NAME

       erl_error - Utilities for reporting errors.

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()

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

              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

              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

Ericsson AB                                      stdlib 4.3.1.3                                  erl_error(3erl)