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  1. OMG Issue

OCL23 — OCL 2.1 Implicit Conversion to Collection Literal

  • Key: OCL23-25
  • Legacy Issue Number: 14981
  • Status: closed  
  • Source: Model Driven Solutions ( Dr. Edward Willink)
  • Summary:

    The specification is very vague as to how and when a non-collection is converted to a collection.

    11.2.3 states that 'If the source is the null literal, it is implicitly converted to Bag{}'.

    7.5.3 states that 'Such a single object can be used as a Set as well. It then behaves as if it is a Set containing the single object. The usage as a set is done through the arrow followed by a property of Set.'

    The collection type inconsistency between Bag{} and Set


    makes compile-time type resolution difficult, since the null-ness of x cannot be known till run-time. It would be better to have either Set or Bag uniformly as the conversion collection type. Bag{} as the least useful seems more appropriate.

    When a conversion is applicable is unclear.

    In the example, self.manager->isEmpty() is intended to be interpreted as

    if self.manager.isUndefined() then Set{} else Set



    so that null->isEmpty is also valid.

    However is


    equivalent to


    and so true?

    A simple policy of 'wherever a collection is required and a non-collection is provided perform an implicit conversion to collection-literal' resolves this.

    However are

    null = Bag{}


    Bag{} = null

    both equivalent to

    Bag{} = Bag{}

    and so true?

    In this case we have a problem of asymmetric overloading.

    null = Bag{} satisfies the signature for OclVoid::=(OclAny) and so returns false.

    Bag{} = null could satisfy the signature for Bag::=(Bag) and so return true with the help of an implicit conversion.

    Presumably an additional OclVoid::=(Collection) overload is needed to restore symmetry.

  • Reported: OCL 2.2 — Sun, 17 Jan 2010 05:00 GMT
  • Disposition: Resolved — OCL 2.3
  • Disposition Summary:

    Resolved by Issue 15009, where the explicit synthesis of oclAsSet() avoids the troublesome conversions above.
    Disposition: See issue 15009 for disposition

  • Updated: Fri, 6 Mar 2015 20:58 GMT