Dynamic instantiation

TLDR

Difference between other languages and ReGIna is dynamic instantiation. In short, properties in classes will be assigned only after their dependencies are assigned, allowing recursive properties of same class A inside class A.

Idea

Classes can have references to other classes inside them as properties. These property-classes are evaluated dynamically. Take a look at this example:

class Node export rect {
    // export = false - can deny export
    iter = if (parent) parent.iter + 1 else 0

    childNode = if (iter == 5) Nothing() else Node()

    position = if (childNode is Node) \
    Position(childNode.position.x + 1, childNode.position.y) \
    else Position(0, 0)
}

fun main() {
    Node()
}

This will create following svg:

<svg>
    <rect x="4" y="0"/>
    <rect x="3" y="0"/>
    <rect x="2" y="0"/>
    <rect x="1" y="0"/>
    <rect x="0" y="0"/>
</svg>

First algorithm creates empty Node() (we’ll call it Node0) from main(). Then, starting from top to bottom:

1. algorithm assigns iter of Node0 to 0 because parent returns 0 as an equivalent of null.
2. After that, childNode is assigned a new Node (Node1).
3. position cannot be assigned because childNode.position is not yet assigned. Algorithm goes to childNode.position, that is Node1.position and tries to assign it. However, childNode is required, so we go to Node1.childNode, which needs iter.

It’s not important to consider while assigning values, but it shows why there cannot be any cyclic dependencies for properties.

How not to write code with dynamic instantiation

Bad class function

class FunctionOveruse {
    prop = make()

    fun make() {
        FunctionOveruse()
        return prop
    }
}

Both lines in make() will execute forever.

To make functions as expressive as possible, it is important to allow class instantiating inside them. If

We either make internal class functions (which is purely decompositional thing) or make instantiating inside functions possible (and fun main() as an entry point)

Main | Writings | Drawings