listOf1

listOf1 is a term used in programming to denote a function or constructor that creates a list containing exactly one element. The idea is to produce a singleton list: a list whose length is one and whose only item is the provided value. The name listOf1 is not universal, but the concept appears across languages as a way to emphasize a non-empty result.

Definition and semantics: Given an element x of type A, listOf1(x) returns a List<A> containing x as its sole element. This is useful when a function or operation must return a list that is guaranteed to have at least one item, avoiding the need to handle empty lists in downstream processing. In languages with strong type systems, a listOf1-like construct can support safe operations such as head, tail, or pattern matching, since the non-empty property is established at construction time.

Language variants and examples: In Kotlin, a one-element list can be created with listOf(x). In Java, Collections.singletonList(x) yields a single-element, immutable list. In Python, [x] constructs a one-element list. Some libraries introduce a named function like listOf1 to make the singleton pattern explicit, especially in functional code that uses non-empty list semantics or in the context of a NonEmpty or List1 data type.

Relation to non-empty lists and data types: In functional programming, non-empty list types (often called NonEmpty or List1) pair with singleton constructors to avoid empty-list errors.listOf1 serves as a bridge between plain lists and non-empty variants by producing a guaranteed singleton when exactly one element is required.

See also: singleton list, NonEmpty list, List1, listOf, Collections.singletonList.