Difference between revisions of "Without /except"

Revision as of 04:59, 18 September 2007

Research: This page describes research about Eiffel, not the actual language specification.

Introduction

By allowing covariant feature redefinition and hiding of features, the Eiffel language introduces the problem of cat-calls (Changed Availability or Type). The restrict mechanism prevents this by introducing new derived types whenever a feature is covariantly redefined or the export status is restricted. These derived types are then used to prevent cat-calls through the conformance rules to the original types.

This mechanism is a variant on forget/keep as originally envisaged by Mark Howard. This variant was worked out by us when he explained to me his ideas about forget/keep.

Example classes

The following classes will be used for illustration:

class ANIMAL
 
feature
 
  eat (f: FOOD)
 
  sleep
 
end

class CAT
 
inherit
 
  ANIMAL
    redefine
      eat
    export
      {NONE} sleep
    end
 
feature
 
  eat (f: CAT_FOOD)
 
end

class FOOD
 
end
 
 
class CAT_FOOD
 
inherit
 
  FOOD
 
end

class LIST [G]
 
feature
 
  has (g: G): BOOLEAN
 
  put (g: G)
 
  item: G
 
end

The problem

The problem of cat-calls has two variants, one because a feature is covariantly redefined:

local
  a: ANIMAL
  c: CAT
  food: FOOD
do
 
  a := c
    -- illegal since `eat' for type CAT takes arguments only of type CAT_FOOD
  a.eat (food)
 
end

And one because the visibility of features change:

local
  a: ANIMAL
  c: CAT
do
 
  a := c
    -- `sleep' is hidden for type CAT and should not be called
  a.sleep
 
end

The idea

To solve the problem of cat-calls, the idea is that whenever you covariantly redefine a feature of a parent type, your new classes conforms to a virtual class of the same type as the parent type except that it lacks the covariant feature. The same goes for changing the export status of a feature.

For instance, if the type CAT inherits from the type ANIMAL but covariantly redefines the feature eat then the type CAT does not conform to ANIMAL anymore, but only to a type ANIMAL without the feature eat.

Restrict mechanism

The restrict mechanism introduces a new keyword restrict which can be used to covariantly redefine features. By removing features from the class interface, a new type is introduced. The new type has all the features that the original type had, except those listed in the restrict clause - these latter features are present in the class, but are not part of the conforming type. If restrict all is used, all features in the original will be removed from the new conforming type. This produces a conforming type with no features at all. We also introduce another new keyword except which can only follow restrict all (therefore it does not need to be a completely reserved word).

Default behavior

In the restrict mechanism the normal declaration of a type will have all the features of a type from which it inherits (that is, it conforms to the type). This implies that all subtypes which reduce an export status or covariantly redefine a feature don't conform to this type anymore.

local
    -- normal declaration means we have all the features in the conforming type
  animal: ANIMAL
  cat: CAT
do
    -- legal since the feature `eat' is in the normal type
  animal.eat (food)
 
    -- legal since the feature `sleep' is in the normal type
  animal.sleep
 
    -- illegal since CAT does not conform to ANIMAL anymore.
    -- N.B. The declaration of CAT given above is illegal under this proposal. 
    -- Adding the restrict keyword will make it legal again, and emphasizes the lack of conformance.
  animal := cat
end

Syntax

-- normal declaration is an object with all features and no
  -- subtype which has a covariantly redefined feature conforms to this type
a1: ANIMAL
 
  -- a type which lacks any conforming features, thus all 
  -- subtypes conform to this type.
a2: ANIMAL restrict all end
 
  -- a type which only restricts the features `eat' and `sleep'. All subtypes which only redefine (but not restrict) `eat' or change the export status 
  -- of `sleep' will conform to this type.
a3: ANIMAL restrict eat export {NONE} sleep end
 
 -- subtypes of the following type conform to it provided they do not restrict (that is, covariantly redefine) `eat' (or restrict its export status).
a4: ANIMAL restrict all except eat end

Covariant feature redefinition

Cat-call problem

With covariant feature redefinition you run into cat-call problems as this example shows:

local
  a: ANIMAL
  c: CAT
  food: FOOD
do
 
  a := c
    -- eat for type CAT takes arguments only of type CAT_FOOD
  a.eat (food)
 
end

Restrict mechanism

Using the restrict mechanism the default behavior for a type is to conform to its parent with respect to all features. Types which want to have covariant redefined features will not conform to the original type. You do not mention these features in a redefine clause, but in a restrict clause.

class CAT
 
inherit
 
  ANIMAL 
    restrict eat end
 
 
feature -- Covariant redefinitions
 
  eat (f: CAT_FOOD) is
   do
    ...
   end
 
end

Now the cat-call example with the new restrict types:

local
  a: ANIMAL
  c: CAT
do
    -- illegal assignment, ANIMAL and CAT don't conform
    -- since CAT inherits from (and therefore conforms to) ANIMAL restrict eat
  a := c
  a.eat (food)
 
end

Feature hiding

Cat-call problem

By restricting the export status of features, a cat-call problem is introduced:

local
  a: ANIMAL
  c: CAT
do
 
  a := c
    -- sleep is hidden for type CAT and should not be called
  a.sleep
 
end

Restricting the export status

Types which reduce the export status of features will not conform to the original type.

Now the cat-call example:

local
  a: ANIMAL
  c: CAT
do
    -- illegal assignment, ANIMAL and CAT don't conform
   a := c
  a.sleep
 
end

local
  a: ANIMAL export {NONE} sleep end
  c: CAT
do
    -- legal, CAT conforms to ANIMAL export {NONE} sleep
  a := c
 
    -- illegal, ANIMAL export {NONE} sleep doesn't export feature sleep
  a.sleep
 
end

Note the declaration of a. It is desirable that any sort of inheritance restriction (such as listing creation procedures) be allowed on declarations.

Generics

Conformance of generic classes (two generic classes conform if their base classes conform and their generic parameters conform) introduces another kind of covariantly redefined features. Every feature which has a generic argument, e.g. put (g: G) in class LIST [G] can be regarded as covariantly redefined since put in LIST [ANY] takes an argument of type ANY and put in LIST [ANIMAL] takes an argument of type ANIMAL. But LIST [ANIMAL] conforms to LIST [ANY], thus the feature put is actually covariantly redefined in LIST [ANIMAL].

Cat-call problem

The conformance rules of generics also introduce cat-call problems:

local
    -- feature `put' has argument type ANY
  any_list: LIST [ANY]
    -- feature `put' has argument type ANIMAL
  animal_list: LIST [ANIMAL]
do
    -- animal_list conforms to any_list, but arguments of the feature
    -- `put' are different
  any_list := animal_list
 
    -- since any type can be put into any_list this will add
    -- an integer to the animal_list
  any_list.put (5)
 
end

Conformance

A generic class X [G] conforms to another generic class X [H] if G and H are the same class, or if G conforms to H and H's conforming type does not include any features taking generic arguments, or any features that in turn (recursively) make use of features taking generic arguments.

Using restrict mechanism

local
  any_list: LIST [ANY]
  animal_list: LIST [ANIMAL]
do
    -- illegal since animal_list does not conform to any_list anymore
  any_list := animal_list
 
  any_list.put (5)
 
end

local
  any_list: like usable_list
  animal_list: LIST [ANIMAL] 
  animal: ANIMAL
do
  -- legal since LIST [ANIMAL] has `put'
  animal_list.put (animal)
 
    -- legal since animal_list conforms to `usable_list'
  any_list := animal_list
 
    -- illegal since `usable_list' does not have `put'
  any_list.put (animal)
 
    -- illegal, since `usable_list' does not have `has'
  any_list.has (animal)
 
end
 
feature {NONE} -- Anchors
 
  usable_list: LIST [ANY] restrict put, has, item end
      -- Anchor - in practice this anchor is defined in a class which is imported using non-conforming inheritance

--Colin Adams and Mark Howard