Where PL/I is used and how a program is built

Why PL/I exists

In the early 1960s, business shops used COBOL and scientific shops used FORTRAN. IBM designed PL/I alongside System/360 to do both: decimal arithmetic for money, floating point for engineering, plus pointers, strings and structured error handling. It never displaced COBOL, but many large organisations adopted it and never left.

Where you will find it

• Banks and insurers, particularly in parts of Europe, where core account and policy systems were written in PL/I.
• Utilities, airlines and government agencies with long-lived batch systems.
• System tools and vendor products, because PL/I handles pointers and strings comfortably.
• Mixed shops where PL/I and COBOL programs call each other, sharing Language Environment at run time.

The current IBM compiler is Enterprise PL/I for z/OS. PL/I runs in batch, under CICS and IMS, and with embedded SQL for DB2, much as COBOL does.

The shape of a program

PL/I is free-form: statements end with a semicolon, not a period, and layout is up to you. Comments sit between /* and */. Keywords can be abbreviated, so DECLARE is often written DCL and PROCEDURE as PROC.

 HELLO: PROCEDURE OPTIONS(MAIN);
   DCL NAME   CHAR(20) INIT('WORLD');
   DCL COUNT  FIXED BIN(31) INIT(0);

   COUNT = COUNT + 1;
   PUT SKIP LIST('HELLO', NAME, COUNT);

   CALL SHOW_TOTAL(COUNT);

   SHOW_TOTAL: PROC(N);
     DCL N FIXED BIN(31);
     PUT SKIP LIST('TOTAL IS', N);
   END SHOW_TOTAL;
 END HELLO;

Blocks and scope

PL/I has two kinds of block. A PROCEDURE is entered only when called. A BEGIN block is entered when execution reaches it, and is used to create local variables or to group statements in an ON-unit. Variables declared in a block are visible in that block and in blocks nested inside it, unless redeclared.

Declare everything

PL/I will invent a declaration for an undeclared name, using defaults based on its first letter. A typing mistake therefore becomes a new variable instead of a compile error. The compiler warns about implicit declarations; treat those warnings as errors, and follow your site standard if it enforces explicit declarations through compiler options.