Difference between revisions of "Forget / Keep Mechanism"

Revision as of 07:35, 20 February 2007

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Introduction

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Example

The following classes will be used for illustration:

class ANIMAL
 
feature
 
  eat (f: FOOD)
 
end

class CAT
 
inherit
 
  ANIMAL
    redefine
      eat
    end
 
feature
 
  eat (f: CAT_FOOD)
 
end

class FOOD
end
 
class CAT_FOOD
 
inherit
 
  FOOD
 
end

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

class LIST [G]
 
inherit 
 
  SET [G]
 
end

Forget mechanism

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Default behaviour

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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
do
 
  a := c
  a.eat (food)
 
end

With forget mechanism

Types which have covariant redefined features will not be conform. A feature which is covariantly redefined changes its inherit clause implicitly to inherit from a parent class which has the redefined feature in a forget clause.

class CAT
 
inherit
 
  ANIMAL
    redefine
      eat
    end
 
feature
 
  eat (f: CAT_FOOD)
 
end

class CAT
 
inherit
 
  ANIMAL
    forget 
      eat
    redefine
      eat
    end
 
feature
 
  eat (f: CAT_FOOD)
 
end

The conformance between ANIMAL and ANIMAL forget eat and ANIMAL forget all is as follows:

local
  normal_animal: ANIMAL
  forget_eat_animal: ANIMAL forget eat end
  forget_all_animal: ANIMAL forget all end
do
    -- this assignment is legal since all features present in the
    -- forget types are also present in the normal type
  forget_eat_animal := normal_animal
  forget_all_animal := normal_animal
 
    -- this assignment is illegal since the forget_animal lacks
    -- the eat feature and thus cannot be used as an ANIMAL
  normal_animal := forget_eat_animal
  normal_animal := forget_all_animal
 
    -- this assignment is legal since the forget all type
    -- has fewer or equal features than the forget eat type
  forget_all_animal := forget_eat_animal
 
    -- this assignment is only legal if only the feature `eat'
    -- is covariantly redefined in the type ANIMAL and thus
    -- forget eat is equivalent to forget all
  forget_eat_animal := forget_all_animal
 
end

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

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

local
  a: ANIMAL forget all end
  c: CAT
do
    -- legal, CAT conforms to ANIMAL forget all
  a := c
 
    -- illegal, ANIMAL forget all doesn't have a feature eat
  a.eat (food)
 
end

Arguments with like

A declaration like Current is a covariant redefinition for all subtypes. Since this occurs in ANY.is_equal and ANY.copy, every class has a covariantly redefined fetaures.

class ANY
 
feature
 
  is_equal (other: like Current): BOOLEAN
 
  copy (other: like Current)
 
end

Generics

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Syntax

The first variant has the default on NOT forgetting features, thus breaking conformance between the type and its subclasses which use covariant redefinition.

-- 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 marked as `poly' will loose all covarianlty redefined features
  -- and all subtypes conform to this type
a2: poly ANIMAL
 
  -- a type which only forgets the features `eat' and `drink' but not other
  -- covarianlty redefined features. all subtypes which only redefine `eat' or
  -- `drink' will conform to this type
a3: ANIMAL forget eat, drink end
 
  -- a type which forgets all covariantly redefined features. equivalent to `poly'
a4: ANIMAL forget all end

The second variant has the default on forgetting all features which are covariantly redefined, thus keeping conformance with subtypes.

-- normal declaration is an object which forgets all covariantly
  -- redefined features. all subtypes conform to this type
b1: ANIMAL
 
  -- a type which keeps all features and looses conformance from subtypes which
  -- covariantly redefine features
b2: mono ANIMAL
 
  -- a type where all subtypes conform except those who covariantly redefine
  -- feature `eat'
b3: ANIMAL keep eat end
 
  -- a type which keeps all features. equivalent to `mono'
b4: ANIMAL keep all end

Consequences

ANY

Since ANY has features with like Current arguments, the default mechanism for forgetting or keeping features has an impact on the bevaiour of ANY and thus all types.

If the default is forgetting all features (poly) than you will be able to assign all types to ANY, but the features is_equal and copy will not be available since they are covariantly redefined.

If the default is keeping all featurs (mono) than no type will conform to ANY and thus all assignments to ANY from any other type are illegal.

Solution: change the argument of is_equal and copy to type ANY and introduce add a precondition to require conformance of argument and object.

SET.has

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