rust-types

Rust Types — Strengthen the Domain Model

Push runtime checks into the type system. Every runtime assert!, if !valid, or "this should never happen" comment is a type waiting to be born.

Preparation

1. Read the target code and identify the domain concepts.
2. Look for primitive types carrying domain meaning (String for emails, u64 for IDs, bool for states).

Strengthening Steps

Step 1: Find Primitive Obsession

Run in the shell:

rg --type rust 'fn \w+\(.*: (String|&str|u32|u64|i32|i64|bool|f64)' src/ -n | head -30

For each function taking raw primitives that represent domain concepts:

• String for an email → struct Email(String) with validation in constructor
• u64 for an ID → struct UserId(u64), struct OrderId(u64) (distinct types!)
• f64 for money → struct Currency { amount: i64, code: CurrencyCode } (never float for money)

Step 2: Replace Booleans with Enums

Run in the shell:

rg --type rust 'fn \w+\(.*: bool' src/ -n

For each boolean parameter:

// Before: process(data, true, false)  — what do these mean?
// After:
enum Direction { Forward, Backward }
enum OutputMode { Raw, Formatted }
fn process(data: &Data, dir: Direction, mode: OutputMode)

Step 3: Make Illegal States Unrepresentable

Look for:

• Structs with fields that can be in inconsistent states → use an enum
• Option<A> + Option<B> where exactly one must be Some → use an enum
• State transitions modeled with mutable fields → use the typestate pattern

// Before: can be in invalid state (title set but no body)
struct Post { title: Option<String>, body: Option<String>, published: bool }

// After: invalid states are compile errors
enum Post {
    Draft { title: String },
    Review { title: String, body: String },
    Published { title: String, body: String, url: Url },
}

Step 4: Validated Constructors

For every newtype, ensure construction goes through validation:

pub struct Email(String);

impl Email {
    pub fn new(s: impl Into<String>) -> Result<Self, ValidationError> {
        let s = s.into();
        if !s.contains('@') { return Err(ValidationError::InvalidEmail); }
        Ok(Self(s))
    }
}