'overloaded' mappings analoguous to Java operation overloads.
e.g.
mapping A::doIt() : AA {...}
mapping B::doIt() : BB {...}
mapping C::doIt() : CC {...}
where C and B extend A
I presume that in QVTo we choose the most derived analoguously to Java.
I'm inclined to clarify this situation by defining such overloads as implicit disjuncts, so that all colliding names contribute to a disjunct
The example is then equivalent to
mapping A::doIt() : AA
disjuncts B::doIt, C::doIt {...}
mapping B::doIt() : BB {...}
mapping C::doIt() : CC {...}
This could make disjuncts extensible since an extending transformation could supply additional name collisions for the explicitly/implicitly disjuncted mapping.
In order to give deterministic dispatch, the order of disjuncts is:
explicit disjuncts first, then implicit disjuncts successively ordered by the following criteria
- most derived source type,
- most derived first argument type,
- most derived second argument type,
- etc
- alphabetically by containing class name,
- alphabetically by containing class' containing package name,
- etc
The signatures of refinements are unclear. Since a mapping is explicitly invoked, we clearly require that disjuncting mappings have the same number, position and direction of arguments and a covariant
(same/derived) return type. (An explicit disjunct may omit trailing in
arguments.)
Since we are invoking mappings with predicates rather than invoking operations, we may allow disjuncting mappings to also have covariant or even contravariant arguments. When a disjuncting mapping has a distinct in/inout argument type, it is equivalent to an oclIsKindOf() when predicate for that argument type with a subsequent oclAsType() in the body.
Regards
Ed Willink
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