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Erlang - Exceptions
Exception handpng is required in any programming language to handle the runtime errors so that normal flow of the apppcation can be maintained. Exception normally disrupts the normal flow of the apppcation, which is the reason why we need to use Exception handpng in our apppcation.
Normally when an exception or error occurs in Erlang, the following message will be displayed.
{"init terminating in do_boot", {undef,[{helloworld,start,[],[]}, {init,start_it,1,[]},{init,start_em,1,[]}]}}
Crash dump will be written to −
erl_crash.dump init terminating in do_boot ()
In Erlang, there are 3 types of exceptions −
Error − Calpng erlang:error(Reason) will end the execution in the current process and include a stack trace of the last functions called with their arguments when you catch it. These are the kind of exceptions that provoke the runtime errors above.
Exists − There are two kinds of exits: internal exits and external exits. The internal exits are triggered by calpng the function exit/1 and make the current process stop its execution. The external exits are called with exit/2 and have to do with multiple processes in the concurrent aspect of Erlang.
Throw − A throw is a class of exception used for cases that the programmer can be expected to handle. In comparison with exits and errors, they don t really carry any crash that process! intent behind them, but rather they control the flow. As you use throws while expecting the programmer to handle them, it s usually a good idea to document their use within a module using them.
A try ... catch is a way to evaluate an expression while letting you handle the successful case as well as the errors encountered.
The general syntax of a try catch expression is as follows.
Syntax
try Expression of SuccessfulPattern1 [Guards] -> Expression1; SuccessfulPattern2 [Guards] -> Expression2 catch TypeOfError:ExceptionPattern1 -> Expression3; TypeOfError:ExceptionPattern2 -> Expression4 end
The Expression in between try and of is said to be protected. This means that any kind of exception happening within that call will be caught. The patterns and expressions in between the try ... of and catch behave in exactly the same manner as a case ... of.
Finally, the catch part – here, you can replace TypeOfError by either error, throw or exit, for each respective type we ve seen in this chapter. If no type is provided, a throw is assumed.
Following are some of the errors and the error reasons in Erlang −
Error | 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 appped 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 pnk to a non-existing process. |
Following is an example of how these exceptions can be used and how things are done.
The first function generates all possible types of an exception.
Then we write a wrapper function to call generate_exception in a try...catch expression.
Example
-module(helloworld). -compile(export_all). generate_exception(1) -> a; generate_exception(2) -> throw(a); generate_exception(3) -> exit(a); generate_exception(4) -> { EXIT , a}; generate_exception(5) -> erlang:error(a). demo1() -> [catcher(I) || I <- [1,2,3,4,5]]. catcher(N) -> try generate_exception(N) of Val -> {N, normal, Val} catch throw:X -> {N, caught, thrown, X}; exit:X -> {N, caught, exited, X}; error:X -> {N, caught, error, X} end. demo2() -> [{I, (catch generate_exception(I))} || I <- [1,2,3,4,5]]. demo3() -> try generate_exception(5) catch error:X -> {X, erlang:get_stacktrace()} end. lookup(N) -> case(N) of 1 -> { EXIT , a}; 2 -> exit(a) end.
If we run the program as helloworld:demo(). , we will get the following output −
Output
[{1,normal,a}, {2,caught,thrown,a}, {3,caught,exited,a}, {4,normal,{ EXIT ,a}}, {5,caught,error,a}]Advertisements