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).
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 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 |
| 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 be bound to a variable 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.
The call-stack back trace (stacktrace)
The stack back-trace (stacktrace) is a list of {Module,Function,Arity,Location} tuples. The field Arity in the first tuple can be the argument list of that function call instead of an arity integer, depending on the exception.
Location is a (possibly empty) list of two-tuples that can indicate the location in the source code of the function. The first element is an atom describing the type of information in the second element. The following items can occur:
- file
- The second element of the tuple is a string (list of characters) representing the filename of the source file of the function.
- line
- The second element of the tuple is the line number (an integer > 0) in the source file where the exception occurred or the function was called.
Developers should rely on stacktrace entries only for debugging purposes.
The VM performs tail call optimization, which does not add new entries to the stacktrace, and also limits stacktraces to a certain depth. Furthermore, compiler options, optimizations and future changes may add or remove stacktrace entries, causing any code that expects the stacktrace to be in a certain order or contain specific items to fail.
The only exception to this rule is the class error with the reason undef which is guaranteed to include the Module, Function and Arity of the attempted function as the first stacktrace entry.
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 throw outside 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]}.