Declarative

Enhanced Definition

In the context of mainframe computing, a **declarative** approach focuses on *specifying what* needs to be achieved rather than *how* to achieve it. It describes the desired outcome or state, leaving the underlying system (like the z/OS operating system, a database manager, or a compiler) to determine the optimal execution path. This paradigm emphasizes high-level expression and abstraction.

Key Characteristics

- Focus on Outcome: Describes the end result or desired state, allowing the system to manage the procedural steps and control flow.
- High Abstraction: Hides the complex, low-level implementation details from the programmer, simplifying code and logic.
- System Optimization: Enables the underlying system (e.g., DB2 optimizer, JCL interpreter) to make intelligent decisions about execution for efficiency and performance.
- Readability and Maintainability: Often results in code that is easier to read, understand, and maintain due to its focus on intent rather than intricate steps.
- Non-Procedural: Unlike imperative programming, it does not explicitly define the sequence of operations or the exact control flow.

Use Cases

- JCL (Job Control Language): JCL is inherently declarative, specifying what programs to run, what datasets to use, and how they should be allocated, without detailing the CPU instructions or I/O operations.
```
jcl
//STEP01 EXEC PGM=MYPROG
//SYSOUT DD SYSOUT=*
//SYSIN DD DSN=MY.INPUT.FILE,DISP=SHR
```
This declares the program, output, and input without specifying *how* to load the program or read the file.
- SQL (Structured Query Language) in DB2/IMS DB: When querying a DB2 database, SQL statements like SELECT declare *what data* is needed, leaving the DB2 optimizer to determine the most efficient access path (e.g., index usage, table scans, join order).
```
sql
SELECT EMP_ID, EMP_NAME FROM EMP_TABLE WHERE DEPT_ID = 'SALES';
```
This declares the desired employee data, not the specific steps to retrieve it.
- COBOL DECLARATIVES: COBOL's DECLARATIVES section allows programmers to specify special procedures to be executed when certain conditions occur (e.g., AT END for file processing, ON EXCEPTION for I/O errors), without explicitly coding the check at every I/O operation.
`cobol DECLARATIVES.