Concatenatable

Concatenatable describes a type or data structure that supports a binary operation to merge two values of that type into a single value of the same type, typically by placing one after the other. The operation is commonly referred to as concatenation or append, and the resulting value preserves the order of the inputs.

In practice, concatenatable types are sequences: strings, lists, vectors, byte arrays, and other collections that can be extended by attaching one sequence to another. Examples include concatenating strings "Hello" and "World" to obtain "HelloWorld," or combining two lists [1, 2] and [3, 4] to yield [1, 2, 3, 4]. Some streaming or buffer types are also treated as concatenatable when they support efficient appending.

Formal perspective often treats concatenation as a monoid operation. A type with a concatenation operation is concatenatable if the operation is associative and there exists an identity element (such as the empty string, empty list, or empty buffer) that leaves a value unchanged when concatenated with it. When these properties hold, the type can be described as a monoid under the concatenation operation, enabling generic reasoning and compositional design.

In programming languages, concatenation is usually implemented as an operator or a method (for example, a plus or append function). Not all types are inherently concatenatable; some require explicit definitions or abstractions. For types that are not naturally concatenatable, developers may define a custom concatenation operation or refrain from using concatenation altogether.

See also: concatenation, monoid, sequence, append.