Substitution Test

A substitution test is useful to determine what the generic components of a program are:

Can the program be modified by a substituting a different component for an existing one, without changing the other components?

In this example, we could produce different but related programs by substitution of components:

• If the input were an array rather than a linked list, a different iterator would be used.

• A different database structure could be used, e.g. an AVL tree rather than an Alist.

• The product of the integers could be accumulated rather than their sum.

• By changing the glue code, we could change the key and accumulated value, e.g. to accumulate symbols associated with each numeric value.

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