TypeIsList

TypeIsList is a predicate used in type systems, reflection utilities, and code-generation tools to determine whether a given type represents a list-like container. In practice, TypeIsList(T) is true for types that behave as sequences or collections whose elements can be accessed by index or iteration, and false for scalar types, maps, or non-sequence containers in many definitions. The exact scope of what counts as a list varies by language and library: some definitions include arrays and generic list types, while others extend to any type that implements a common sequence interface. Strings are often treated as non-lists to avoid conflating textual data with list-like structures.

Implementation and usage vary by language. In statically typed languages with generics, TypeIsList can be a compile-time trait or type alias that checks whether T is a list or array type. In languages with reflection, a runtime function TypeIsList(t: Type) may test whether t implements a list-like interface or matches a known list-type pattern. These checks support conditional logic, serialization, validation, and code generation that must adapt to sequences differently from single values.

Examples in common languages illustrate the idea. In TypeScript, a simple form is type TypeIsList<T> = T extends any[] ? true : false. In C#, a runtime helper might return true if a type is an array or implements a generic IList<T>. In Python, a predicate might verify that a type is list-like via checks against list, tuple, or collections.abc.Sequence, excluding strings.

Limitations include ambiguity across collection types, differences between arrays, lists, and other iterables, and potential erasure of type information at runtime. TypeIsList is most reliable when its definition is anchored to a specific language’s standard collection interfaces.

See also: IsIterable, IsSequence, List, Array, Collection, Type predicates.