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Patterns in Types: A Look at Reader, Writer and State in Scala

This is the base project for "Pattern in Types" jam, for the LambdaJam Australia conference.

note: please test your environment before you arrive so we can get started quickly on the day.

note: test you environment as soon as possible, but please grab the latest copy of this repository the day before the jam with a git pull origin master.

The Challenges

Challenge 0

Challenge 0 contains no coding exercise, but is just there for you to become somewhat familiar with the type class hierarchy provided.


  • Functor
  • Monad
  • Monoid
  • MonadTrans

Challenge 1

Implement the missing methods from the Result data type.

Result is a straight forward sum-type with three cases:

  • An explosion (i.e. an exception from somewhere), or
  • A failure message, or
  • A success value

It could be thought of as a newtype for: Exception / String / A.

Challenge 2

Implement the Reader data type.

Challenge 3

Implement the Writer data type.

Challenge 4

Implement the State data type.

Challenge 5

Implement the ReaderT data type.

Challenge 6

Implement the WriterT data type.

Challenge 7

Implement the StateT data type.

Challenge 8

This challenge presents two styles in 8a and 8b.

8a uses a function to encode reader/writer/state/result for a Http data type.

8b uses a transformer stack to encode reader/writer/state/result for a Http data type.

Working with the code.

  • Challenges are in roughly increasing difficulty.

  • Challenges can be done in any order (except 8b which depends on 5, 6, 7.

  • There is a branch that answers each challenge. If you want to skip a challenge (for example, you wanted to do 7 and 8b, you can merge in the answers for the other exercises with:

git merge origin/challenge5
git merge origin/challenge6
  • If you want to have a sneak peek you can just do a diff:
git diff origin/challenge2 src/main/scala/challenge2
  • You are encouraged to jump in at a level which you feel comfortable. So starting at 1 or at 8 is fine.

  • If you are really confident, I challenge you to implement all exercises in the allocated time. It is possible ;)

  • If at any point you skipped ahead and are having trouble, try going back and doing some of the early challenges.

Getting started

Before you attend you will need to get a few things ready and ensure everything is setup properly. sbt is going to do all the heavy lifting though, so hopefully it is all straight forward, if not, send us an email via


  1. A valid install of java 6+
  2. git
  3. if you are windows only install sbt using the msi installer

Getting scala and validating your environment (for unix):

git clone
cd lambdajam-patterns-in-types
./sbt test

Getting scala and validating your environment (for windows):

git clone
cd lambdajam-patterns-in-types
sbt test

For either platform this may take a few minutes. It will:

  1. Download the sbt build tool.
  2. Download the required versions of scala.
  3. Compile the main and test source code.
  4. Run the tests.

You should see green output, no errors and, an exit code of 0.

Working with scala.

Any good text editor will be sufficient for the course. If you prefer an IDE, you can use the eclipse based scala-ide or intellij with the scala and sbt plugins installed.

You can generate project files for intellij with:

./sbt 'gen-idea no-classifiers'

You can generate project files for eclipse with:

./sbt eclipse

Just note that if you choose eclipse or intellij, have a backup texteditor as well, because there won't be enough time to debug any editor issues.


Base project for "Patterns in Types" jam, for LambdaJam Australia conference.






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