COBOL Nested Programs

Code Example

## COBOL Nested Programs

Nested programs are complete programs contained within another program. They provide encapsulation and modularity without separate compilation units.

### Structure

\`\`\`cobol
IDENTIFICATION DIVISION.
PROGRAM-ID. OUTER-PROGRAM.
...
IDENTIFICATION DIVISION.
PROGRAM-ID. INNER-PROGRAM.
...
END PROGRAM INNER-PROGRAM.
END PROGRAM OUTER-PROGRAM.
\`\`\`

### Key Concepts

1. **COMMON Clause**: Makes nested program callable by siblings
2. **GLOBAL**: Makes data items accessible to nested programs
3. **Scope**: Inner programs can access outer GLOBAL items
4. **Separate Compilation**: Alternative to nesting

### Visibility Rules

- Nested program sees its own items
- Nested program sees GLOBAL items from outer
- Outer cannot see nested program's items
- Siblings can call COMMON programs

### Benefits

- Encapsulation of helper routines
- Single compilation unit
- Shared global data without linkage
- Better organization of related functions