Difference between revisions of "Forget / Keep Mechanism"
| Line 3: | Line 3: | ||
===Example=== | ===Example=== | ||
| + | {| class="top-aligned" | ||
| + | | | ||
<e> | <e> | ||
class ANIMAL | class ANIMAL | ||
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end | end | ||
</e> | </e> | ||
| − | + | | | |
<e> | <e> | ||
class CAT | class CAT | ||
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end | end | ||
</e> | </e> | ||
| + | | | ||
| + | <e> | ||
| + | class FOOD | ||
| + | end | ||
| + | |||
| + | class CAT_FOOD | ||
| + | |||
| + | inherit | ||
| + | |||
| + | FOOD | ||
| + | |||
| + | end | ||
| + | </e> | ||
| + | |} | ||
| − | cat-call | + | ===cat-call=== |
<e> | <e> | ||
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</e> | </e> | ||
| − | forget-mechanism | + | ===forget-mechanism=== |
| − | Types which have covariant redefined features will not be conform. | + | 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=" | + | {| class="top-aligned" |
|- | |- | ||
| − | + | ! what you write | |
| − | + | ! what is implied | |
|- | |- | ||
| | | | ||
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end | end | ||
| − | + | feature | |
| + | |||
| + | eat (f: CAT_FOOD) | ||
| + | |||
| + | end | ||
</e> | </e> | ||
| | | | ||
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end | end | ||
| − | + | feature | |
| + | |||
| + | eat (f: CAT_FOOD) | ||
| + | |||
| + | end | ||
</e> | </e> | ||
|} | |} | ||
| − | + | The conformance between <e>ANIMAL</e> and <e>ANIMAL forget eat</e> and <e>ANIMAL forget all</e> is as follows: | |
<e> | <e> | ||
local | local | ||
normal_animal: ANIMAL | normal_animal: ANIMAL | ||
| − | + | forget_eat_animal: ANIMAL forget eat end | |
| + | forget_all_animal: ANIMAL forget all end | ||
do | do | ||
| − | -- this assignment is legal | + | -- 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 | -- this assignment is illegal since the forget_animal lacks | ||
-- the eat feature and thus cannot be used as an ANIMAL | -- the eat feature and thus cannot be used as an ANIMAL | ||
| − | normal_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 | end | ||
</e> | </e> | ||
| − | Now the cat-call example: | + | Now the cat-call example with the new forget types: |
<e> | <e> | ||
| Line 107: | Line 145: | ||
do | do | ||
-- illegal assignment, ANIMAL and CAT don't conform | -- illegal assignment, ANIMAL and CAT don't conform | ||
| − | -- since CAT inherits from ANIMAL forget eat | + | -- since CAT implicitly inherits from ANIMAL forget eat |
a := c | a := c | ||
a.eat (food) | a.eat (food) | ||
| + | |||
end | end | ||
</e> | </e> | ||
| Line 120: | Line 159: | ||
-- legal, CAT conforms to ANIMAL forget all | -- legal, CAT conforms to ANIMAL forget all | ||
a := c | a := c | ||
| + | |||
-- illegal, ANIMAL forget all doesn't have a feature eat | -- illegal, ANIMAL forget all doesn't have a feature eat | ||
a.eat (food) | a.eat (food) | ||
| + | |||
end | end | ||
</e> | </e> | ||
Revision as of 15:39, 19 February 2007
Warning: Warning: Article under development
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
