Notes (TRPL 06/21): Enums and Pattern Matching

6 Enums and Pattern Matching

Cool, so now we’ve got sum types.

6.1 Defining an Enum

see enums/src/main.rs

This section is showing us the pattern that:

(a || b) && c == a && c || b && c

Or in context:

(V4 || V6) && String == V4 && String || V6 && String

Or with numbers:

(1 + 1) * 1 = 1 * 1 + 1 * 1

Still, it’s a very useful pattern!

The Option Enum and Its Advantages Over Null Values

This little piggy had roast beef, This little piggy had none,

-Mother Goose

You know what my favorite thing about the Option type in Rust is? It’s that Some and None are both four letters long. This is also the case in OCaml and it often makes the vertical alignment of code much prettier, than e.g. Haskell’s Just and Nothing type constructors for the Maybe type.

Here’s a nice post describing the monadic structure of the Option type: https://hoverbear.org/2014/08/12/option-monads-in-rust/

6.2 The match Control Flow Operator

see enums/src/main2.rs

Good old pattern matching. It’s the best.

Matches are Exhaustive

Compile-time totality checking is always helpful. I bet there’s a compiler flag somewhere to turn this off though.

The `_` Placeholder

Ah _, my old friend. Man, for a systems language Rust really does use a lot of recognizable functional idioms.

6.3 Concise Control Flow with `if let`

see enums/src/main3.rs

Interesting that 0u8 is 0 : u8, that’s a convenient shorthand syntax for literals.