Exception handling syntax is the set of keywords and/or structures provided by a computer programming language to allow exception handling, which separates the handling of errors that arise during a program's operation from its ordinary processes. Syntax for exception handling varies between programming languages, partly to cover semantic differences but largely to fit into each language's overall syntactic structure. Some languages do not call the relevant concept "exception handling"; others may not have direct facilities for it, but can still provide means to implement it.
Most commonly, error handling uses a <code>try...[catch...][finally...]</code> block, and errors are created via a <code>throw</code> statement, but there is significant variation in naming and syntax.
Most assembly languages will have a macro instruction or an interrupt address available for the particular system to intercept events such as illegal op codes, program check, data errors, overflow, divide by zero, and other such. IBM and UNIVAC mainframes had the STXIT macro. Digital Equipment Corporation RT11 systems had trap vectors for program errors, i/o interrupts, and such. DOS has certain interrupt addresses. Microsoft Windows has specific module calls to trap program errors.
One can set a trap for multiple errors, responding to any signal with syntax like:
An On Error goto/gosub structure is used in BASIC and is quite different from modern exception handling; in BASIC there is only one global handler whereas in modern exception handling, exception handlers are stacked.
C does not provide direct support to exception handling: it is the programmer's responsibility to prevent errors in the first place and test return values from the functions.
In any case, a possible way to implement exception handling in standard C is to use / functions:
Two types exist:
Example of SEH in C programming language:
A <code>try</code> block must have at least one <code>catch</code> or <code>finally</code> clause and at most one <code>finally</code> clause.
In C++, a resource acquisition is initialization technique can be used to clean up resources in exceptional situations. C++ intentionally does not support . The outer braces for the method are optional.
Adobe ColdFusion documentation
Adobe ColdFusion documentation
Added to the standard syntax above, CFML dialects of Railo and Lucee allow a <code>retry</code> statement.
This statement returns processing to the start of the prior <code>try</code> block.
CFScript example:
Tag-syntax example:
In D, a clause or the resource acquisition is initialization technique can be used to clean up resources in exceptional situations.
In addition to the OCaml-based <code>try...with</code>, F# also has the separate <code>try...finally</code> construct, which has the same behavior as a try block with a <code>finally</code> clause in other .NET languages.
For comparison, this is a translation of the C# sample above.
For comparison, this is translation of the OCaml sample below.
Haskell does not have special syntax for exceptions. Instead, a ///. interface is provided by functions.
prints
(1,42)
in analogy with this C++
Another example is
In purely functional code, if only one error condition exists, the type may be sufficient, and is an instance of Haskell's class by default. More complex error propagation can be achieved using the or monads, for which similar functionality (using ) is supported.
A <code>try</code> block must have at least one <code>catch</code> or <code>finally</code> clause and at most one <code>finally</code> clause. Java distinguishes between <code>Exceptions</code> (errors reasonable to catch) and <code>Error</code> (errors, usually more severe, unreasonable to catch), both of which are descendants of <code>Throwable</code> (the base class of any object which may be thrown).
If multiple resources are acquired, the correct way to deal with them is with nested try blocks. For this reason and others, try-with-resources was added to the language to almost entirely replace finally clauses. Resources acquired in a parentheses after the try keyword will be cleaned up automatically. Classes used in these statements must implement an interface called <code>java.lang.AutoCloseable</code>. This is similar to the "resource acquisition is initialization" pattern common in languages like C++, where resources are cleaned after leaving scope.
The design of JavaScript makes loud/hard errors very uncommon. Soft/quiet errors are much more prevalent. Hard errors propagate to the nearest <code>try</code> statement, which must be followed by either a single <code>catch</code> clause, a single <code>finally</code> clause, or both.
If there is no <code>try</code> statement at all, then the webpage does not crash. Rather, an error is logged to the console and the stack is cleared. However, JavaScript has the interesting quirk of asynchronous externally-invoked entry points. Whereas, in most other languages, there is always some part of the code running at all times, JavaScript does not have to run linearly from start to end. For example, event listeners, Promises, and timers can be invoked by the browser at a later point in time and run in an isolated but shared context with the rest of the code. Observe how the code below will throw a new error every 4 seconds for an indefinite period of time or until the browser/tab/computer is closed.
Another interesting quirk is polymorphism: JavaScript can throw primitive values as errors.
Note that the <code>catch</code> clause is a catch-all, which catches every type of error. There is no syntaxical ability to assign different handlers to different error types aside from experimental and presently removed Gecko extensions from many years ago. Instead, one can either propagate the error by using a <code>throw</code> statement inside the <code>catch</code> statement, or use multiple conditional cases. Let us compare an example in Java and its rough equivalents in JavaScript.
Another aspect of exceptions are promises, which handle the exception asynchronously. Handling the exception asynchronously has the benefit that errors inside the error handler do not propagate further outwards.
Also observe how event handlers can tie into promises as well.
Lastly, note that, as JavaScript uses mark-and-sweep garbage-collection, there is never any memory leakage from throw statements because the browser automatically cleans dead objects—even with circular references.
In TypeScript, <code>catch</code> blocks cannot specify types, but this is traditionally done using <code>instanceof</code>.
Kotlin error handling syntax is very similar to Java.
Kotlin does not force catching checked exceptions, but can still force Java callers to catch/declare the checked exception.
Kotlin supports <code>try</code>-expressions:
Lua uses the <code>pcall</code> and <code>xpcall</code> functions, with <code>xpcall</code> taking a function to act as a <code>catch</code> block.
"tor" is try-or operator. In case of any exception when evaluating the argument on the left, evaluates to the argument on the right.
The Perl mechanism for exception handling uses to throw an exception when wrapped inside an block. After the , the special variable contains the value passed from .
Perl 5.005 added the ability to throw objects as well as strings. This allows better introspection and handling of types of exceptions.
The pseudo-signal can be trapped to handle calls to . This is not suitable for exception handling since it is global. However it can be used to convert string-based exceptions from third-party packages into objects.
The forms shown above can sometimes fail if the global variable is changed between when the exception is thrown and when it is checked in the statement. This can happen in multi-threaded environments, or even in single-threaded environments when other code (typically called in the destruction of some object) resets the global variable before the checking code. The following example shows a way to avoid this problem (see https://archive.today/20130415214802/http://www.perlfoundation.org/perl5/index.cgi?exception_handling or https://stackoverflow.com/a/10343025; cf. http://mvp.kablamo.org/essentials/die-eval/). But at the cost of not being able to use return values:
Several modules in the Comprehensive Perl Archive Network (CPAN) expand on the basic mechanism:
Exception handling is available in PowerBuilder versions 8.0 and above.
<pre> TRY // Normal execution path CATCH (ExampleException ee) // deal with the ExampleException FINALLY // This optional section is executed upon termination of any of the try or catch blocks above END TRY </pre>
try { % code that might throw an exception } catch SomeError: { % code that handles this exception } catch SomeOtherError: { % code that handles this exception } finally % optional block { % This code will always get executed }
New exceptions may be created using the function, e.g., new_exception ("MyIOError", IOError, "My I/O Error"); will create an exception called as a subclass of . Exceptions may be generated using the throw statement, which can throw arbitrary S-Lang objects.
The general mechanism is provided by the message . Exceptions are just normal objects that subclass , you throw one by creating an instance and sending it a message, e.g., . The handling mechanism () is again just a normal message implemented by . The thrown exception is passed as a parameter to the handling block closure, and can be queried, as well as potentially sending to it, to allow execution flow to continue.
Exception handling is supported since Swift 2.
Since Tcl 8.6, there is also a try command:
Exception handling syntax is very similar to Basic. Error handling is local on each procedure.
Example of specific (non official) implementation of exception handling, which uses object of class "Try".
A <code>Try</code> block must have at least one clause <code>Catch</code> or <code>Finally</code> clause and at most one <code>Finally</code> clause.