Topic: variant data types

Summary

Variant types are union types which share most of their components. For instance employee records may have male and female variants. Variant fields are identified by a tag component which can be case discriminated. The tag component is an enumerated type. In use, variants are usually fixed once a variable is assigned. In Pascal, a major weakness is uncontrolled assignment to variant fields instead of dynamic type checking or required variant discrimination. (cbb 5/80)

Subtopic: case type and variants

Quote: for Euclid case types, either a case for every value or an otherwise clause [»barnDT3_1978]

Quote: Euclid allows assignment to a variant only within a discriminating case [»veneT3_1978]

Subtopic: tag field

QuoteRef: wirtN1_1971 ;;37 record variants by tag field eg alloc ( p[tag])

Subtopic: unchecked tag field

Quote: Pascal does not check when assigning values to record variants

Subtopic: tag modification

Quote: Pascal errors caused by changing the tag of a variant record without initializes related fields

Quote: Ada requires a new record value if the discriminant or tag of a variant record is changed [»maclBJ_1987]

QuoteRef: gescCM8_1977 ;;552 "It seems that we hardly ever change the variant of a record once it has been initialized

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