Union type
A union type is a type whose values can be any one of several specified alternative types or variants.
Essential TypeScript 5, Third Edition
A union type is a TypeScript type that allows a value to conform to any one of multiple constituent types and exposes only members shared by those types.
- How it is used
- Allows variables, function results, parameters, and arrays to accept several specified types while retaining type checking.
- How deep it goes
- Works through syntax, shared members, property types, narrowing, nullable values, and limits that motivate overloads or generics.
- Examples
- Uses a tax function returning strings or numbers, nullable tax results, and an array mixing objects with shared properties.
- What you can do after
- Build it from scratch: annotate mixed values and safely access constituent-specific members after narrowing.
Read how Essential TypeScript 5, Third Edition explains union type →
Pro Angular 16
A union type is a TypeScript type that permits a value to belong to one of several specified types.
- How it is used
- Restricts function parameters, variables, and mixed array elements to specified alternatives without using `any`.
- How deep it goes
- Works through annotations, compiler errors, type checks, missing values, and array grouping; stops short of compiler internals.
- Examples
- Uses a number-or-string function, a nullable variable, an optional parameter, and a mixed-type array.
- What you can do after
- Apply it via TypeScript annotations to permit selected alternatives and check a value before using type-specific methods.
F# in Action: A Revised Edition of Get Programming with F#
A discriminated union is an F# type whose values are selected from a fixed set of named cases, each potentially carrying different data.
- How it is used
- Models domain alternatives where exactly one named case can exist, complementing records and tuples for coexisting data.
- How deep it goes
- Explains declaration, construction, pattern matching, nesting, and exhaustiveness, then applies unions to domain states and errors.
- Examples
- Uses customer contact methods, local versus international SMS numbers, loan decisions, validation errors, and musical instruments.
- What you can do after
- Apply unions to encode bounded domain alternatives and use exhaustive pattern matches to handle each case safely.
Read how F# in Action: A Revised Edition of Get Programming with F# explains union type →
Functional Design and Architecture: Examples in Haskell
A sum type represents a value that can be one of several alternative constructors or variants.
- How it is used
- Models domain concepts whose valid values fall into distinct alternative cases.
- How deep it goes
- Defines sum types as ADT components and illustrates variant declarations without detailing pattern matching or representation internals.
- Examples
- Illustrates an Actions type with SendMoney, InviteToParty, and ScheduleMeeting variants.
- What you can do after
- Evaluate/understand how alternative variants can encode different forms of a domain action.
Read how Functional Design and Architecture: Examples in Haskell explains union type →
Kafka Streams in Action, Second Edition
A union type is a construct whose value or field may conform to one of several specified types.
- How it is used
- Keeps related Kafka event types on one topic while constraining values to a declared set.
- How deep it goes
- Explains schema declaration, reference registration, serializer settings, consumer configuration, and concrete-type dispatch.
- Examples
- Uses PlaneEvent, TruckEvent, DeliveryEvent, an all_events.avsc union, and a TransportationEvent record.
- What you can do after
- Operate Avro producers and consumers that serialize, retrieve, and dispatch multiple event types from one Kafka topic.
Read how Kafka Streams in Action, Second Edition explains union type →
React in Depth
A discriminated union is a TypeScript union type whose alternatives are distinguished by a shared property with distinct literal values.
- How it is used
- Constrains React component props so property values select valid configuration-specific requirements.
- How deep it goes
- Defines interfaces, unions them, narrows with literal checks, and notes that extracted rest properties lose discriminator correlation.
- Examples
- Uses an e-commerce product card with regular and on-sale variants, sale price, and expiry date.
- What you can do after
- Build it from scratch by modeling React component configurations and safely accessing variant-specific properties after narrowing.
Modern C, Third Edition
A C union is a user-defined type whose members occupy overlapping storage and are represented by the same memory location.
- How it is used
- Uses overlapping member storage to inspect an object's representation and examine individual bytes.
- How deep it goes
- Explains storage overlay, representation digits, sizeof sizing, and implementation-defined byte order, but stops short of broader union operations.
- Examples
- Uses an unsigned value initialized to 0xAABBCCDD and prints its overlaid bytes to reveal machine byte order.
- What you can do after
- Evaluate object representations and investigate implementation-defined byte order with a union and unsigned-char array.
Modern C
A union is a C type whose members share the same memory location, so different members overlay one another.
- How it is used
- Uses unions to inspect an object's individual bytes through an alternate type interpretation.
- How deep it goes
- Defines overlay semantics and works through byte output, while deferring broader alignment and representation limits.
- Examples
- Demonstrates an unsigned value overlaid with an unsigned-char array and prints its bytes in machine-dependent order.
- What you can do after
- Apply it via C union declarations to inspect an unsigned object's representation and compare observed byte order.
Python How-To
A union type specifies that a value may belong to one of several permitted data types.
- How it is used
- Documents function parameters that accept values from several data types.
- How deep it goes
- Explains vertical-bar syntax, nested container annotations, and IDE feedback without covering runtime validation.
- Examples
- Uses statistics over a list-or-tuple parameter, multiway annotations, and unions nested inside set or list annotations.
- What you can do after
- Apply it via Python type hints to document flexible function parameters and receive development-time incompatibility warnings.
Learn Haskell by Example
A sum type is a data type whose value is one of several alternative constructors, with only one constructor choice present at a time.
- How it is used
- Models distinguishable choices in a function interface, such as selecting left or right string padding.
- How deep it goes
- Defines constructors and demonstrates pattern matching through a complete padding function, without descending into runtime internals.
- Examples
- Demonstrates PadMode with PadLeft and PadRight, then applies both choices to padding strings.
- What you can do after
- Apply it via Haskell data declarations and case expressions to model and handle a fixed set of alternatives.
Related concepts
- result typeincludes · A type that represents an operation’s outcome as either a successful value or a failure value.
- System.Text.Jsoncontrasts with · System.Text.Json is a .NET library providing APIs for serializing, deserializing, reading, writing, and processing JSON data.
- single-case discriminated unionincludes · A single-case discriminated union is a discriminated union with exactly one named case, typically used to create a distinct domain-specific type around another value.
- parameterized typekind of · A parameterized type is a type definition or constructor that takes one or more type parameters to produce a specialized or concrete type.
- algebraic data typekind of · An algebraic data type is a composite type whose values are formed from product structures, finite alternatives, or both through type constructors.
- in-memory formatuses · An in-memory format is the structure and representation used to store data or values in computer memory during program execution.
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