Articles A Commentary on APL Development Kenneth E. Iverson Because A Dictionary of APL [1] was designed for concise and convenient reference, it is important that it be supplemented by other treatments of the language, concerning matters such as style and literacy [2], discussion of the means used to eradicate anomalies that have crept into the language [3], illustrations of analytic uses of the language [4], and explorations of the uses of particular verbs, adverbs, and conjunctions [5]. The present paper Ireats the rationale for many of the less familiar facilities defined in the dictionary, rectifies the definition of rank, provides simpler definitions for the conjuctions denoted by cup and cap, and discusses the matter of treating sets and set-like operations. The order of treatment parallels that of the dictionary. Although some of the facilities in APL have been introduced in response to demands from programmers, most have been motivated by knowledge of their utility in some branch of mathematics. Moreover, they have invariably been generalized from the special cases on which they were based. For example, reduction, inner product, and outer product were not demanded by programmers and are still not demanded by programmers in other languages.
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