Despite a lot of recent interest in purely functional data structures, for example [Ada93, Oka95, BO96, Oka96, OB97, Erw97], few have been benchmarked. Of these, even fewer have their performance qualified by how they are used. But how a data structure is used can significantly affect performance. This paper makes three original contributions. (1) We present an algorithm for generating a benchmark according to a given use of data structure. (2) We compare use of an automated tool based on this algorithm, with the traditional technique of hand-picked benchmarks, by benchmarking six implementations of random-access list using both methods. (3) We use the results of this benchmarking to present a decision tree for the choice of random-access list implementation, according to how the list will be used.