APLGOL, an Experimental Structured Programming Language

An experimental programming language called APLGOL adds structured programming facilities to the existing framework of APL. The conventional semantics of APL is unaltered and only minor changes are incorporated in the syntax. The advantages of the proposed interstatement structuring and control are outlined. Programs designed and written using " structured " programming techniques have been demonstrated to be more readily produced, more reliable, and more easily maintained than unstructured programs [ 1 1. These techniques essentially involve arranging the application into principal components, which in turn, are further organized to produce a set of highly structured procedures. For this purpose, key programming language constructs have been used in conjunction with many languages to highlight interstatement structuring and control. Since structured programming techniques have been successful when applied to programming in other languages , they should be equally advantageous for APL programming efforts. In APL the compact and concise operators extend to vector or array operands, and single expressions often can subsume the equivalent of several statements in a language such as ALGOL or PLh. However , even in spite of the famous APL " one-liners " , many APL programs do require quite a few statements and frequently utilize rather complicated control flow. If this were highlighted by structured programming language constructs, the resulting programs should prove easier to write and debug, and more importantly, easier for others to read and understand. Consequently, the APLGOL language [ 2 ] , based on a notation of Abrams [3], is an experiment to add struc-tured programming language constructs to the basic APL framework. In adding the structured statements, a deliberate effort has been made to augment APL in the areas where these constructs are absent, and to avoid establishing other constructs, e.g., a new rule for name scope, that would tend to compete with existing APL facilities. Close attention has been paid to the relation between the new APLGOL facilities and existing APL operations. In this sense, APL was considered the target machine language to which APLGOL source programs could be compiled readily without materially affecting the speed or size of the object program or the compiler, or otherwise distorting the APL system. APLGOL semantics Before any new features were incorporated in APLGOL, it was decided first that all the semantic functions of APL should appear in APLGOL without change, since removing or altering any of them would be a step in …