dependenttypelike

Dependenttypelike is a term used in discussions of programming language type systems to describe features that approximate the behavior of dependent types without requiring a language to implement full dependent type theory. In a true dependent type system, a type can be parameterized by a value, allowing types to express invariants that depend on runtime data. Dependenttypelike features attempt to capture some of that expressiveness through restricted or modular mechanisms that tie values and types more closely together.

Common patterns include length- or content-indexed types (for example, a vector type that carries its length in the type), dependent pair types, and limited type-level computation where properties of values are reflected at the type level.

Language examples: Haskell with GADTs and DataKinds enables many dependent-like patterns; Rust's const generics allow types to depend on compile-time constants; some surveying languages like Idris and Agda implement full dependent types but inspire dependenttypelike approaches in other ecosystems.

Advantages and challenges: dependenttypelike features can encode invariants and safety guarantees at compile time, improving API safety and correctness. They also increase type complexity and can lead to longer compilation, potential for challenging type errors, and a steeper learning curve. In more aggressive forms, they raise concerns about decidability of type checking in unrestricted use.

Relation to dependent types: dependenttypelike is not a formal standard term; it denotes a pragmatic spectrum between parametric polymorphism and full dependent type systems. It is used to discuss gradual or partial adoption of dependent-type ideas in language design and embeddings.