Difference between revisions of "Conversion rules"

Revision as of 10:46, 19 January 2007

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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.

Possible solution

Instead of restricting VYCQ(2) and VYCP(2) to non-generic types we allow them 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 SUPER which replaces every formal generic with NONE. (If used here one assumes that NONE conforms to expanded types as well.)

The new version could look like this:

• VYCP'(2) SUPER(SOURCE) does not conform to CT.
• VYCQ'(2) SUPER(CT) does not conform to TARGET.

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.