discriminated union

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discriminated union

The discriminated union of two sets A and B is

A + B = inA, a) | a in U inB, b)| b in

where inA and inB are arbitrary tags which specify which summand an element originates from.

A type (especially an algebraic data type) might be described as a discriminated union if it is a sum type whose objects consist of a tag to say which part of the union they belong to and a value of the corresponding type.
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Another user @raboof noted, "Pleasantly surprised to see Apple's #swift embracing discriminated unions http://goo.
q=%23swift&src=hash) #swift embracing discriminated unions (http://t.