Recursive Constraints with Generics

Written by Team Kodeco

Generics in Swift allow you to write code that is flexible and reusable by abstracting over types. Sometimes, you might need to define a generic type that has a recursive relationship with itself. In this case, you can use recursive constraints to define the relationship.

Here’s an example of a binary tree data structure that uses recursive constraints. A binary tree is a tree data structure in which each node has at most two children, which are referred to as the left child and the right child.

class BinaryTree<T> {
  var value: T
  var left: BinaryTree?
  var right: BinaryTree?
  
  init(_ value: T) {
    self.value = value
  }
}

In this example, the BinaryTree class is defined as a generic class with a type parameter T. The left and right properties are optional instances of BinaryTree, which means that each node in the tree can have at most two children.

However, this isn’t enough to constrain the type of children of the binary tree, since left and right are BinaryTree? which means that they can be of any types. You can add a constraint to the BinaryTree class to ensure that the type of the children is the same as the type of the parent.

class BinaryTree<T> {
  var value: T
  var left: BinaryTree<T>?
  var right: BinaryTree<T>?
  
  init(_ value: T) {
    self.value = value
  }
}

Now the class is constrained to only allow instances of BinaryTree with the same type as the parent.

In this way, you used recursive constraints to define a generic BinaryTree class that can have children of the same type as the parent. This allows you to create a binary tree with nodes of any type while maintaining the integrity of the data structure.