Difference between revisions of "Forget / Keep Mechanism"
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===Arguments with like=== | ===Arguments with like=== | ||
| − | A declaration <e>like Current</e> is a covariant redefinition for all subtypes. | + | A declaration <e>like Current</e> is a covariant redefinition for all subtypes. |
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==Feature hiding== | ==Feature hiding== | ||
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===ANY=== | ===ANY=== | ||
| − | + | <e>ANY</e> has the features <e>ANY.is_equal</e> and <e>ANY.copy</e> with a <e>like Current</e> argument: | |
| + | |||
| + | <e> | ||
| + | class ANY | ||
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| + | feature | ||
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| + | is_equal (other: like Current): BOOLEAN | ||
| + | |||
| + | copy (other: like Current) | ||
| + | |||
| + | end | ||
| + | </e> | ||
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| + | Since all types are descendants of <e>ANY</e>, all types have covariantly redefined features. The choice of default for the forget mechanism has thus an impact on the whole type hierarchy: | ||
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 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. | ||
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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. | 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. | ||
| − | '''''Solution''''': change the argument of <e>is_equal</e> and <e>copy</e> to type <e>ANY</e> | + | '''''Solution''''': change the argument of <e>is_equal</e> and <e>copy</e> to type <e>ANY</e>. Then either introduce a precondition to require conformance of argument and object or add a type checks to the actual implementation of the features. |
===SET.has=== | ===SET.has=== | ||
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Revision as of 08:17, 20 February 2007
Warning: Warning: Article under development
Contents
Introduction
By allowing covariant feature redefinition and hiding of features, the Eiffel language introduces the problem of cat-calls. The forget mechanism tries to prevent this by introducing new derived types whenever a feature is covariantly redefined or the export status restricted. These derived types are then used to prevent cat-calls through the conformance rules to the original types.
Example
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 SET [G] feature has (g: G): BOOLEAN put (g: G) item: G end |
class LIST [G] inherit SET [G] end |
Forget mechanism
...
Conformance
The conformance between ANIMAL, ANIMAL forget eat, ANIMAL forget sleep and ANIMAL forget all is as follows:
local normal_animal: ANIMAL forget_eat_animal: ANIMAL forget eat end forget_sleep_animal: ANIMAL forget sleep end forget_all_animal: ANIMAL forget all end do -- these assignments are legal since all features present in the -- forget types are also present in the normal type forget_eat_animal := normal_animal forget_sleep_animal := normal_animal forget_all_animal := normal_animal -- these assignment are illegal since the forget types lack -- at least one feature and thus cannot be used as an ANIMAL normal_animal := forget_eat_animal normal_animal := forget_sleep_animal normal_animal := forget_all_animal -- these assignments are legal since the forget all type -- has fewer or equal features than the other forget types forget_all_animal := forget_eat_animal forget_all_animal := forget_sleep_animal -- these assignments are illegal since the forget all type -- lacks one feature that the other types have (eat or sleep -- respectively) forget_eat_animal := forget_all_animal forget_sleep_animal := forget_all_animal -- these assignments are illegal since both types lack -- a feature which the other type has forget_eat_animal := forget_sleep_animal forget_sleep_animal := forget_eat_animal end
Default behaviour
...
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
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.
| 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 |
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.
Feature hiding
Cat-call problem
By changing 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
With forget mechanism
When you change the export status of a feature, you implicitly change the inherit clause to inherit from a type which has the specific feature in a forget clause.
| what you write | what is implied |
|---|---|
class CAT inherit ANIMAL export {NONE} sleep end end |
class CAT inherit ANIMAL forget sleep export {NONE} sleep end end |
Now the cat-call example:
local a: ANIMAL c: CAT do -- illegal assignment, ANIMAL and CAT don't conform -- since CAT implicitly inherits from ANIMAL forget sleep a := c a.sleep 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 sleep a.sleep 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
ANY has the features ANY.is_equal and ANY.copy with a like Current argument:
class ANY feature is_equal (other: like Current): BOOLEAN copy (other: like Current) end
Since all types are descendants of ANY, all types have covariantly redefined features. The choice of default for the forget mechanism has thus an impact on the whole type hierarchy:
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. Then either introduce a precondition to require conformance of argument and object or add a type checks to the actual implementation of the features.
SET.has
...
