Real World Haskell solutions

These are solutions to exercises from me reading the first edition of the the Real World Haskell book. Publishing the solutions like this helps me in being more thorough in reading the book. I also hope they will be helpful in your journey to the world of Haskell.

The goal is to provide solutions to all the exercises. I'm adding them gradually as I'm progressing through the book.

There is often more than one solution to an exercise. If your solution is different but meets the assignment, it is the right one.

My original intention was for the exercises to be solved as by a beginner, with all the things that a seasoned Haskell programmer would have done differently. The solutions should have mostly used only knowledge from the preceding parts of the book. Exceptions to this were cases where such approach would lead to too many additional lines of code or to distracting the reader from the main idea of an exercise. Advanced topics from the subsequent chapters should have been avoided, though.

However, in about two thirds of the book I finally had to admit to myself that it was too challenging and got stuck. Then I took a detour to other Haskell books that were more suitable for beginners. Now I returned and I'm going to see how much difference it has made and whether I will be able to continue in the exercises.

Running the code on new systems

The book was published in 2008. Even though it's old, most of the code can still be compiled with recent versions of GHC without any change. Only a few parts required minor modifications or additions. These are described in the comments in the source files of the exercise solutions.

Where more work was needed, I wanted to keep the code in its original form as much as possible instead of rewriting it for new GHC or libraries. These cases are described here.

Chapter 23: GUI programming with gtk2hs

This chapter uses more libraries than the other chapters and I was not able to install all of them using recent stack resolvers (sets of GHC and specific package versions) and Cabal versions.

I decided to create a reproducible environment consisting of an older GHC and compatible versions of the required libraries. Eventually, I was able to compile and run the application.

See the README in the ch23/support directory.

Exercises

There are a few kinds of exercises:

• Describe what happens when you run the code
• Answer a question
• Write an implementation

Solutions to the first two kinds of exercises are provided in comments in Haskell source files.

Ambiguity and mistakes in the assignments

Assignments of the exercises are taken exactly as they are in the printed version of the book without correcting any mistakes. Those are corrected in comments immediately following the assignments.

Some exercises are unclear or they seem to contain contradicting information. In these cases I describe my additional assumptions and thought process in comments.

Solutions using code from the examples

Some solution implementations use the example code from the book. The complete examples repository these solutions are base on is added as a submodule in the 'examples' directory.

If a solution of an exercise depends on more source code from the examples, the whole files are copied to a directory of that solution.

If not much code is needed, then just the required parts are copied directly to the file with implementation and marked by comments.

Platform

The solutions are implemented and tested in a Linux environment. This also includes solutions that could have been made multiplatform.

List of the exercises

Format of an exercise label is '{Module_}\<chapter>_\<group>_\<exercise>'. Some chapters contain more groups of exercises. To differentiate those, an exercise group letter is included (the exact letters don't have a connection to the book). To make the exercise groups more visible in the list the first exercise of a group is in bold italics.

If a solution is intended to be used in solutions to other exercises, to reduce code duplication, it is imported as a Haskell module. For this to work, the naming conventions for Haskell modules need to be followed. The names of such source files are prefixed with 'Module_'.

Bug reports

If you find an error in a solution or anything that should be described more clearly, feel free to let me know by opening an issue here or a pull request here.