The Inspect protocol is responsible for converting any Elixir data structure into an algebra document. This document is then formatted, either in pretty printing format or a regular one.

The inspect/2 function receives the entity to be inspected followed by the inspecting options, represented by the struct Inspect.Opts.

Inspection is done using the functions available in Inspect.Algebra.

Examples

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

defimpl Inspect, for: MapSet do
  import Inspect.Algebra

  def inspect(dict, opts) do
    concat(["#MapSet<", to_doc(MapSet.to_list(dict), 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<" (all strings are valid algebra documents that keep their formatting when pretty printed), the document returned by Inspect.Algebra.to_doc/2 and the other string ">".

Since regular strings are valid entities in an algebra document, an implementation of inspect may simply return a string, although that will devoid it of any pretty-printing.

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.

You can however access the underlying error by invoking the Inspect implementation directly. For example, to test Inspect.MapSet above, you can invoke it as:

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