Revision as of 09:43, 28 September 2006

Unfolding

Definition: Coupled name

Motivation

There are several situations in which the ECMA standard uses unfolded forms as a vehicle to describe semantics. When this unfolded forms need names, like in Precursor, inline agents and not isolated features. These names have an influence on the semantics of the system. An example:

class
   B
feature
   f 
      do 
         (agent do g := g + 1; print (g) end).call ([]) 
      end
   g: INTEGER
end

class
   D
inherit
   B
      rename f as f1, g as g1, select f1, g1 end
   B
      rename f as f2, g as g2 end
end

It feels natural to unfold class B first and then inherit D from its unfolded form before D is unfolded:

class
   B
feature
   f 
      do 
         (agent fict_name).call ([]) 
      end
   g: INTEGER
 
   fict_name 
      do 
         g := g + 1; print (g) 
      end
end

class
   D
inherit
   B
      rename f as f1, g as g1, select f1, g1 end
   B
      rename f as f2, g as g2 end
end

The call-equivalent of the inline-agent (here named fict_name) has a call to g which has several potential versions in D. Hence this is not a valid system. The same problem can occur with calls to Precursor. The programmer cannot do anything about it since he has no knowledge of the fictitious name of the call-equivalent. There should have been some coupling between the name f and the name of the call-equivalent of the inline-agent. The unfolded form of a renaming would then also rename all the coupled name (a precise definition follows). Our final example would become:

class
   B
feature
   f 
      do 
         (agent fict_name).call ([]) 
      end
   g: INTEGER
 
   fict_name 
      do 
         g := g + 1; print (g) 
      end
end

class
   D
inherit
   B
      rename f as f1, fict_name1, g as g1 
      redefine f1, fict_name1, g1
      select f1, fict_name1, g1 end
   B
      rename f as f2, fict_name as fict_name2, g as g2 
      redefine f2, fict_name2, g2 end
feature
   f1 do (agent fict_name1).call ([]) end
   fict_name1 do g1 := g1 + 1; print (g1) end
   g1: INTEGER
 
   f2 do (agent fict_name2).call ([]) end
   fict_name2 do g2 := g2 + 1; print (g2) end      
   g2: INTEGER
end

The redefinitions of f1, fict_name1, g1, f2, fict_name2 and g2 come with the unfolded form of not isolated features. Please note that the unfolded form of D needs to select fict_name1 or fict_name2 for the system to be valid. But this select has no semantic influence.

Definition

A feature name n can be coupled to another feature name .

Change on renaming

The unfolded form of a renaming introduces renamings to new fictitious names for all the names coupled to one of the renamed names.

Informal:

• Example Let feature names fc1 and fc2 be coupled to name f and feature name gc be coupled to g. The following renaming:

rename f as f', g as g'

Has the unfolded form:

rename f as f', fc1 as fc1', fc2 as fc2', g as g', gc as gc'

Whereas fc1', fc2' and gc' are new unique names.

Change on select

The unfolded form of a select introduces selects of all names coupled to one the originally selected names.

A complex example with precursor

class
   A
feature
   a: INTEGER   
   f do a := a + 1 end
   g do f end

class
   B
inherit
   A redefine g end
feature
   g do Precursor end
end

class
   D
inherit
   B 
      rename a as a1, f as f1, g as g1 
      select a1, f1, g1 end
   B
      rename a as a2, f as f2, g as g2 end
feature
end

Unfolded forms of A and B:

class
   A
feature
   a: INTEGER   
   f do a := a + 1 end
   g, gp do f end

class
   B
inherit
   A redefine g end
feature
   g do gp end
end

Unfolded form of D:

class
   D
inherit
   B 
      rename a as a1, f as f1, g as g1, gp as gp1
      redefine f1, g1, gp1 end
      select a1, f1, g1, gp1 end
   B
      rename a as a2, f as f2, g as g2, gp as gp2
      redefine f2, g2, gp2 end
feature
   f1 do a1 := a1 + 1 end
   g1 do gp1 end
   gp1 do f1 end
 
   f2 do a2 := a2 + 1 end
   g2 do gp2 end
   gp2 do f2 end
end

New Behaviour of renaming and select

The unfolded form of a renaming is the renaming itself plus the unfolded forms of the renamings of all the coupled names.

Precursor

The current definition of the Precursor semantics in the ECMA standard (8.10.11)

Agent types

Motivation

The classes ROUTINE, PROCEDURE, FUNCTION and the new class PREDICATE do not nicely fit in to the Eiffel language. Reasons:

• They do not reflect the fact, that an agent can refer an attribute. An attribute is not a ROUTINE.
• They do contradict the uniform access principle.
• They are difficult to define. It is difficult to understand, why it is so difficult to define an agent type when it is so easy in functional languages!
  • Specially the first generic Parameter of ROUTINE is difficult to understand and, if I am not wrong, never used.
• The standard conformance rules simply do not work nice for agent types. They introduce a type hole that can only be detected at runtime, and even then, only if precondition assertions are enabled. Other than cat calls this typing problem doesn't give the language more power.

An other mechanism:

I will give an example of an other mechanism:

Instead of writing

f: FUNCTION [ANY, TUPLE [STRING], INTEGER]

we could write

f (STRING): INTEGER

Calling this f would be easy:

i: INTEGER ... i := f ("hello")

We could also write:

i: INTEGER ... i := agent f ("hello")

Instead of writing:

f: FUNCTION [ANY, TUPLE [STRING], ANY]

we would write:

f (STRING): ANY

Inherited, redeclared and immediate features

Definition 8.5.1 should be changed to something with the following semantics:

8.5.1 Definition: Inherited, immediate; origin; redeclared; replicated introduce

Any feature f of a class C is of one of the following four kinds:

1. If C obtains f from one of its parents, f is an inherited feature of C. In this case any declaration of f in C (adapting the original properties of f for C) is a redeclaration.
2. If a declaration appearing in C applies to a feature that is not inherited, the feature is said to be immediate in C. Then C is the origin (short for “class of origin”) of f, and is said to introduce f.