m05-type-driven
CRITICAL: Use for type-driven design. Triggers: type state, PhantomData, newtype, marker trait, builder pattern, make invalid states unrepresentable, compile-time validation, sealed trait, ZST, 类型状态, 新类型模式, 类型驱动设计
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Install
Works with every agent the skills CLI supports.
npx skills add https://github.com/actionbook/rust-skills --skill m05-type-driven- 01
Install
Run the command in your project. The CLI asks which agents to install for.
- 02
Describe the task
Ask your agent for something this skill covers. It loads m05-type-driven automatically when relevant.
- 03
Review the result
The agent follows the SKILL.md steps, scripts and references bundled with the skill.
SKILL.md
The instructions your agent reads when the skill is triggered.
Type-Driven Design
Layer 1: Language Mechanics
Core Question
How can the type system prevent invalid states?
Before reaching for runtime checks:
- Can the compiler catch this error?
- Can invalid states be unrepresentable?
- Can the type encode the invariant?
Error → Design Question
| Pattern | Don't Just Say | Ask Instead |
|---|---|---|
| Primitive obsession | "It's just a string" | What does this value represent? |
| Boolean flags | "Add an is_valid flag" | Can states be types? |
| Optional everywhere | "Check for None" | Is absence really possible? |
| Validation at runtime | "Return Err if invalid" | Can we validate at construction? |
Thinking Prompt
Before adding runtime validation:
-
Can the type encode the constraint?
- Numeric range → bounded types or newtypes
- Valid states → type state pattern
- Semantic meaning → newtype
-
When is validation possible?
- At construction → validated newtype
- At state transition → type state
- Only at runtime → Result with clear error
-
Who needs to know the invariant?
- Compiler → type-level encoding
- API users → clear type signatures
- Runtime only → documentation
Trace Up ↑
When type design is unclear:
"Need to validate email format"
↑ Ask: Is this a domain value object?
↑ Check: m09-domain (Email as Value Object)
↑ Check: domain-* (validation requirements)
| Situation | Trace To | Question |
|---|---|---|
| What types to create | m09-domain | What's the domain model? |
| State machine design | m09-domain | What are valid transitions? |
| Marker trait usage | m04-zero-cost | Static or dynamic dispatch? |
Trace Down ↓
From design to implementation:
"Need type-safe wrapper for primitives"
↓ Newtype: struct UserId(u64);
"Need compile-time state validation"
↓ Type State: Connection<Connected>
"Need to track phantom type parameters"
↓ PhantomData: PhantomData<T>
"Need capability markers"
↓ Marker Trait: trait Validated {}
"Need gradual construction"
↓ Builder: Builder::new().field(x).build()
Quick Reference
| Pattern | Purpose | Example |
|---|---|---|
| Newtype | Type safety | struct UserId(u64); |
| Type State | State machine | Connection<Connected> |
| PhantomData | Variance/lifetime | PhantomData<&'a T> |
| Marker Trait | Capability flag | trait Validated {} |
| Builder | Gradual construction | Builder::new().name("x").build() |
| Sealed Trait | Prevent external impl | mod private { pub trait Sealed {} } |
Pattern Examples
Newtype
struct Email(String); // Not just any string
impl Email {
pub fn new(s: &str) -> Result<Self, ValidationError> {
// Validate once, trust forever
validate_email(s)?;
Ok(Self(s.to_string()))
}
}
Type State
struct Connection<State>(TcpStream, PhantomData<State>);
struct Disconnected;
struct Connected;
struct Authenticated;
impl Connection<Disconnected> {
fn connect(self) -> Connection<Connected> { ... }
}
impl Connection<Connected> {
fn authenticate(self) -> Connection<Authenticated> { ... }
}
Decision Guide
| Need | Pattern |
|---|---|
| Type safety for primitives | Newtype |
| Compile-time state validation | Type State |
| Lifetime/variance markers | PhantomData |
| Capability flags | Marker Trait |
| Gradual construction | Builder |
| Closed set of impls | Sealed Trait |
| Zero-sized type marker | ZST struct |
Anti-Patterns
| Anti-Pattern | Why Bad | Better |
|---|---|---|
| Boolean flags for states | Runtime errors | Type state |
| String for semantic types | No type safety | Newtype |
| Option for uninitialized | Unclear invariant | Builder |
| Public fields with invariants | Invariant violation | Private + validated new() |
Related Skills
| When | See |
|---|---|
| Domain modeling | m09-domain |
| Trait design | m04-zero-cost |
| Error handling in constructors | m06-error-handling |
| Anti-patterns | m15-anti-pattern |
FAQ
What does the m05-type-driven skill do?
CRITICAL: Use for type-driven design. Triggers: type state, PhantomData, newtype, marker trait, builder pattern, make invalid states unrepresentable, compile-time validation, sealed trait, ZST, 类型状态, 新类型模式, 类型驱动设计
How do I install m05-type-driven?
Run npx skills add https://github.com/actionbook/rust-skills --skill m05-type-driven in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/m05-type-driven/ (personal) or .claude/skills/m05-type-driven/ (project).
Which AI agents can use m05-type-driven?
Any agent that supports the open Agent Skills format (a folder with a SKILL.md file), including Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, Windsurf, Cline, OpenCode and more.
Is m05-type-driven safe to install?
No partner security audit is available yet. Skills can run commands and scripts, so read the SKILL.md and any bundled files before installing.
Who maintains m05-type-driven?
It is published by actionbook in actionbook/rust-skills. It has 3,401 recorded installs.
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