11 Errors and Error Handling

11.1 Terminology

Errors can roughly be divided into four different types:

• Compile-time errors
• Logical errors
• Run-time errors
• Generated errors

A compile-time error, for example a syntax error, does not cause much trouble as it is caught by the compiler.

A logical error is when a program does not behave as intended, but does not crash. An example is that nothing happens when a button in a graphical user interface is clicked.

A run-time error is when a crash occurs. An example is when an operator is applied to arguments of the wrong type. The Erlang programming language has built-in features for handling of run-time errors.

A run-time error can also be emulated by calling erlang:error(Reason) or erlang:error(Reason, Args) (those appeared in Erlang 5.4/OTP-R10).

A run-time error is another name for an exception of class error.

A generated error is when the code itself calls exit/1 or throw/1. Notice that emulated run-time errors are not denoted as generated errors here.

Generated errors are exceptions of classes exit and throw.

When a run-time error or generated error occurs in Erlang, execution for the process that evaluated the erroneous expression is stopped. This is referred to as a failure, that execution or evaluation fails, or that the process fails, terminates, or exits. Notice that a process can terminate/exit for other reasons than a failure.

A process that terminates emits an exit signal with an exit reason that says something about which error has occurred. Normally, some information about the error is printed to the terminal.

11.2 Exceptions

Exceptions are run-time errors or generated errors and are of three different classes, with different origins. The try expression (new in Erlang 5.4/OTP R10B) can distinguish between the different classes, whereas the catch expression cannot. They are described in Expressions.

Class	Origin
error	Run-time error, for example, 1+a, or the process called erlang:error/1,2 (new in Erlang 5.4/OTP R10B)
exit	The process called exit/1
throw	The process called throw/1

Table 11.1: Exception Classes.

An exception consists of its class, an exit reason (see Exit Reason), and a stack trace (which aids in finding the code location of the exception).

The stack trace can be retrieved using erlang:get_stacktrace/0 (new in Erlang 5.4/OTP R10B) from within a try expression, and is returned for exceptions of class error from a catch expression.

An exception of class error is also known as a run-time error.

11.3 Handling of Run-time Errors in Erlang

Error Handling Within Processes

It is possible to prevent run-time errors and other exceptions from causing the process to terminate by using catch or try, see Expressions about catch and try.

Error Handling Between Processes

Processes can monitor other processes and detect process terminations, see Processes.

11.4 Exit Reasons

When a run-time error occurs, that is an exception of class error. The exit reason is a tuple {Reason,Stack}, where Reason is a term indicating the type of error:

Reason	Type of Error
badarg	Bad argument. The argument is of wrong data type, or is otherwise badly formed.
badarith	Bad argument in an arithmetic expression.
{badmatch,V}	Evaluation of a match expression failed. The value V did not match.
function_clause	No matching function clause is found when evaluating a function call.
{case_clause,V}	No matching branch is found when evaluating a case expression. The value V did not match.
if_clause	No true branch is found when evaluating an if expression.
{try_clause,V}	No matching branch is found when evaluating the of-section of a try expression. The value V did not match.
undef	The function cannot be found when evaluating a function call.
{badfun,F}	Something is wrong with a fun F.
{badarity,F}	A fun is applied to the wrong number of arguments. F describes the fun and the arguments.
timeout_value	The timeout value in a receive..after expression is evaluated to something else than an integer or infinity.
noproc	Trying to link to a non-existing process.
{nocatch,V}	Trying to evaluate a throwoutside a catch. V is the thrown term.
system_limit	A system limit has been reached. See Efficiency Guide for information about system limits.

Table 11.2: Exit Reasons

Stack is the stack of function calls being evaluated when the error occurred, given as a list of tuples {Module,Name,Arity} with the most recent function call first. The most recent function call tuple can in some cases be {Module,Name,[Arg]}.