Variables

To allow Eisen to operate at the C/C++ level, it is necessary to make the distinction between two different concepts: allocated object and references to objects.

An object can be allocated using the let keyword. This sets aside a given amount of space for the object on the stack.

struct Object {
    ...
}

fn main() {
    let myObj = Object()
}

The reference myObj can be used to manipulate the memory it refers to. But it is also bound to this single instance of Object, as it refers to a single and particular memory allocation. There are many cases where it is useful or necessary to have some type of reference which we can reassign.

let o1: Object
let o2: Object

var bestCandidate: Object

if (...) {
    bestCandidate = o1
}
else {
    bestCandidate = defaultObj
}

// common functionality using the bestCandidate

In the example above, we want to be able to determine the bestCandidate and then use this Object. We don't need a new memory allocation, we just need a variable that can refer to existing memory allocations, and be assigned, or ever reassigned to the proper one.

Eisen give us this ability with the var keyword. Whereas the let keyword establishes a new memory allocation, the var keyword serves the purpose of a pointer. It provides access to some pre-existing memory allocation. It can be assigned or reassigned as needed without restriction.

Primitives

Note that primitive types do not share this same problem because we can in fact modify the memory of the primitive object in place. Primitive types are the simplest examples of Data Structs.