README.md

Logging with the Writer effect

The Writer[L, ?] effect lets the computation emit additional values of type L as a side-effect of computation. It is commonly used to functional logging, but you could also view the log value as an appendix or supplimentary information about the computation.

This exercise is a use-case of functional logging and how "logs-as-values" lets us easily write unit tests around the log output.

Tasks

🔍 Study Code

• The effect stack R includes Writer[Log], where Log is a sealed hierarchy of Log events at different levels.

• In PathScan.scan, the tell operator is used to emit Log values. Because tell just emits a log, it has type Eff[R, Unit]. So an underscore is used on the lefthand-side of the for (eg _ <- tell(x)).

• In main, the interpretation of the Eff program now includes a runWriterUnsafe step. This is one of several approaches to logging offerred by Eff, where we send a side-effecting handler function (eg println) into the interpreter, that logs each event as it is emitted, rather than returning an accumulation. It's not pure, but it has the advantage of not accumulating data in memory during execution.

▶️ Run Code

Run the tests. They now verify not just the program output but the logs. They should fail because they expect log output for each file visited by the scan.

📝 Write Code

Make the test pass by adding the appropriate tell statement to the File case in pathScan.

▶️ Run Code

run the scanner on a real directory tree and check the logging works as expected.

📝 Write Code

• Measure the time the overall scan took. Take start and end times in scanReport, and use a tell to log the elapsed millis. Ensure your calls to the clock are wrapped in taskDelay effects to ensure the clock timings are taken when the program runs, and not when it is created.

• Try changing the interpretation in main to use the plain runWriter. What does the program return now? How can you print the logs?