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Custom Effects

In previous examples, we've been using Task to wrap code that is side-effecting, deferring its effects until the the Eff program is interpreted, following the style of Haskell's IO.

This style, whereby all IO is wrapped in a single catch-all type, is coming under increasing challenge. It ought to be possible to be more descriptive about what type of external effects a program has, beyond simply acknowledging that they exist.

In this example, we'll look at how custom effects can be used for this goal. We'll take the filesystem related operations that the program uses and factor them into a filesystem effect. We'll leave the interactions with the console (via println) and the clock (via System.currentTimeMillis) wrapped in Task, showing that custom IO effects can be introduced gradually into a program.

🔍 Study Code

  • The sealed trait FilesystemCmd[A] defines the operations supported by the custom effect. There are 3 subclasses, one for each operation. The generic type A represents the value returned when this operation is run.

  • The FilesystemCmd companion object also defines three combinators that introduce filesystem effects into an Eff program. Notice that the all work similarly, creating an instance of a FilesystemCmd, and then calling Eff.send to add the instance into the Eff stack of effects to be resolved at interpret-time.

  • As well as introducing effects, we need a way to resolve them. The abstract Filesystem class includes an interpreter that resolves filesystem effects out of the stack. This has been left incomplete as an exercise for you.

  • Note the presence of a _filesystem member constraint in the main pathScan method.

📝 Write Code

  • In Filesystem, complete the match clause inside runFilesystemCmds. You'll need to define logic for interpreting each type of FilesystemCmd.

▶️ Run Code

Run tests to verify your implementation works correctly.

🔍 Study Code

  • Examine the unit tests. How does the use of a custom test change the testing approach?

  • What other effect is not needed in this version as a result?