General Asides

More General CS Asides

These are points about broader concepts in programming that come up in the tutorial but aren't quite isolated to Rust.

Aside: Sum Types

This, to the uninitiated, must be strange: the enum seems to be able to reference other fields, like the index or the description. This is not a struct. In type theory, this is a sum type.

Sum types are "labelled choices." The choices may be different per label too, and different labels can have the same "choices." In particular: for our Instruction, there are 4 labels: Add, Remove, Modify, and Print. Each label has different choices: such as the usize index in Remove.

These are called enums since Java, C, and C++ enumerations are the labels without the choices necessarily enforced. Another similar construct is a union from C and C++, but those are the choices without the labels enforced. Rust enums are both. Have your labels and choices too!

If you'd like to read more about sum types, know that there is not much of an end to what you'd be reading. As a teaser, here's some magic:

pub enum BinaryTree<T> {
    Node(Box<BinaryTree<T>>, T, Box<BinaryTree<T>>),
    Empty
}

If you read Box as "pointer to", you may recognise a binary tree structure. Such recursive trees are broadly applicable as parts of finite state machines or programming language parse trees. But there's more! These structures arise in a confluence of catagory theory and type theory while appearing in useful code.

This appears under various names in various languages. In particular:

Alias	Where it Appears	Link
Disjoint Union	Set theory	Wikipedia
Tagged Union	It's a synonym for sum type	Wikipedia which includes various implementations of it too
Coproduct	Category theory	Wikipedia

Aside: The Builder Pattern

The above structure where . are chained to form a representation of some data and then a final .get function converts or uses the representation, providing a more useful form to the user of the library. This is a useful pattern for libraries to expose complicated and flexible functions while staying legible its to users.

Here is a Java-based tutorial and here is a boarder tutorial.