The Inspect protocol converts an Elixir data structure into an algebra document.

This is typically done when you want to customize how your own structs are inspected in logs and the terminal.

This documentation refers to implementing the Inspect protocol for your own data structures. To learn more about using inspect, see Kernel.inspect/2 and IO.inspect/2.

Inspect representation

There are typically three choices of inspect representation. In order to understand them, let's imagine we have the following User struct:

defmodule User do
  defstruct [:id, :name, :address]
end

Our choices are:

1. Print the struct using Elixir's struct syntax, for example: %User{address: "Earth", id: 13, name: "Jane"}. This is the default representation and best choice if all struct fields are public.

2. Print using the #User<...> notation, for example: #User<id: 13, name: "Jane", ...>. This notation does not emit valid Elixir code and is typically used when the struct has private fields (for example, you may want to hide the field :address to redact person identifiable information).

3. Print the struct using the expression syntax, for example: User.new(13, "Jane", "Earth"). This assumes there is a User.new/3 function. This option is mostly used as an alternative to option 2 for representing custom data structures, such as MapSet, Date.Range, and others.

You can implement the Inspect protocol for your own structs while adhering to the conventions above. Option 1 is the default representation and you can quickly achieve option 2 by deriving the Inspect protocol. For option 3, you need your custom implementation.

Deriving

The Inspect protocol can be derived to customize the order of fields (the default is alphabetical) and hide certain fields from structs, so they don't show up in logs, inspects and similar. The latter is especially useful for fields containing private information.

The supported options are:

• :only - only include the given fields when inspecting.

• :except - remove the given fields when inspecting.

• :optional - (since v1.14.0) do not include a field if it matches its default value. This can be used to simplify the struct representation at the cost of hiding information.

Whenever :only or :except are used to restrict fields, the struct will be printed using the #User<...> notation, as the struct can no longer be copy and pasted as valid Elixir code. Let's see an example:

defmodule User do
  @derive {Inspect, only: [:id, :name]}
  defstruct [:id, :name, :address]
end

inspect(%User{id: 1, name: "Jane", address: "Earth"})
#=> #User<id: 1, name: "Jane", ...>

If you use only the :optional option, the struct will still be printed as %User{...}.

Custom implementation

You can also define your custom protocol implementation by defining the inspect/2 function. The function receives the entity to be inspected followed by the inspecting options, represented by the struct Inspect.Opts. Building of the algebra document is done with Inspect.Algebra.

Many times, inspecting a structure can be implemented in function of existing entities. For example, here is MapSet's inspect/2 implementation:

defimpl Inspect, for: MapSet do
  import Inspect.Algebra

  def inspect(map_set, opts) do
    concat(["MapSet.new(", Inspect.List.inspect(MapSet.to_list(map_set), opts), ")"])
  end
end

The concat/1 function comes from Inspect.Algebra and it concatenates algebra documents together. In the example above it is concatenating the string "MapSet.new(", the document returned by Inspect.Algebra.to_doc/2, and the final string ")". Therefore, the MapSet with the numbers 1, 2, and 3 will be printed as:

iex> MapSet.new([1, 2, 3], fn x -> x * 2 end)
MapSet.new([2, 4, 6])

In other words, MapSet's inspect representation returns an expression that, when evaluated, builds the MapSet itself.

Error handling

In case there is an error while your structure is being inspected, Elixir will raise an ArgumentError error and will automatically fall back to a raw representation for printing the structure. Furthermore, you must be careful when debugging your own Inspect implementation, as calls to IO.inspect/2 or dbg/1 may trigger an infinite loop (as in order to inspect/debug the data structure, you must call inspect itself).

Here are some tips:

• For debugging, use IO.inspect/2 with the structs: false option, which disables custom printing and avoids calling the Inspect implementation recursively

• To access the underlying error on your custom Inspect implementation, you may invoke the protocol directly. For example, we could invoke the Inspect.MapSet implementation above as:

  Inspect.MapSet.inspect(MapSet.new(), %Inspect.Opts{})