# Tutorial 6 - SideEffects

The current say() method has a Concern that modifies its value. What if we instead want the value to be intact, but log that value to System.out? That would be considered a side-effect of the say() method, and should hence not be done in a Concern. It would be better to implement this in a SideEffect. SideEffects are executed after the Mixin and all Concerns for a method are done, which means that the final result has been computed. A SideEffect can access this result value, and then use that for further computation, but it should not change the value or throw an exception.

SideEffects can be either typed or generic, just like Concerns. In the typed case we are interested in specifying SideEffects for one or more particular methods, whereas in the generic case the SideEffect is not really relying on what method is being invoked. Both are useful in different scenarios.

The easiest way to implement a typed SideEffect is to subclass the SideEffectOf class. This gives you access to the result of the real method invocation by using the "result" field, which has the same type as the interface of the method you want the code to be a side-effect of. Note that calling "result" does not actually do anything, it only returns the value (or throws the exception, if one was thrown from the original method) that has already been computed. Similarly, since the method is already done, you can return anything from the SideEffect method. The framework will simply throw it away, and also ignore any exceptions that you throw in your code.

To declare that the SideEffect should be used you add the @SideEffects annotation to either the TransientComposite type, the Mixin type, or the Mixin implementation. Either works.

Steps for this tutorial:

1. Create the SideEffect class that logs the result of say() to System.out.
2. Add a @SideEffects annotation with the SideEffect to the HelloWorldComposite interface.
3. Remove the Concern from the previous step.
4. Move the HelloWorldStateMixin from the HelloWorldState to the HelloWorldComposite interface.