discriminated union

theory The discriminated union of two sets A and B is

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

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.

(1995-04-25)