APL (A Programming Language) is an interpretive, array-oriented general-purpose computer language that has been implemented on a wide range of hardware from PCs to 68000s to 3090s. The language is highly transportable across implementations. (Some of my utilities are 18 years old and have been run, without modification, on 360s, DEC 2060s, 68000s, PCs and 3090s.) APL is mature enough to have acquired an ISO standard. The interpreters currently available for the PC (8086 family) include two superb implementations from IBM and STSC, as well as public domain, shareware, and demo versions (albeit stripped down) of the commercial products. Additionally, there are two interpreters available for 386 machines from STSC and Dyalog. IBM includes a 386-only interpreter with its APL2/PC.

Some of the features of APL that will appeal to C programmers are bounds checking, dynamic arrays, and the lack of an operator hierarchy. APL statements are executed from right to left. Parenthesis are used to change the order of execution if required. The equal function in APL is a question rather than a statement of fact — there is a separate assignment operator that is not easily confused with the equal function. Variables do not have to be declared but they do have to be initialized before being referenced. Functions do not have to be explicitly prototyped. A function to calculate the average of its right argument can be written to work equally well with a vector of two integers or a 10x10 matrix of floating-point numbers.

While the symbols of APL may look confusing at first, I can guarantee that most programmers are familiar with the symbols of a significant number of APL functions. The plus, minus, divide, and times functions use the symbols most of us have used since grammar school (and before programming). The symbols (and their meanings) for the relational functions would be immediately recognizable. An additional benefit of the use of symbols in APL is that their use makes APL more of an international language. Unfortunately, most of these familiar symbols cannot be printed on a standard ASCII terminal.

APL*PLUS provides three facilities useful to the C programmer developing external functions. One is a software interrupt that I discuss below. The other two are APL QUAD functions (the QUAD symbol indicates an APL reserved name):

• QUADSTPTR returns a pointer to the segment address of an APL object. It is a dyadic function, which means that it takes both right and left arguments. Other possibilities for APL functions are niladic (no arguments) or monadic (right argument only). Some dyadic functions can be ambivalent, which means that the left argument is optional, and will take default values if not specified.
• QUADCALL calls the external function in the right argument (which has previously been read into the workspace as an APL variable). It first sets the registers (AX, BX, CX, DX, BP, SI and DI) to the values specified in the (optional) left argument. Registers not explicitly defined in the left argument are set to 0. The DS register is set to the start of the APL-interpreter data segment and the CS and ES registers are set to point to the APL object being executed. IP is set to point to the first data byte within the object. Execution will continue until either a far return is encountered (in which case the APL session resumes, returning the register values at exit from the routine), an INT 86h is executed (APL terminates), or an INT 0 is executed (APL signals a Domain Error).

(256,QUADSTPTR' FO0') QUADCALL
   phoobah

ES:[SI] +0 byte- data type
      (1 = char,2 = int,8 = float)
   + 1 byte - rank of object
   +2 word - first dimension
   +4 word - second dimension
   .. .. nth ..

1. Name the interface module MAIN. This will satisfy the requirements of the C compiler for something named main.

While the interface can be coded in C, I have found that it is best coded in assembly language for a number of reasons. One is that important details can be hidden in a C implementation. The major reason, however, is that C code can be difficult to follow on the machine-code level.

Source-code information is not available when running under APL. External routines may be debugged by:

• inserting INT 3 statements in the code (another plus for using assembly
• bringing up APL under your favorite debugger and pressing the GO key
• running the external routine normally in APL.

• Link PC databases (such as BTRIEVE or Paradox) with APL routines to handle complicated scientific, actuarial, or financial calculations. While it may be possible to beat the execution speed of the APL primitives in C, the longer development time may be more of a problem than execution time.
• Provide compiled routines in those unavoidable situations where APL would normally loop. The screen-read is a perfect example of this. A QUAD function is provided that will read one screen block, but reading multiple blocks requires a loop. The sample program runs significantly faster than the equivalent APL function.
• Provide support for operations that are difficult with an array-oriented language — for example string searches without regard to case or white-space characters.

TryAPL2 (free), APL2PC ($500): APL Products IBM Santa Teresa Dept. M46/B25 P.O. Box 49023 San Jose, CA 95161-9023.

Dyalog APL ($1495): Dyadic Systems, Ltd. Riverside View, Basing Road Old Basing, Basingstoke Hants, RG23 OAL, England, UK. U.S. distributor: MIPS, 33493 W. 14 Mile Road, Farmington Hills, MI 48331.

I-APL (Public Domain), Documentation ($22.50) Edward Cherlin, I-APL Ltd, 6611 Linville Drive, Weed, CA 96094.

James A. Brown, Sandra Pakin and Raymond P. Polivka, APL2 at a Glance Brown, Prentice Hall, 1988.

Jerry R. Turner, APL is Easy, John Wiley & Sons, 1987 (included with STSC's APL*Plus).