Affiliation:
1. Department of Computer Science, 3190 MEB, University of Utah, Salt Lake City, UT
Abstract
A record data type can be
extended
by addition of more fields. The extended type is a subtype of the original, in that any value of the extended type can be regarded as a value of the original type by ignoring the additional fields. This results in a type hierarchy.
Milner [3] has proposed a polymorphic type system. With the Milner approach, the type of a function may contain type variables. This also results in a type hierarchy.
In a language with a polymorphic type system, if it is anticipated that a record type will need to be extended, then the record type can be defined to have a dummy
extension field
. In the parent type, the extension field will have
null
contents of type
void
. The type of the extension field can differ with different subtypes.
The approach can be extended to allow a type to be subtype of two or more parent types.
To a limited extent, this approach can be used in Ada and other languages with generic program units.
Publisher
Association for Computing Machinery (ACM)
Reference7 articles.
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