The error handler module defines what happens when certain types of errors occur.
undefined_function(Module, Function, Args) -> any()
This function is called by the run-time system if a call is made to Module:Function(Arg1,.., ArgN) and Module:Function/N is undefined. Note that undefined_function/3 is evaluated inside the process making the original call.
This function will first attempt to autoload Module. If that is not possible, an undef exception will be raised.
If it was possible to load Module and the function Function/N is exported, it will be called.
Otherwise, if the function '$handle_undefined_function'/2 is exported, it will be called as '$handle_undefined_function'(Function, Args).
Defining '$handle_undefined_function'/2 in ordinary application code is highly discouraged. It is very easy to make subtle errors that can take a long time to debug. Furthermore, none of the tools for static code analysis (such as Dialyzer and Xref) supports the use of '$handle_undefined_function'/2 and no such support will be added. Only use this function after having carefully considered other, less dangerous, solutions. One example of potential legitimate use is creating stubs for other sub-systems during testing and debugging.
Otherwise an undef exception will be raised.
raise_undef_exception(Module, Function, Args) -> no_return()
Raise an undef exception with a stacktrace indicating that Module:Function/N is undefined.
undefined_lambda(Module, Fun, Args) -> term()
This function is evaluated if a call is made to Fun(Arg1,.., ArgN) when the module defining the fun is not loaded. The function is evaluated inside the process making the original call.
If Module is interpreted, the interpreter is invoked and the return value of the interpreted Fun(Arg1,.., ArgN) call is returned.
Otherwise, it returns, if possible, the value of apply(Fun, Args) after an attempt has been made to autoload Module. If this is not possible, the call fails with exit reason undef.
The code in error_handler is complex and should not be changed without fully understanding the interaction between the error handler, the init process of the code server, and the I/O mechanism of the code.
Changes in the code which may seem small can cause a deadlock as unforeseen consequences may occur. The use of input is dangerous in this type of code.