Difference between revisions of "Forget / Keep Mechanism"
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− | + | -- normal declaration is an object with all features and does | |
− | + | -- not conform to subtypes which have covariantly redefined features | |
− | + | a1: ANIMAL | |
− | + | -- a type marked as `poly' will loose all covarianlty redefined features | |
− | + | -- and be conform to all subtypes | |
− | + | a2: poly ANIMAL | |
− | + | -- a type which only forgets the features `eat' and `drink' but not other | |
− | + | -- covarianlty redefined features | |
− | + | a3: ANIMAL forget eat, drink end | |
− | + | -- a type which forgets all covariantly redefined features. equivalent to `poly' | |
− | + | a4: ANIMAL forget all end | |
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− | + | -- normal declaration is an object which forgets all covariantly | |
− | + | -- redefined features and is conform to all subtypes | |
− | + | b1: ANIMAL | |
− | + | -- a type which keeps all features and looses conformance to subtypes which | |
− | + | -- covariantly redefine features | |
− | + | b2: mono ANIMAL | |
− | + | -- a type which conforms to all subtypes 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 | |
</e> | </e> |
Revision as of 17:06, 19 February 2007
Warning: Warning: Article under development
Contents
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.
what you write | what is implied |
---|---|
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
...
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 does -- not conform to subtypes which have covariantly redefined features a1: ANIMAL -- a type marked as `poly' will loose all covarianlty redefined features -- and be conform to all subtypes a2: poly ANIMAL -- a type which only forgets the features `eat' and `drink' but not other -- covarianlty redefined features 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 and is conform to all subtypes b1: ANIMAL -- a type which keeps all features and looses conformance to subtypes which -- covariantly redefine features b2: mono ANIMAL -- a type which conforms to all subtypes 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