# `Mutare.Mutators.PatternSwap`
[🔗](https://github.com/foxbenjaminfox/mutare/blob/v0.2.1/lib/mutare/mutators/pattern_swap.ex#L1)

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.

# `pattern_mutations`

```elixir
@spec pattern_mutations([Macro.t()], MapSet.t()) :: [[Macro.t()]]
```

Returns every single-swap variant of the structural pattern position.

Each variant exchanges two distinct variables within the same container.
`used_outside` does not affect swaps because variable usage is unchanged.
Returns `[]` when no eligible pair exists.

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*Consult [api-reference.md](api-reference.md) for complete listing*