EDT:Language Overview

Introduction

This essay gives an overview of the EGL language. 

Compilation

In relation to any modern programming language, an automated process converts source code from a relatively abstract form to a more concrete, platform-specific one. For example, the logic might be compiled into machine code, or into byte codes that can be read by a virtual machine. 

[ summarize typical compilation.  identify how the EGL tech is different and why. ]

Types and values

In general usage, a type such as integer or string defines a set of values and a set of operations that can be applied to those values. For example, integers are whole numbers that can be added, subtracted, and so forth; and the number 5 is a value of that type.

The meaning is much the same in programming, where every value is “of a type.” The type defines the structure of the value and the set of operations that can be applied to the value.

Reference and value types

In general, types are of two kinds, reference and value:

• A reference type defines an object, which is a value in a memory area that was allocated specifically to hold the value. The object is referenced from some logic and is an instance of the type. In this case, the words "value," "object," and "instance" are interchangeable.
• A value type defines a value that is embedded in an object rather than being referenced from it.

In general, a field declaration is a coded statement that has two effects:

• Declares a value of a type.
• Names a memory area that provides access to the value. If the value is based on a reference type, the memory area holds an address or other detail that points to the value. If the value is based on a value type, the memory area contains the value itself.

Turning now to the syntax of EGL, here are field declarations:

   // variable declaration
   NumberOfCars INT;    
   
   // constant declaration
   const MINIMUMNUMBER INT = 2; 

The type is each case is INT, which is a value type for four-byte integers. The first statement declares a value variable, the second declares a value constant.

For a second example, you might declare a list of five integers by coding a statement like one of these:

   // variable declaration
   NumberOfVehicles INT[5];

   // constant declaration
   const MINIMUMNUMBERS INT[5] = [1,2,3,4,5];

The type in this case is INT List or INT[], which is a reference type. The first statement declares a reference variable, which means that you can assign a different list to NumberOfVehicles in a later assignment. Incidentally, you can also change the values inside the list and can change the number of elements.

The second statement declares a reference constant, which means that you cannot assign a different list to MINIMUMNUMBERS in a later assignment. However, even in this case, you can alter the values inside the list and can change the number of elements.

The behavior is consistent because each declaration in the second example identifies a memory area that contains an address of a second memory area. The constant is only constant in the sense that the area identified as MINIMUMNUMBERS must always contain the same address, which refers to the same list. The following, subsequent assignment is not valid even though the values are the same:
// An invalid assignment
MINIMUMNUMBERS = [1,2,3,4,5];