Replication

Author: Matthias Konrad

Known problems of the following text: Calls to targets of type like Current do not include usage of an attribute when it is assigned to.

One definition of replication is: the action or process of reproducing or duplicating.

Both ETL, OOSC2 and the standard use the term replication as if its semantics were common knowledge. I disagree to that, since I was not able to get consistent information about replication from notable experts. Hence it is certainly a term to be defined much more carefully.

Although not explicitly stated, it seems that a replicated feature has an unfolded form. That would mean, that we could get rid of replication by reducing it to other language mechanisms (As it is done for precursors). This is how the current ISE compiler (5.7 and earlier) handles replication (won't help us here since it doesn't comply to the standard).

If an unfolded form cannot be found, this has a huge impact on the standards text. The semantics of unqualified calls in the context of replicated features would for example not be defined.

Talking about features

Before digging into the replication rules we need a clear understanding of a feature.

There is a one to one connection between feature and Feature_body. Every feature has exacly one body. Every body belongs to exactly one feature. Meaning that, two different bodies belong to two different features and two different features have two different bodies.

Have a look at this example:

class
   B
feature
   f do ... end
end

class
   D
inherit
   B
      rename f as f1 select f1 end
   B
      rename f as f2 redefine f2 end
feature
   f2 do ... end
end

The above system has two features. Where the feature named f1 in class D and the feature named f in class B are one and the same feature. Its body is defined in class B. It is valid to say: Feature f1 of class D and feature f of class B are the same feature. The second feature in the system is feature f2 of class D or the feature named f2 in class D. This feature has its body defined in class D.

By saying feature, we indirectly reference a certain Feature_body. Indirectly, because we can say: "feature f of a class D", when the Feature_body of f belongs to an Ancestor class A of D (Given that f is not redeclared on the inheritance path between A and D).

The name of feature f is always the name stated in its Feature_body. A feature may have different names in different classes (through renaming). It is thus legal to say: "Feature f with name fn in class D, when feature f is renamed to fn somewhere along the according inheritance path.

When say feature bodies we are not strictly boud to the bodies occuring in the programms text. We may also denote the bodies that are results of folded forms. Examples are the body of a synonym or the body of a call-equivalent of an inline-agent or the body of a replicate as we will later see.

Version

A version of a feature f is a feature f2 different from f that is a redeclaration from either f or a version of f.

New and modified rules

In the following a set of rules are given that are meant to replace rules 8.16.2 through 8.16.5.

8.16.2 Semantics: Repeated Inheritance rule

This rule will be obsolete but some thing similar might be introduced as informal text.

8.16.3 Definition: Replication

A feature f of class D is replicated in D if one of the following is true:

• Feature f has more then one name in D.
• There is a feature f2 in class D such that f2 is a version of f.

Informal:

• Example:

class
   B
feature
   f do ... end
end

class
   C
inherit
   B
      rename f as f1 redefine f1 end
feature
   f1 do ... end
end

class
   D
inherit
   C
      select f1 end
   B 
      rename f as f2 redefine f2 end
   B
      rename f as f3 end
feature
   f2 do ... end
end

• Feature f1 of class D (whose body is in class C) is not replicated in class D.
• Feature f2 of class D (whose body is in class D) is not replicated in class D.
• Feature f3 of class D (whose body is in class B) is replicated in class D, since feature f2 of class B is a version of it.

8.16.X Validity: replicated feature

Let feature f with name fn in class D be replicated in D. Let P1...Pn be the parent clauses through which D inherits feature f with name fn.

Feature f is valid in class D if and only if it satisfies the following:

Every feature m that is used in f on a target of type like Current must be inherited through clauses P1...Pn with the same name.

Informal:

• Example:

class
   B
feature
   f do g end
   g do ... end
 
end

class
   D
inherit
   B
      rename f as f1, rename g as g1, select f1, g1 end
   B
      rename f as f2, g as g2 end 
   B
      rename f as f2, g as g3 end
feature
   f1 do ... end
end

There are several features that are replicated in D: f1, g1, f2, g2, g3. Only f2 is not a valid replicated feature in D. It is inherited through the second and third parent part and has a call to g (with target of type like Current). Since g of class B is inherited with a different name through the second and third parent part the system is not valid.

8.16.X Semantics: Replication

Let:

• feature f with name fn be replicated in class D.
• P1...Pn be the parent clauses through which D inherits feature f with name fn.
• B be the class that contains the body of f.
• CP be one of the conformance paths from B to D that enter D through the parent parts P1.

The unfolded form of feature f consists of a new redefine fn clause in each of P1,...,Pn and the corresponding new feature that is further called the replicate of f and defined as follows:

• The replicate of f and f have the same signature.
• The name of the replicate is fn.
• The body of the replicate is a modified copy of the body of f. The feature names that are used with targets of type like current are renamed according to the renamings that occur on the conformance path CP.

Informative:

• The validity rule of not isolated features warrant, that all the conformance paths from B over P1..Pn to D yield the the same renamings.

• Example:

class
   B
feature
   f do g end
   g do end
end

class
   D
inherit
   B
      rename f as f1, g as g1 redefine f1 select f1, g1 end
      rename f as f2, g as g2 end
feature
   f1 do ... end
end

The not isolated features of class D are g1, f1 and g2. The following snippet shows class D with all its not isolated features unfolded:

class
   D
inherit
   B
      rename f as f1, g as g1 redefine f1, g1 select f1, g1 end
      rename f as f2, g as g2 redefine f2, g2 end
feature
   f1 do ... end 
   g1 do end    --I am a replicate!
   f2 do g2 end --Me to
   g2 do end    --Me to
end

8.16.4 Validity: Call Sharing rule

(The rule described here would replace 8.16.4. Maybe an other name is needed for the rule)

It is valid for a feature f declared in class B and isolated in class D to use feature g of A on a target of type like Current if and only if there is exactly one version of g in D.

Informative:

• This rule implies one very simple property of calls to targets of type like Current, they are not influenced by selects.
• Example of an invalid System according to this rule:

class
   B
feature
   f
      do g end
   g  do ... end
end

class
   D
inherit
   B
      rename g as g1 redefine g1 select g1 end
   B
      rename g as g2 redefine g2 end
feature
   g1 do ... end
   g2 do ... end
end

Feature f is inherited by class D from ancestor A. The feature involves an unqualified call to feature g. Since feature g has two versions in D (g1 and g2) the system is invalid. If f would be renamed in one or both the parent parts, the system could be made valid.

8.16.5 Semantics: Replication Semantics rule

Rather than a semantic rule this is a consequence of all the not isolated features of a class being unfolded.