What is the name of this programming language feature and are there any real-world languages that support it?

Question

Let a tree data structure be defined as such:

A tree has one Node as its root. A Node is either a Leaf or it is an Inner Node which has one or more Nodes as its children.

In some kind of pseudo OO programming language we may define a tree like this:

Node := InnerNode | Leaf

Leaf {

    isLeaf() : TRUE

}

InnerNode {

    isLeaf() : FALSE
    children() : List<Node>

}

Tree {
    root() : Node
}

Now we can define two functions, 'bad_code' and 'good_code'. The function 'bad_code' does not compile, the other function does:

function bad_code(Node anyNode) : void {

    // this will give a compile time error "type Node does not define method children()"
    anyNode.children();
}

function good_code(Node anyNode) : void {

    // the compiler understands that all Nodes must have a method called isLeaf() which 
    // returns a boolean
    let b : boolean <- anyNode.isLeaf();

    if (b == FALSE) {

        // this will not give a compile time error because the compiler can deduce that 
        // anyNode must be of type InnerNode which has the method children()
        anyNode.children();
    }
}

Question:

1. Is the above an example of a language feature that has been defined / described in some official way?
2. If so: what is this language feature officially called?
3. Are there any real-world programming languages which implement this language feature?
4. Can this language feature be implemented as a compile time check with zero costs at runtime?

Answer 1

What you're describing is that the compiler uses the control-flow graph to narrow the type of a variable, so that when an if statement tests a condition which relates to the type of a variable, a more specific type for the same variable can be inferred for the body of the if statement.

This is called control-flow type narrowing, and it's done in e.g. Typescript. It is purely a static check, done at compile-time with no runtime penalty; in fact, types in Typescript are not available at runtime at all.

type TreeNode = InnerNode | Leaf

interface Leaf {
    isLeaf: true
}

interface InnerNode {
    isLeaf: false
    children: Node[]
}

function bad_code(anyNode: TreeNode): void {
    // type error: Property 'children' does not exist on type 'TreeNode'.
    console.log(anyNode.children);
}

function good_code(anyNode: TreeNode): void {
    if (!anyNode.isLeaf) {
        // narrowed type to anyNode: InnerNode
        console.log(anyNode.children);
    }
}

Note that Typescript requires you to do this in a particular way; we test anyNode.isLeaf directly rather than storing it in a variable b: boolean first, because Typescript doesn't keep track of the relationship between the two variables b and anyNode:

function bad_in_typescript(anyNode: TreeNode): void {
    let b: boolean = anyNode.isLeaf;

    if (!b) {
        // type error: Property 'children' does not exist on type 'TreeNode'.
        console.log(anyNode.children);
    }
}

Also, in the above code isLeaf is a property instead of a method. Typescript does have a related feature called user-defined type guards which allow a method's return type to be something like this is Leaf, indicating that the method returns true only when called on something of type Leaf:

type TreeNode = InnerNode | Leaf

interface BaseNode {
    isLeaf(): this is Leaf
    isInner(): this is InnerNode
}

interface Leaf extends BaseNode {}

interface InnerNode extends BaseNode {
    children(): Node[]
}

However, Typescript is still a bit more limited than your example; we have to test anyNode.isInner() because !anyNode.isLeaf() won't necessarily do the same narrowing. (Typescript uses structural types, so in fact this Leaf is a supertype of InnerNode, which causes some problems for the union type. If you give Leaf a property like value: number which InnerNode doesn't have, then !anyNode.isLeaf() works how you would expect.)

• Typescript Playground Link for version with properties

• Typescript Playground Link for version with methods