Difference between revisions of "Forget / Keep Mechanism"
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Since <e>ANY</e> has features with <e>like Current</e> arguments, the default mechanism for forgetting or keeping features has an impact on the bevaiour of <e>ANY</e> and thus all types. | Since <e>ANY</e> has features with <e>like Current</e> arguments, the default mechanism for forgetting or keeping features has an impact on the bevaiour of <e>ANY</e> 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 <e>is_equal</e> and <e>copy</e> will not be available since they are covariantly redefined. | + | If the default is forgetting all features (poly) than you will be able to assign all types to <e>ANY</e>, but the features <e>is_equal</e> and <e>copy</e> 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. | + | If the default is keeping all featurs (mono) than no type will conform to <e>ANY</e> and thus all assignments to <e>ANY</e> from any other type are illegal. |
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Revision as of 07:58, 20 February 2007
Warning: Warning: Article under development
Contents
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 |
covariant feature redefinition
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
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
...
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
like and 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.
...