Difference between revisions of "Conversion rules"
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Instead of restricting VYCQ(2) and VYCP(2) to non-generic types we allow generic types too. As VYC*(2) is even using the notion of ''current type'', it might indeed be possible that it was the authors original intention. | Instead of restricting VYCQ(2) and VYCP(2) to non-generic types we allow generic types too. As VYC*(2) is even using the notion of ''current type'', it might indeed be possible that it was the authors original intention. | ||
| − | We define an additional function '' | + | We define an additional function ''FTN'' which replaces every formal generic with NONE. |
(If used here one assumes that NONE conforms to expanded types as well.) | (If used here one assumes that NONE conforms to expanded types as well.) | ||
| Line 147: | Line 147: | ||
The new version could look like this: | The new version could look like this: | ||
| − | * VYCP'(2) '' | + | * VYCP'(2) ''FTN(SOURCE)'' does not conform to ''CT'' |
| − | * VYCQ'(2) '' | + | * VYCQ'(2) ''FTN(CT)'' does not conform to ''TARGET'' |
To complete and take the | To complete and take the | ||
Revision as of 20:45, 19 January 2007
Warning: Warning: Article under development
Contents
Introduction
This article discusses issues recently discovered for the following two validity rules:
- 8.15.7 Validity: Conversion Procedure rule, Validity code: VYCP
- 8.15.8 Validity: Conversion Query rule, Validity code: VYCQ
There are cases where both of them violate the conversion principle:
- 8.15.3 Validity: Conversion principle: No type may both conform and convert to another.
Examples
As the conversion rules are strongly dual, each example can be transformed to show the issue for its sibling.
Example 1
We have a conversion to the current type of the class. It should not be allowed. Currently no rule rejects this code.
- eweasel test: convert-to-current-type
class A[G] convert to_a: {A[G]} feature to_a: A[G] do end end
The conversion to A[G] should indeed not be valid because they are conform.
The only rule that matters in our case is VYCQ(3).
What VYCQ(3) asks is the following
Is A[ANY] conform to A[G]?
As we answer this question with no, the conversion is valid because all the other rules do not object either.
Example 2
This example shows a special case which is valid under the current rule but can possibly lead to a conflict between conformance and conversion.
- eweasel test: convert-to-possible-actual-type
class A[G] convert to_a: {A[STRING]} feature to_a: A[STRING] do end end
In the case where G's actual type parameter is a subtype of STRING it yields in a situation where the two types are conform again.
The interesting rule is again VYCQ(3):
Is A[ANY] conform to A[STRING]?
This question is answered with no too. Therefore the conversion is valid.
Example 3
- eweasel test: convert-to-base-class
class A[G,H] convert to_a: {A[G,G]} feature to_a: A[G,G] do end end
VYCQ(3):
Is A[ANY,ANY] conform to A[G,G]?
The answer is no and thus not violating the rule, but it should.
Example 4
This is an example which is valid under the current rules and should remain valid. Even though we inherit from A[ANY] the conversion to A[STRING] should be valid.
- eweasel test: convert-to-base-class-inherited
class A[G] end class B[G] inherit A[ANY] convert to_b: {A[STRING]} feature to_b: A[STRING] do end end
VYCQ(3):
Is B[ANY] conform to A[STRING]?
This question is answered with no too. Therefore the conversion is valid, which is desired.
Example 5
This is the second example which is valid under the current rules. The code is valid as the Conversion principle cannot possibly be violated.
- eweasel test: convert-to-base-class-inherited
class A[G,H] end class B[G->INTEGER,H->DOUBLE] inherit A[G,H] convert to_a: {A[DOUBLE,INTEGER]} feature to_a: A[DOUBLE,INTEGER] do end end
VYCQ(3):
Is B[INTEGER,DOUBLE] conform to A[DOUBLE,INTEGER]?
The answer is no and therefore the code is regarded as valid.
Understanding the matter
If we take the inheritance hierarchy of an Eiffel system it can be abstracted to a directed acyclic graph.
The following illustration shows where conversion is valid and where not.
Possible solution
Instead of restricting VYCQ(2) and VYCP(2) to non-generic types we allow generic types too. As VYC*(2) is even using the notion of current type, it might indeed be possible that it was the authors original intention.
We define an additional function FTN which replaces every formal generic with NONE. (If used here one assumes that NONE conforms to expanded types as well.)
We define CTC as the type obtained from CT by replacing every formal generic parameter by its constraint.
The new version could look like this:
- VYCP'(2) FTN(SOURCE) does not conform to CT
- VYCQ'(2) FTN(CT) does not conform to TARGET
To complete and take the
New rule applied to examples
Example 1 for VYCQ'(2):
Is A[NONE] conform to A[G]?
The answer is yes and the validity rule is violated, which is good.
Example 2 for VYCQ'(2):
Is A[NONE] conform to A[STRING]?
The answer is yes and we reject the code.
Example 3 for VYCQ'(2):
Is A[NONE,NONE] conform to A[G,G]?
The answer is again yes and therefore the code not valid.
Example 4 for VYCQ'(2):
Is B[NONE] conform to A[STRING]?
As B only inherits from A[ANY] the answer is no and we're fine.
Example 5 for VYCQ'(2):
Is B[NONE,NONE] conform to A[DOUBLE,INTEGER]?
The answer is no and therefore the code valid.
