simple-db-knowledge.md

Knowledge

This section explains concepts necessary to solve the simpleDB exercise.

We also recommend using the official Rust documentation to figure out unfamiliar concepts. If you ever feel completely stuck, or if you haven’t understood something specific, please hail the trainers quickly.

Derives

#[derive(PartialEq, Eq)]

This enables comparison between 2 instances of the type, by comparing every field/variant. This enables the assert_eq! macro, which relies on equality being defined. Eq for total equality isn’t strictly necessary for this example, but it is good practice to derive it if it applies.

#[derive(Debug)]

This enables the automatic generation of a debug formatting function for the type. The assert_eq! macro requires this for testing.

Control flow and pattern matching, returning values

This exercise involves handling a number of cases. You are already familiar with if/else and a basic form of match. Here, we’ll introduce you to if let, and let else:

if let Some(message) = message.strip_prefix("PREFIX:") {
    // Executes if the above pattern is a match.
}
// The variable `message` is NOT available here.

let Some(message) = message.strip_prefix("PREFIX:") else {
    // Executes if the above pattern is NOT a match.
    // Must have an early return in this block.
}
// The variable `message` is still available here.

When to use what?

if let is like a pattern-matching match block with only one arm. So, if your match only has one arm of interest, consider an if let or let else instead (depending on whether the pattern match means success, or the pattern match means there's an error).

match can be used to handle more fine grained and complex pattern matching, especially when there are several, equally ranked possibilities. The match arms may have to include a catch all _ => arm, for every possible case that is not explicitly spelled out. The order of the match arms matter: The catch all branch needs to be last, otherwise, it catches all…

Returning Values from branches and match arms

All match arms always need to produce a value the same type (or they diverge with a return statement).