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The objective of the APL IL Interpreter Generator is to solve the problem of creating APL interpreters for different machines at a minimum cost. The objective has been accomplished by writing an APL interpreter in a specially designed programming language (IL) that has very low semantics but high-level syntax. The interpreter is translated to each target machine language by easily built compilers that produce high-performance code. The paper describes IL, the APL interpreters written in IL, and the final systems generated for seven different target machines and operating systems. Some of these systems have been generated in an extremely short time.
Among the many languages used to write programs, APL and its successor, APL2, are very powerful. They support highly structured data of several different internal types and recognize a large number of primitive functions and operators, some of which (for example, execute,)(Example omitted) are extremely complicated for some arguments. The existence of these primitives makes it very difficult for APL to be compiled (except for subsets of the language or through the inclusion of an interpreter in the machine code). Thus full APL and APL2 systems have to be interpretive. These interpreters are very large programs, consisting of tens of thousands of instructions.
Since interpreted programs normally run at least an order of magnitude slower than their compiled equivalents, programs written in APL or APL2 start with a speed handicap as compared to programs written in, say, C. However, the designers of APL and APL2 and the implementers of the interpreters have tried to reduce this effect in two different ways:
* By extending the language with ever more powerful primitives. In a single stroke, these perform complex operations that, in other languages, would require complicated algorithms. In this way, the time for interpretation is minimized with respect to the time for execution. The fact that most APL primitives apply to entire arrays also helps in this direction.
* By programming the interpreters in very low-level languages that make the best possible use of the resources of the machine or the operating system.
As a result, APL and APL2 interpreters were usually written in assembly languages, with the consequent loss of portability. It has been estimated several times that,...