# Handle Errors

There is a difficult balance to cut between informative errors and enabling simple reactions to those errors. Since many extensions may need to work with and/or adapt their behavior based on errors coming from Ash, we need rich error messages. However, when you have a hundred different exceptions to represent the various kinds of errors a system can produce, it becomes difficult to say something like "try this code, and if it is invalid, do x, if it is forbidden, do y. To this effect, exceptions in Ash have one of four classes mapping to the top level exceptions.

## Error Classes

- forbidden - `Ash.Error.Forbidden`
- invalid - `Ash.Error.Invalid`
- framework - `Ash.Error.Framework`
- unknown - `Ash.Error.Unknown`

Since many actions can be happening at once, we want to support the presence of multiple errors as a result of a request to Ash. We do this by grouping up the errors into one before returning or raising.
We choose an exception based on the order of the exceptions listed above. If there is a single forbidden, we choose `Ash.Error.Forbidden`, if there is a single invalid, we choose `Ash.Error.Invalid` and so on. The actual errors will be included in the `errors` key on the exception. The exception's message will contain a bulleted list of all the underlying exceptions that occurred. This makes it easy to react to specific kinds of errors, as well as to react to _any/all_ of the errors present.

An example of a single error being raised, representing multiple underlying errors:

```elixir
AshExample.Representative
|> Ash.Changeset.new(%{employee_id: "the best"})
|> AshExample.Api.create!()
 ** (Ash.Error.Invalid) Input Invalid
 * employee_id: must be absent.
 * first_name, last_name: at least 1 must be present.
    (ash 1.3.0) lib/ash/api/api.ex:534: Ash.Api.unwrap_or_raise!/1
```

This allows easy rescuing of the major error classes, as well as inspection of the underlying cases

```elixir
try do
  AshExample.Representative
  |> Ash.Changeset.new(%{employee_id: "dabes"})
  |> AshExample.Api.create!()
rescue
  e in Ash.Error.Invalid ->
    "Encountered #{Enum.count(e.errors)} errors"
end

"Encountered 2 errors"
```

This pattern does add some additional overhead when you want to rescue specific kinds of errors. For example, you may need to do something like this:

```elixir
try do
  AshExample.Representative
  |> Ash.Changeset.new(%{employee_id: "dabes"})
  |> AshExample.Api.create!()
rescue
  e in Ash.Error.Invalid ->
    case Enum.find(e.errors, &(&1.__struct__ == A.Specific.Error)) do
      nil ->
        ...handle errors
      error ->
        ...handle specific error you found
    end
end
```

## Generating Errors

When returning errors in your application, you can a few different things:

## Return a keyword list in changes and validations

A shortcut for creating errors is to return a keyword list containing `field`
and `message`. This works in changes and validations. For example:

```elixir
# in a change, you use `Ash.Changeset.add_error/2`
def change(changeset, _, _) do
  if under_21?(changeset) do
    Ash.Changeset.add_error(changeset, field: :age, message: "must be 21 or older")
  else
    changeset
  end
end

# in a validation, you return the error in an `{:error, error}` tuple.
def change(changeset, _, _) do
  if under_21?(changeset) do
    {:error,  field: :age, message: "must be 21 or older"}
  else
    :ok
  end
end
```

## Using a Builtin Exception

These are all modules under `Ash.Error.*`. You can create a new one with `error.exception(options)`, and the options are documented in each exception. This documentation is missing in some cases. Go to the source code of the exception to see its special options. All of them support the `vars` option, which are values to be interpolated into the message, useful for things like translation.

For example:
```elixir
def change(changeset, _, _) do
  if some_condition(changeset) do
    error = Ash.Error.Changes.Required.new(
      field: :foo,
      type: :attribute,
      resource: changeset.resource
    )

    Ash.Changeset.add_error(changeset, error)
  else
    changeset
  end
end
```

## Use a Custom Exception

You can create a custom exception like so. This is an example of a builtin exception that you could mirror to build your own

```elixir
defmodule Ash.Error.Action.InvalidArgument do
  @moduledoc "Used when an invalid value is provided for an action argument"
  use Ash.Error.Exception

  def_ash_error([:field, :message, :value], class: :invalid)

  defimpl Ash.ErrorKind do
    def id(_), do: Ash.UUID.generate()

    def code(_), do: "invalid_argument"

    def message(error) do
      """
      Invalid value provided#{for_field(error)}#{do_message(error)}

      #{inspect(error.value)}
      """
    end

    defp for_field(%{field: field}) when not is_nil(field), do: " for #{field}"
    defp for_field(_), do: ""

    defp do_message(%{message: message}) when not is_nil(message) do
      ": #{message}."
    end

    defp do_message(_), do: "."
  end
end
```