Hi
Thanks. Three interesting points. We have 6 days before this gets
semi-frozen in a Preview.
It is not my intention to require any run-time
functionality to move to comple-time. I think I just highlight that
unspecified but necessary functionality is at run-time.
The "usual object-oriented virtual call semantics" is:
execute the function corresponding to the derived type.
My suggested MappingCallExp semantics is a prioritized (run-time)
disjunct search:
if A's predicates are satisfied doA
else if B's predicates are satisfied doB
...
else null
I observe that some static analysis can eliminate some of the cases.
(In the case of unknown transformation extension, the static
analysis could be at load-time, or JIT for each new invocation.)
In your example perhaps your internal doIt' is my internal list of
candidates. Reifying the internal list may make the exposition
clearer.
---
Given a body for both candidate and disjuncting mapping, when is
each executed? Perhaps the disjuncting body (default null) is
executed only when no candidate is found.
---
If a MappingCallExp.referredOperation references X::doA, it is
currently unspecified in OCL and so in QVTo whether this invokes
precisely X::A or the overload applicable to the actual source type.
In OCL it should nearly always be the overload, though just
occasionally as in super-calls it would be nice to suppress dynamic
dispatch. Perhaps a Boolean isStaticDispatch flag is needed and set
by the presence of a fully qualified name in the source code.
If a MappingCallExp used the "usual object-oriented virtual call
semantics" then it is precisely the same problem. But it's different
although with the same ambiguity. Does the the AS reference to X::A
trigger the disjunct search for X::A or just call X::A regardless?
Mostly it should do the disjunct search, but just X::A if we want a
super-call. (A more optimized implementation looks up the
precomp[uted source type specific list of candidate mappings.)
---
Tracing has a similar problem.
Is the disjuncting mapping traced or is the actual selected
candidate mapping traced? Both are of some use. See my AMT 2015
talk. The most important usage is in resolveIn where the user cannot
sensibly reference the candidate mapping, therefore resolveIn must
at least work for a reference to the disjuncting mapping. However,
particularly given that a candidate mapping may actually be
candidate for multiple disjuncting mappings, it should also be
possible to invoke resolveIn with respect to the actual candidate
mapping. This is probably difficult and hard to support unless the
trace record contains both disjuncting and candidate mapping
identities. Both are needed internally, an arbitrary choice could be
made for the qvttrace file for which the QVTo specification might
evolve to dismiss as a proprietary convenience. Certainly nothing
that is QVTr compatible and fully supporting incremental
re-execution.
Regards
Ed
On 07/10/2015 11:31, Adolfo
Sanchez-Barbudo Herrera wrote:
Hi Ed,
In principle, the implicit disjuncts seems a good idea, an there
are some interesting clarifications in your proposal, but I need
to point out a couple of concerns.
1. The disjuncting mapping itself is a kind of "intermediate"
mapping (no logic goes there). So in your example:
mapping A::doIt() : AA {...}
mapping B::doIt() : BB {...}
mapping C::doIt() : CC {...}
The equivalent should be something like the following:
mapping A::doIt'() : AA
disjuncts B::doIt, C::doIt, A::doIt {...}
mapping A::doIt() : AA {...}
mapping B::doIt() : BB {...}
mapping C::doIt() : CC {...}
At compilation time any static call to A::doIt(), should be
changed to the A::doIt'()
2. More seriously, you are moving a run-time dispatch mechanism,
to a compile-time, which it should turn to be reflected in the AS.
That in principle sounds OK for single transformations, i.e. for
inter-transformation mappings extensions, but it doesn't seem so
good in the inter-transformation reuse scenario, because we need
cross-references from the extended transformation to the different
extending ones. Compiled transformations wouldn't make less sense,
or we would require many AS compiled copies/variants of a extended
transformation, depending on how these implicit disjuncts come up
from the extending ones.
I reluctant to replace the traditional virtual mapping call
mechanism (performed at runtime) by this new implicit disjuncts
mechanism (performed at compile time). That said, I think that
much of what you have tried clarify, can be useful explain for
instance, how signature of extending mappings should be, or how
the virtual mapping call mechanism should be performed in QVT
rather than "This follows usual object-oriented virtual call
semantics" (page 93) .
I hope you find my comments useful.
Regards,
Adolfo.
On 07/10/2015 10:41, Ed Willink
wrote:
Hi
A first attempt at a proposal. I think it says what is needed,
but perhaps some of it should be moved/duplicated in 8.2.1.21
MappingCallExp.
I considered "abstract" since it is in the Concrete Syntax and
another issue requests it. I'm not sure whether it is really
that useful.
Regards
Ed Willink
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.
On 06/10/2015 13:26, Christopher
Gerking wrote:
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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