YOW Lambdajam Sydney 2017 workshop on programming with the Eff framework.

The workshop consists of a series of practical exercises which teach the use of Eff. Each exercise is an alternate implementation of the same use case:

Ever had a full disk? Where does the space go? Implement a program that can find the largest N files in a directory tree

• Wifi/Internet required.

• You will need Java 8+ and Simple Build Tool (sbt) installed.

• While SBT will download Scala and the Eff libraries on-demand, this can be a slow process. Before the workshop, it is recommended to run sbt update in the base directory to pre-download the required libraries. This may take a few minutes up to 1 hour, depending what you have cached locally in ~/.ivy2/cache.

• Import the base SBT project into your IDE: Intellij, Eclipse ScalaIDE or Ensime.

• Or work with any editor and the SBT command line if you prefer.

  Be warned that IDE presentation compilers don't correctly handle some Eff code, and may flag valid code as invalid. Try your code with the full Scala compiler via SBT command line before concluding there is a problem.

The SBT base project contains five exercise projects, each with a README with instructions to attempt. Each of them contains a different implementation of a file scanner program. Do the exercises in order.

The instruction pages are best viewed in a browser; reach them here:

• exercise1 - File Scanning without using Eff
• exercise2 - Using Eff Reader effect for dependency injection
• exercise3 - Using Eff Either effect for error handling
• exercise4 - Using Eff Task effect for asynchronous & concurrent execution
• exercise5 - Using Eff Writer effect for logging

There are three types of tasks you'll encounter

• 🔍 Study Code Study existing application and test code
• 📝 Write Code Adding missing code or changing existing code at an indicated line or method.
• ▶️ Run Code Run the file scanner (eg exercise1/run) or the unit tests (eg exercise1/test) from SBT prompt.

Each project can be compiled, run or tested separately; errors in one project won't affect the others.

There is a solutions subfolder containing 5 corresponding solution subprojects.

There is learning value in attempting a hard problem, getting stuck, then reviewing the solution. Use the solutions if you get blocked!

Start SBT in the base directory and then operate from the SBT prompt. Invoking each SBT command from the shell (eg sbt exercise1/compile) is slower due to JVM startup costs.

/Users/ben_hutchison/projects/GettingWorkDoneWithExtensibleEffects $ sbt
Getting org.scala-sbt sbt 0.13.13 ...
..further sbt loading omitted..
>

To list all 5 exercise and 5 solution subproject names:

> projects

Try running the file scanner (ie main method) of subproject solutionExercise1 on the current directory.

> solutionExercise1/run .`

To compile sources in subproject exercise1:

> exercise1/compile`

To run any unit tests (in src/test/scala/*) under subproject exercise1

> exercise1/test

SBT commands should be scoped to a subproject (eg exercise1/test). Running eg test at the top level will load 10 copies of the classes into the SBT JVM, potentially leading to OutOfMemoryError: Metaspace

This project teaches Extensible Effects in practice; what it feels like to code with the Eff framework.

It doesn't make any attempt to cover the complex, subtle theory behind Eff, a refinement of 25 years experience of programming with monads, and isn't a complete picture of Eff by any means. At the time of writing however, there are more resources available covering the theory, than practice, of Eff, including:

• The original paper Extensible effects: an alternative to monad transformers in Haskell and followup refinement Freer Monads, More Extensible Effects.

• Video presentation of the above material by Oleg Kiselyov

• The Eff monad, one monad to rule them all by Eff library creator Eric Torreborre

• My own video Getting Work Done with the Eff Monad in Scala