NPL: highlights of a new programming language
Communications of the ACM
A programming language
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Description of trees and strings - both composed of lexical values - matching of trees and strings against partially or totally defined pattern and operations on trees and strings form the essential part of the Processor. The notion of string has been extended to strings embedded in or extracted from trees, and a modified version of the COMIT rewriting rule applied to these strings. Variables ranging over lexical values, strings and trees can be introduced and operated on according to a "pushdown" principle. Besides, variables can be operated either ir the "connected" or "autonomous" mode, depending whether their connections with some parent structure are remembered or not. Variable value structures can be matched against patterns or compared among themselves. Transformations on trees and strings are defined, allowing for the development of a given data structure into a new one. All these features and devices were defined by extracting the elementary steps with which linguists compose their operations and by generalizing these steps reasonably. The resulting language specifications are proposed for discussing the solution of a class of non-numerical programming problems.