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License: CC-BY-SA The ExpenseReport legacy code refactoring kata in various languages.

This is an example of a piece of legacy code with lots of code smells. The goal is to support the following new feature as best as you can:

  • Add Lunch with an expense limit of 2000.


  1. 📚 Read the code to understand what it does and how it works.
  2. 🦨 Read the code and check for design and code smells. Make a list of all code and design smells that you find.
  3. 🧑‍🔬 Analyze what you would have to change to implement the new requirement without refactoring the code.
  4. 🧪 Write a characterization test. Expand your list of code and design smells. Add those smells that you missed earlier and discovered now because they made your life writing a test miserable.
  5. 🔧 Refactor the code.
  6. 🔧 Refactor the test.
  7. 👼 Test-drive the new feature.

Supported Languages

The ExpenseReport example currently exists in the following languages:

Planned languages

(in no particular order and with no guarantee)

  • ABAP
  • Amiga E
  • APL
  • More Assembler implementations (arm, aarch64, i686, powerpc, powerpc64, mpis, mips64, sparc, sparc64, risc-v, etc, usually on Linux)
  • Carbon
  • Cluster
  • Concurnas
  • Curry
  • Dark
  • Eiffel
  • Elm
  • Erlang
  • Flix
  • Forth
  • Gleam
  • Gosu
  • Hack
  • Hare
  • Java Byte Code / JVM Assembler
  • Lobster
  • Logo
  • Mercury
  • Modula-2 (once the linker starts working again)
  • Mojo
  • Oberon
  • OCaml
  • Pony
  • PureScript
  • R
  • Reason
  • Scratch
  • Solidity (maybe)
  • Strongtalk
  • TeX
  • Unison
  • WebAssembly

Languages explicitly not planned

  • Brainfuck
  • False
  • Malbolge
  • Whitespace

Undecided Languages

There are many more programming languages than in the list above. Find a more complete list on Wikipedia.

Other plans

  • Make sure that all languages are providing the identical challenge. To be practical, this will require the removal of the timestamp side-effect.
  • Provide the time-stamp side-effect on a separate branch.
  • Provide a test setup (without test) on a separate branch so that folks can choose whether they want to include the setup work in the kata or not.
  • Provide a level 2 challenge for creating an HTML report besides the Plain Text report.


To see solutions, switch to the solutions branch.

Warning The solutions branch will be rebased!

Credits and License

I first encountered the ExpenseReport example during a bootcamp at Equal Experts. I also have seen the ExpenseReport example being used by Robert "Uncle Bob" C. Martin. However, he seems to not be the original author ( I have tried to research its origins but so far I have failed. If you know who has first come up with this example, please get in touch with me.

As I was asked for the license conditions, I have decided to put this under CC-BY-SA, with the following considerations:

  • Please credit me, last but not least to retain the opportunity for people to experience this exercise in as many programming languages as they are interested in.
  • Share it under the same license.
  • Using this as training material should be allowed, even if the trainer is charging money for their time.

Disclaimer: As stated above, I did not come up with the original version. The original author is unknown to me. The CC-BY-SA license thus can only cover my contribution: Recreating the Java version from memory, creating the versions in other programming languages than Java, and keeping them consistent.