View Source Returning Errors

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One or more errors for a field can be returned in a single {:error, error_value} tuple.

error_value can be:

A simple error message string.
A map containing :message key, plus any additional serializable metadata.
A keyword list containing a :message key, plus any additional serializable metadata.
A list containing multiple of any/all of these.
Any other value compatible with to_string/1.

basic-errors
Basic Errors

A simple error message:

{:error, "Something bad happened"}

Multiple error messages:

{:error, ["Something bad", "Even worse"]}

Single custom errors (note the required :message keys):

{:error, message: "Unknown user", code: 21}
{:error, %{message: "A database error occurred", details: format_db_error(some_value)}}

Three errors of mixed types:

{:error, ["Simple message", [message: "A keyword list error", code: 1], %{message: "A map error"}]}

Generic handler for interoperability with errors from other libraries:

{:error, :foo}
{:error, 1.0}
{:error, 2}

You may want to look at the Absinthe ErrorPayload package.