# erl_error

###
Module

erl_error

###
Module Summary

Utilities for reporting errors.

###
Since

Module erl_error was introduced in OTP 24.0.

###
Description

This module provides functions for pretty-printing errors and exceptions. It is used by both the shell and by proc_lib to print exceptions.

It is possible for the module raising an error to provide additional information by calling error/3 with extra error information. More details about this mechanism is described in EEP-54.

###
Data Types

####
format_options() =
#{column => column(),
stack_trim_fun => stack_trim_fun(),
format_fun => format_fun()}

A map with formatting options.

####
stack_trim_fun() =
fun((module(), atom(), arity()) -> boolean())

A fun used to trim the end of the stacktrace. It is called with module, function, and arity from an entry from the stacktrace. The fun is to return true if the entry should be trimmed, and false otherwise. The default value is:

    fun(_, _, _) -> false end

####
format_fun() = fun((term(), column()) -> iolist())

A fun used to format function arguments for BIF and function calls. By default the following fun will be used:

    fun(Term, I) -> io_lib:print(Term, I, 80, 30) end

####
column() = integer() >= 1

Start column number. Default is 1.

###
Exports

####
format_exception(Class, Reason, StackTrace) -> unicode:chardata()
OTP 24.0

####
format_exception(Class, Reason, StackTrace, Options) ->
unicode:chardata()
OTP 24.0

### Types

Class = error | exit | throw
Reason = term()
StackTrace = erlang:stacktrace()
Options = format_options()

Format the error reason and stack back-trace caught using try ... catch in the same style as the shell formats them.

Example:

    try
        do_something()
    catch
        C:R:Stk ->
            Message = erl_error:format_exception(C, R, Stk),
            io:format(LogFile, "~ts\n", [Message])
    end

If error_info is provided with the exception, format_exception will use that information to provide additional information about the exception.

Example:

    try
      erlang:raise(badarg,[],[{error_info,#{}}])
    catch
        C:R:Stk ->
            Message = erl_error:format_exception(C, R, Stk),
            io:format(LogFile, "~ts\n", [Message])
    end

See erlang:error/3 for details on how to raise an exception with error_info included.

###
Callback Functions

The following functions are to be exported from an Error Info handler.

###
Exports

####
Module:format_error(Reason, StackTrace) -> ErrorDescription
OTP 24.0

### Types

Reason = term()
StackTrace = erlang:stacktrace()
ArgumentPosition = pos_integer()
ErrorDescription =
#{ ArgumentPosition => unicode:chardata(),
general => unicode:chardata(),
reason => unicode:chardata() }

This callback is called when format_exception/4 or similar functionality wants to provide extra information about an error. The Module:Function called is the one specificed by the error_info map.

The function should return a map with additional information about what have caused the exception. The possible keys of the map are:

ArgumentPosition = pos_integer()
  The position of the argument that caused the error starting at 1.
general
  An error that is not associated with any argument caused the error.
reason
  If the Reason should be printed differently than the default way.

If the text returned includes new-lines, format_exception/4 will indent the text correctly.

Example:

    -module(my_error_module).
    -export([atom_to_string/1, format_error/2]).

    atom_to_string(Arg) when is_atom(Arg) ->
      atom_to_list(Arg);
    atom_to_string(Arg) ->
      erlang:error(badarg,[Arg],
                   [{error_info,#{ module => ?MODULE,
                                   cause => #{ 1 => "should be an atom" }}}]).

    format_error(Reason, [{_M,_F,_As,Info}|_]) ->
      ErrorInfo = proplists:get_value(error_info, Info, #{}),
      ErrorMap = maps:get(cause, ErrorInfo),
      ErrorMap#{ general => "optional general information",
                 reason => io_lib:format("~p: ~p",[?MODULE, Reason]) }.

    1> c(my_error_module).
    {ok,my_error_module}
    2> my_error_module:atom_to_string(1).
    ** exception error: my_error_module: badarg
         in function  my_error_module:atom_to_string/1
            called as my_error_module:atom_to_string(1)
            *** argument 1: should be an atom
            *** optional general information