Difference between revisions of "Forget / Keep Mechanism"

Revision as of 16:44, 19 February 2007

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covariant feature redefinition

Example

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

cat-call

local
  a: ANIMAL
  c: CAT
do
 
  a := c
  a.eat (food)
 
end

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

generics

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