# `DeepMerge.Resolver`
[🔗](https://github.com/PragTob/deep_merge/blob/1.0.2/lib/deep_merge/resolver.ex#L1)

Protocol defining how conflicts during deep_merge should be resolved.

As part of the DeepMerge library this protocol is already implemented for
`Map` and `List` as well as a fallback to `Any` (which just always takes the
override).

If you want your custom structs to also be deeply mergable and not just
override one another (default behaviour) you can derive the protocol:

    defmodule Derived do
      @derive [DeepMerge.Resolver]
      defstruct [:attrs]
    end

It will then automatically be deeply merged with structs of its own kind, not
with other structs or maps though.

# `t`

```elixir
@type t() :: term()
```

All the types that implement this protocol.

# `resolve`

Defines what happens when a merge conflict occurs on this struct during a
deep_merge.

Can be implemented for additional data types to implement custom deep merging
behavior.

The passed in values are:
  * `original` - the value in the original data structure, usually left side
  argument
  * `override` - the value with which `original` would be overridden in a
  normal `Map.merge/2`
  * `resolver` - the function used by DeepMerge to resolve merge conflicts,
  i.e. what you can pass to `Map.merge/3` and `Keyword.merge/3` to continue
  deeply merging.

An example implementation might look like this if you want to deeply merge
your struct but only against non `nil` values (because all keys are always there)
if you merge against the same struct (but still merge with maps):

    defimpl DeepMerge.Resolver, for: MyStruct do
      def resolve(original, override = %MyStruct{}, resolver) do
        cleaned_override =
          override
          |> Map.from_struct()
          |> Enum.reject(fn {_key, value} -> is_nil(value) end)
          |> Map.new()

        Map.merge(original, cleaned_override, resolver)
      end

      def resolve(original, override, resolver) when is_map(override) do
        Map.merge(original, override, resolver)
      end
    end

---

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