partialranges

Partial ranges are a family of range-like constructs used in some programming languages to describe a subset of a range when one bound is not specified. They are most commonly associated with Swift, where they enable concise slicing of collections and flexible pattern matching by omitting either the lower bound, the upper bound, or both.

In Swift, there are three main forms of partial ranges, each corresponding to a different way of omitting a bound:

- PartialRangeFrom, represented by expressions like 5..., denotes a range that starts at a lower bound and extends to the end of the collection or sequence.

- PartialRangeThrough, represented by ...5, denotes a range that starts at the beginning and goes through the specified upper bound, inclusive.

- PartialRangeUpTo, represented by ..<5, denotes a range that starts at the beginning and goes up to but not including the specified bound.

These forms conform to the RangeExpression protocol and can be used wherever a range is accepted for subscripting or for pattern matching in switch statements. They enable operations such as extracting a slice from a collection starting at a given index, or selecting a prefix up to a certain point, without requiring explicit end bounds.

Using partial ranges requires the underlying collection to support indexing by the resulting range type and, in the case of strings or other grapheme-based collections, careful handling of indices. When applied to arrays, partial ranges provide straightforward, zero-based indexing semantics and are evaluated according to the collection’s index system.

Overall, partial ranges offer a concise way to express common subranges while preserving the clarity of range-based APIs.