typechecks

Type checks are mechanisms in programming languages and tools to ensure operations are used with compatible types, catching errors before or during runtime. They can be static or dynamic, and are part of a broader type system.

Static type checking occurs at compile time, catching many errors early, enabling optimizations and better tooling. However, some programs may be rejected that would not fail at runtime, depending on the language's type system. Languages that employ static checking include Java, Rust, Haskell, and TypeScript when used in its statically checked mode.

Dynamic type checking runs at runtime; variables can hold values of any type, and type errors occur when mismatches arise during execution. This provides flexibility but incurs runtime cost and potential crashes if checks fail. Dynamic typing is common in languages like Python, JavaScript, and Ruby.

Type inference reduces the need for explicit annotations, as in ML, Haskell, and Rust; gradual typing allows mixing static and dynamic typing, as in TypeScript, Dart, and Python with type hints. Type systems can be sound, meaning well-typed programs do not produce type errors, or partially unsound, depending on design choices and language features.

Practical considerations include performance overhead for dynamic checks and the clarity of error messages. Static type information can improve tooling, such as IDE features, refactoring safety, and compiler optimizations. The choice of type checking approach reflects trade-offs between expressiveness, safety, and developer productivity.

In summary, type checking is a core component of software reliability and language design, balancing safety, performance, and usability.