Exchanges two variables within a container in a structural pattern position:

• {x, y} → {y, x}
• [x, y] → [y, x]
• %{left: x, right: y} → %{left: y, right: x}
• [left: x, right: y] → [left: y, right: x]
• <<x::8, y::16>> → <<y::8, x::16>>

Structural pattern positions include def/defp heads, clause patterns, destructuring match patterns, and routed :binding_pattern macro arguments. Swaps occur within tuples, lists, map and keyword values, and bitstring segment values. Map and keyword keys and bitstring specifiers remain in place. A top-level pattern list, such as a function argument list or multi-pattern clause head, is not itself a swap site.

The two variables must have distinct names. _ and underscore-prefixed names are excluded. Pinned variables may be swapped with another pin or with a binding. Repeated same-name variables are handled by Mutare.Mutators.PatternWildcard instead.

A bitstring value used by another segment as a size is not moved. For example, n in <<n, rest::binary-size(n)>> remains in place.

This family is enabled by default and uses the pattern_swap ignore name.