Elixir v1.9.2 Inspect protocol View Source
The Inspect
protocol converts an Elixir data structure into an
algebra document.
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
.
The inspect/2
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
.
Examples
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(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 the Inspect
protocol 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{})
Deriving
The Inspect
protocol can be derived to hide certain fields from
structs, so they don't show up in logs, inspects and similar. This
is especially useful for fields containing private information.
The options :only
and :except
can be used with @derive
to
specify which fields should and should not appear in the
algebra document:
defmodule User do
@derive {Inspect, only: [:id, :name]}
defstruct [:id, :name, :address]
end
inspect(%User{id: 1, name: "Homer", address: "742 Evergreen Terrace"})
#=> #User<id: 1, name: "Homer", ...>
Link to this section Summary
Functions
Converts term
into an algebra document.
Link to this section Types
Specs
t() :: term()
Link to this section Functions
Specs
inspect(t(), Inspect.Opts.t()) :: Inspect.Algebra.t()
Converts term
into an algebra document.
This function shouldn't be invoked directly, unless when implementing
a custom inspect_fun
to be given to Inspect.Opts
. Everywhere else,
Inspect.Algebra.to_doc/2
should be preferred as it handles structs
and exceptions.