Generic Component: Views

A simple generic component such as sum involves several types, mappings, and a constant:

• T_acc, accumulator type. sum is defined for any type that defines a + operator, including integer, real, boolean, string, and any application type that defines +.

• Constant C₀:T_acc, initial value of the accumulator. C₀ defaults to the zero value of the type T_acc; it is possible to specify a sum with a non-zero initial value (e.g. a taxi meter).

• T_rec, accumulator record type. A component such as sum never ``owns'' its storage; instead, the storage is owned by some outside component and passed to sum as a parameter.

• A mapping M_acc: T_rec &rarr T_acc that specifies how to obtain the data for this instance of sum from data of type T_rec. This mapping must allow both reading and writing of the accumulator value. There could be multiple instances of sum data in a T_rec, e.g., it might be desired to sum both cost and shipping weight at the same time.

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