Evaluation-Semantics.md

Evaluation semantics: deferred evaluation

Being a data-flow language, DScript has a slightly different evaluation semantics from Typscript/JavaScript. In DScript, global declarations are evaluated once and are evaluated in parallel with blocking semantics, while statements in functions are evaluated top-down as usual.

Consider the following example:

function func(caller: string, input: number) : number
{
    Debug.writeLine(`${caller}: ${input}`);
    return input + 1;
}

const x = func("x", 0);
const y = func("y", x);
const z = func("z", x);

This will output:

x: 0
y: 1
z: 1

All right-hand side (rhs) expressions of x, y, and z are evaluated in parallel. Although x is referred twice, its rhs expression is only evaluated once. The evaluations of y's and z's rhs block until the evaluation result of x is available, and thus the output x: 0 always appears first. Because there is no data-flow dependency between y and z, the outputs y: 1 and z: 1 can appear in different orders. The end values after the evaluation are x = 1, y = 2, and z = 2.