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Exception Support

SARL supports exception throwing and catching. The mechanism is similar to the one in Java.

For a description of the exceptions that may be thrown by a function, please see how to declare exceptions in a function prototype.

1. Throwing Exceptions

Throwing objects of type Throwable and the throw keyword have the same semantics and syntax as in Java; see Java Language Specification.

throw new IllegalArgumentException("explanation")

2. Try, Catch, Finally

The try-catch-finally expression is used to handle exceptional situations. Checked exceptions are treated like runtime exceptions and only optionally validated. You can, but do not have to, catch them as they will be silently thrown.

try {
	throw new RuntimeException()
} catch(e : Exception) {
	// Handle the exception
} finally {
	// Do this block after the try block (if no exception thrown), 
	// the matched catch block (if an exception was catched),
	// or before exiting the function (if an exception was thrown
	// but not catched).
}

3. Try-Catch as an Expression

For try-catch, the argument is an expression. Consequently, you can write code like the following and do not have to rely on non-final variables:

def readFromFile : String { } 
def example {
	val name =	try {
					readFromFile
				} catch (e : IOException) {
					"unknown"
				}
	println(name)
}

4. The try-with-resources Statement

The try-with-resources statement is a try statement that declares one or more resources. A resource is an object that must be closed after the program is finished with it. The try-with-resources statement ensures that each resource is closed at the end of the statement. Any object that implements java.lang.AutoCloseable, which includes all objects which implement java.io.Closeable, can be used as a resource.

The following example reads the first line from a file. It uses an instance of BufferedReader to read data from the file. BufferedReader is a resource that must be closed after the program is finished with it:

static def readFirstLineFromFile(path : String) : String {
	try (var br = new BufferedReader(new FileReader(path))) {
		return br.readLine
	}
}

In this example, the resource declared in the try-with-resources statement is a BufferedReader. The declaration statement appears within parentheses immediately after the try keyword. The class BufferedReader implements the interface java.lang.AutoCloseable. Because the BufferedReader instance is declared in a try-with-resource statement, it will be closed regardless of whether the try statement completes normally or abruptly (as a result of the method BufferedReader.readLine throwing an IOException).

5. Acknowledgements

This documentation is inspired by the documentations from the Xtext and Xtend projects.

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