Phantom type
A phantom type is a type parameter present in a type declaration but absent from the runtime representation of its values.
Functional Design and Architecture: Examples in Haskell
A phantom type is a type parameter that appears in a type declaration but not in the runtime representation of its values.
- How it is used
- Separates database computations and selects service implementations at compile time without changing their runtime representation.
- How deep it goes
- Explains marker types, type classes, and type applications, while stopping short of fully hiding runtime channel mismatches.
- Examples
- Uses `ValuesDB` and `AstroDB` markers plus `HttpAstroService` and `TcpAstroService` selectors.
- What you can do after
- Apply it via Haskell type applications and type classes to distinguish compatible-looking database or service computations.
Read how Functional Design and Architecture: Examples in Haskell explains phantom type →
Idiomatic Rust
A type-system technique that attaches a generic parameter to a type without storing a value of that parameter.
- How it is used
- Uses compile-time type tags when a generic distinction matters but its value need not be stored at runtime.
- How deep it goes
- Works through the unused-parameter error, PhantomData, construction, and specialized implementations.
- Examples
- Uses a Dog tagged with empty breed types, including a Poodle named Jeffrey.
- What you can do after
- Build it from scratch by adding PhantomData and defining type-specific Dog implementations.
Related concepts
- Proxyincludes · Proxy is a Haskell type whose value carries type information without containing a runtime value of that type.
- type selectorused by · A type selector is a type-level marker that selects among alternative implementations without carrying a runtime value.
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