Type checking
Type checking is the process of determining whether values, expressions, or operations conform to the rules or constraints of a type system.
Kotlin in Action, 2E
A type check is a Kotlin operation that tests whether a value has a specified runtime type.
- How it is used
- Uses `is` to select subtype-specific behavior while safely accessing members in the successful branch.
- How deep it goes
- Explains smart casts and their stability constraint, then details runtime limits caused by generic type erasure.
- Examples
- Demonstrates recursive `Expr` evaluation, checking an `Any` value for `String`, and distinguishing lists from sets.
- What you can do after
- Apply `is` and smart casts for subtype handling while avoiding invalid assumptions about erased generic arguments.
The Quick Python Book, Fourth Edition
Type checking is the analysis of program values and operations to detect whether they conform to expected types.
- How it is used
- Uses checks to reject mismatched values in functions, methods, and type-restricted collections.
- How deep it goes
- Works through collection initialization and indexed assignment, then flags constructor paths that can bypass assignment checks.
- Examples
- Compares string and integer types and builds a list that rejects elements of the wrong type.
- What you can do after
- Build it from scratch in a typed list, or adapt a built-in collection while checking its insertion paths.
Read how The Quick Python Book, Fourth Edition explains type checking →
Essential TypeScript 5, Third Edition
Type checking is the analysis of program values and expressions to determine whether their types satisfy language rules.
- How it is used
- Uses static analysis to catch incompatible values while developing TypeScript, JavaScript integrations, and React applications.
- How deep it goes
- Explains annotations, inference, unions, narrowing, object shapes, and configuration, while stopping short of compiler implementation internals.
- Examples
- Uses a cart formatter receiving strings, an ensuing toFixed runtime error, and a React tsconfig with strict and noEmit.
- What you can do after
- Diagnose/debug with it by identifying unchecked JavaScript boundaries and separating TypeScript type errors from Babel transformation.
Read how Essential TypeScript 5, Third Edition explains type checking →
Julia as a Second Language
Type checking is the process of verifying that operations are applied to compatible types.
- How it is used
- Enforces compatible argument and field types at runtime while retaining Julia’s dynamic typing.
- How deep it goes
- Demonstrates checks through a parametric method and typed fields, but defers detailed treatment of performance costs.
- Examples
- Uses a linear search that rejects a string search value for an integer vector, plus typed and untyped two-dimensional vector structs.
- What you can do after
- Apply it via Julia parametric methods and structs to require matching argument or field types.
Read how Julia as a Second Language explains type checking →
Refactoring to Rust
Type checking is the process of verifying that values and operations conform to the rules of a type system.
- How it is used
- Uses typed Rust structs to validate JSON data during deserialization.
- How deep it goes
- Explains inference, trait requirements, and runtime mismatch errors, but defers broader Serde configuration options.
- Examples
- Demonstrates typed `Data` structs, compiler diagnostics, and a JSON string whose `name` field incorrectly contains text.
- What you can do after
- Diagnose JSON type mismatches using compiler messages and Serde's line-and-column runtime errors.
Related concepts
- smart castproduces · A smart cast is Kotlin's flow-sensitive automatic refinement of a value's type after a relevant type condition is established.
- TypeScriptused by · TypeScript is a statically typed superset of JavaScript that adds type-system features and is compiled to JavaScript.
- any typecontrasts with · The any type is a TypeScript type that permits values to be treated as having essentially any type, largely disabling static type checking for them.
- type hintsused by · Type hints are source-code annotations that indicate the intended or expected types of program elements and can support documentation, tooling, or type checking.
- TypeScript compilerused by · The TypeScript compiler analyzes and checks TypeScript source code and can emit JavaScript and declaration files.
- Cedarused by · Cedar is an open-source declarative policy language and evaluation engine for expressing and evaluating fine-grained authorization and access-control policies.
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