CHAPTER 1: Introduction

1.1 What is Empress 4GL?

Empress 4GL is an application development environment containing a fundamental programming language and a set of tools to help you develop your application quickly.

The Empress 4GL language is an event-driven language which uses the following basic structures as building blocks to construct complex applications which may be used as prototypes or finished products.

Forms
Windows
Fields
Keys
Scripts

The Empress 4GL development tools consist of:

Default Application Generator
Application Generator
Application Manager
Form Manager
Source Debugger
Executable Builder
Compiler
Linker

1.2 Capabilities of Empress 4GL

Using Empress 4GL you can design and run applications that may access Empress databases. Your application may be simple, allowing you to retrieve information from a single database table, or it may be sophisticated, accessing several databases and processing the data in complex ways. In addition, an application can act as a menu interface to the operating system and to Empress database.

Empress 4GL offers several advantages as a design environment. It is easy to use, it improves productivity, and your applications will be easy to maintain and enhance. It is particularly useful for the development of screen-intensive applications. The facilities for screen control that you will be using are centered on two powerful concepts: screen painting and windowing.

Screen painting is the ability to design a screen format by simply painting it directly on the screen. When the application is run, the screen format will appear just as it was painted. This means that you need not do any programming to position things where you want them.

Windowing is a technique for dividing the screen into independent regions so that you can see and do different things in the different regions. Each region is called a window and shows a format you have painted. Windows can be programmed to appear when required, become larger or smaller at the touch of a key, or even to move around.

If you wish to use Empress 4GL to do simple Query Language operations on a database table, you can generate a default application for the table of your choice. Empress 4GL will create an application that will allow you to select, update, delete and insert records in the table.

If you wish to design a more sophisticated application, Empress 4GL provides a simple language with which you can write scripts to do the processing you require. With scripts, you can access tables in databases, do computations and comparisons on the data, create and control windows, and effectively do anything you could do with another programming language.

1.3 Features of Empress 4GL

The programming language of Empress 4GL is used to write scripts that allow the application to respond to user or system events. Some example are: pressing a function key, moving between fields in a window, and responding to errors and operating system signals.

Empress 4GL provides:

Commands to select, insert, update and delete records from database tables. Additionally, there are commands to define and set variables, evaluate expressions, perform iterative processing, conditional testing and comparison operations.

Convenient and efficient screen handling functions that allow you to define windows which you can display and remove from the screen with single commands.

A convenient means of attaching scripts to function keys, so that users of your application can control windows and processing with a minimum number of keystrokes.

A series of functions designed to facilitate the programming of multiple record displays showing "pages" of records - line movements and paging can be controlled with single commands.

When you start writing applications, you will find it easy to make your applications user-friendly. Here are some of the features of Empress 4GL:

Any command or set of commands may be bound to any function key.

Windows can be shown or removed with a single command. Any number of windows can be displayed at one time and these may overlap on the screen.

Windows can be zoomed or moved on the screen by using simple script commands. These actions can be controlled by key scripts.

Field characteristics can be customized to enable easy data entry. Users do not need to have knowledge of the underlying structure of the database or its tables.

Access to all operating system commands with ability to escape to a sub-shell or SQL.

Reports can be generated by using the Empress Report Writer.

Records can be selected from one or more tables in sorted or unsorted order using the SQL compatible select statement. The selection can be qualified with one or more conditions.

Database access is provided through insert, update, delete and select commands. As well as, support for moving forwards and backwards through a selected context of records.

Using the completely programmable 4GL interface, any combination of functionality can be provided or restricted.

One or more records can be displayed on the screen at once.

Menu applications can be created which can invoke other menus or applications.

1.3.1 Features that do not Require Programming

You can change the screen format of the application. Screen formats are painted on "forms" and these can be edited without having to change the application logic. Text, video regions, boxes and lines can be entered anywhere on a form. The fields through which data interchange occurs can be moved or resized on the form without having to modify the application logic.

Window positions can be altered simply by changing its coordinates in the window definition table.

1.4 Invoking Empress 4GL

In order to invoke Empress 4GL, a database must exist and it must be initialized for Empress 4GL. To initialize a new or existing database, use the following command at the operating system level:

emp4init database

where:

database is any valid database name.

If the database does not exist, emp4init will create the database first then proceed with the initialization.

For example, to initialize the database repairs for Empress 4GL, use the following command:

   emp4init repairs

You will see the following response on your terminal screen as the result of the initialization:

Creating Database 'repairs'

Importing 4GL Tables

   IMPORT V6.0

   Copyright Empress Software Inc. 1983, 1992

sys_4gl_application

sys_4gl_compile

sys_4gl_domain

sys_4gl_field

sys_4gl_form

sys_4gl_key

sys_4gl_link

sys_4gl_script

sys_4gl_window

sys_ag_attribute

sys_ag_definition

sys_ag_function

sys_ag_menu

sys_ag_menu_item

sys_ag_parameter

sys_ag_qual_list

sys_ag_qualification

sys_ag_screen

sys_ag_screen_attr

sys_ag_table

sys_attr_privs

sys_attrssys_dictionary

sys_privs

sys_tables

These are the Empress 4GL system tables where all the information describing an application is stored. If the circumstances require it, you can use Empress SQL commands to acquire information from these tables.

Note: The database only needs to be initialized once. If you apply emp4init on a database which has already been initialized, the following error message will occur:

Importing 4GL Tables

   IMPORT V6.1

   (c) Copyright Empress Software Inc. 1983, 1992

sys_4gl_application

*** User Error *** table 'sys_4gl_application' already exists

sys_4gl_compile

*** User Error *** table 'sys_4gl_compile' already exists

sys_4gl_domain

*** User Error *** table 'sys_4gl_domain' already exists

sys_4gl_field

*** User Error *** table 'sys_4gl_field' already exists

sys_4gl_form

*** User Error *** table 'sys_4gl_form' already exists

sys_4gl_key

*** User Error *** table 'sys_4gl_key' already exists

sys_4gl_link

*** User Error *** table 'sys_4gl_link' already exists

sys_4gl_script

*** User Error *** table 'sys_4gl_script' already exists

sys_4gl_window

*** User Error *** table 'sys_4gl_window' already exists

sys_ag_attribute

*** User Error *** table 'sys_ag_attribute' already exists

sys_ag_definition

*** User Error *** table 'sys_ag_definition' already exists

sys_ag_function

*** User Error *** table 'sys_ag_function' already exists

sys_ag_menu

*** User Error *** table 'sys_ag_menu' already exists

sys_ag_menu_item

*** User Error *** table 'sys_ag_menu_item' already exists

sys_ag_parameter

*** User Error *** table 'sys_ag_parameter' already exists

sys_ag_qual_list

*** User Error *** table 'sys_ag_qual_list' already exists

sys_ag_qualification

*** User Error *** table 'sys_ag_qualification' already exists

sys_ag_screen

*** User Error *** table 'sys_ag_screen' already exists

sys_ag_screen_attr

*** User Error *** table 'sys_ag_screen_attr' already exists

sys_ag_table

*** User Error *** table 'sys_ag_table' already exists

sys_attr_privs

sys_attrs

sys_dictionary

sys_privs

sys_tables

To invoke Empress 4GL, use the following command at the operating system level:

emp4gl database

where:

database is any valid database name previously initialized for use with the 4GL.

For example, to invoke Empress 4GL on the database repairs, type in the following:

   emp4gl repairs

After the copyright messages, the following screen will appear on the terminal:

Figure 1-1 Application Development Main Menu

This is the Empress 4GL Application Development Main Menu. Through this Main Menu, you access all the Empress 4GL tools to create your applications.

Note: The key labels (for example F5, F6, F4, etc.,) are terminal dependent. You may have different key labels displayed on your screen.