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Haskellings

Welcome to Haskellings, an automated Haskell tutorial program (inspired by Rustlings). This program has dozens of small exercises where you can read about the basic ideas of the Haskell language and then implement some simple functions.

Getting Started

To use Haskellings, you must first have Haskell installed on your machine. The simplest way to do this is to install the Haskell Platform.

Next, you'll need to clone this repository and then build the code using stack.

>> git clone https://github.com/MondayMorningHaskell/haskellings.git
>> stack build

Note that you must clone it somewhere within your "home" directory (e.g. ~/haskellings, or ~/haskell_projects/haskellings). You can then install the haskellings executable to your local path using stack install. This will let you run haskellings from anywhere on your system:

>> stack install
>> haskellings run Types1
...

You can also run any haskellings command from the project directory using stack exec like so:

>> stack exec -- haskellings run Types1

Using Nix (Experimental)

Instead of running stack manually, you can use a nix shell to install the dependencies for you. To do this, install Nix.

Then, when in the project's root directory, run stack build. This is necessary to generate haskellings.cabal from package.yaml (hpack will also work). Then you can run nix-shell.

After the build process, you should see a bash prompt like this:

[nix-shell:~]$ 

This shell provides you with an environment that has GHC 8.10.4 and the haskellings binary installed and available in your $PATH - so you can run haskellings and get started!

These dependencies are only available within the nix-shell, so don't worry about contaminating your global system with conflicting installations.

Running Exercises

Within the exercises directory, you can find sub-directories containing all the different exercise modules. You can run individual exercises with the haskellings run command combined with the name of the fine (without the extension).

>> haskellings run Types1

This will run the Types1 exercise.

Initially, exercises will not compile. You'll want to open the file, read the information at the top, and then fill in the code as indicated by the TODO comments. You'll usually be replacing the question marks ??? with your function implementations and type declarations.

You can re-run the exercise, and the output will let you know when you're done!

If you're stuck, each exercise also has a hint you can see by running haskellings hint:

>> haskellings hint Types1
Fill in appropriate type signatures for the expressions at the bottom.

Sometimes you might want to compile and run code you've written for one function without filling in the rest. When this happens, you can usually replace the question marks ??? for function implementations with the phrase undefined. Using undefined lets your code compile, though that particular function won't run:

-- BEFORE:
subtract7 :: Int -> Int
subtract7 = ???

multiplyBy7 :: Int -> Int
multiplyBy7 = ???

-- AFTER:
subtract7 :: Int -> Int
subtract7 x = x - 7

multiplyBy7 :: Int -> Int
multiplyBy7 = undefined

For example, after making the above change, you'll be able to run the tests for the subtract7 function. You'll still see errors while running the tests for multiplyBy7, but they will compile. Sometimes the implementation will start as undefined rather than ???, in which case it's still your job to replace it!

Using the Watcher

The simplest way to ensure that you're doing exercises in order is to use the Watcher. You can invoke this with the command haskellings watch.

When you save your changes to the file, the Watcher will automatically re-compile and re-run the exercise, so you can see if you've completed it!

Each exercise file has the following comment:

-- I AM NOT DONE

After the exercise is passing, you must delete this comment from the file and re-save to indicate to the Watcher that you are ready to move on to the next exercise. It will then automatically load and run your next task!

You can also use hints within the Watcher! Just type 'hint' and press enter!

>> haskellings watch
Couldn't compile : Expressions.hs
... (compilation output)

hint
Fill in numeric values in place of '???' for expression3 and expression4.

Types of Exercises

There are three different types of exercises.

Compile-Only Exercises

These are the most basic exercises. You'll see these with the more basic concepts, especially around create data types. In order to "pass" the exercise, you only have to edit the code so that the file compiles.

Unit Test Exercises

These represent the bulk of exercises. You'll see a main function at the bottom of the exercise file with obvious testing code. For these exercises, you'll need to fill in function definitions to get certain expected results. So pay attention to the instructions! Once the unit tests pass, you can move on!

Executable Exercises

There are also a few executable exercises. These are most prevalent when you are learning about the IO monad. In these exercises, you will fill in the definition of the main function near the bottom of the file, as well as other IO functions in the file. This will be runnable IO code that can print to the terminal and, if necessary, read user input.

When you use haskellings run, it will run your program on one set of possible inputs and check if the output matches what we expect.

If it does not, you can run the executable using haskellings exec:

haskellings exec IO1

You can run this entering your own custom inputs. But to see why it is not "passing", you can enter in the Sample Inputs at the bottom of the file (if there are any). You should then compare your output to the Sample Output. It should usually (but not always) be a direct match.

Contributing

Haskellings is an open source project. If you've found a bug, have any suggestions for improvements or just want to help, let us know! Take a look at the CONTRIBUTING doc to learn how!

Configuration

If you have a normal installation of the Haskell Platform, you probably do not need to read this section. Configuration happens automatically. But feel free to read on if you want to know more about the details and how you can customize them!

In order to run exercises, Haskellings needs to know two paths on the local system.

  1. The path to the appropriate version of GHC (currently 8.8.4)
  2. The path to a Stack snapshot package DB for that GHC version. This package DB must contain any dependencies that are needed for the exercises, particularly tasty and tasty-hunit which are used for unit testing exercise functions.

Default Configuration

By default, Haskellings will search for these in particular locations. We search for the GHC path by starting at Stack's programs:

  1. ~/.stack/programs on Posix systems
  2. %LOCALAPPDATA%\Programs\stack on Windows

Then we do a breadth-first-search of these directories until we locate ghc-8.8.4. Typical results for this look like:

  1. ~/.stack/programs/x86_64-linux/ghc-8.8.4/bin/ghc on Linux
  2. Users\username\AppData\Local\Programs\stack\x86_64-windows\ghc-8.8.4\bin\ghc on Windows

Then to find the Stack snapshots directory, we use similar starting points:

  1. ~/.stack/snapshots on Posix systems
  2. %STACK_ROOT\snapshots on Windows

These directories contain all your snapshots, each indicated by a hash value. Haskellings performs a BFS on these different hashes until it finds one contains lib/{os}-ghc-8.8.4, and then backtracks to find the corresponding pkgdb. Typical results look like:

  1. ~/.stack/snapshots/x86_64-linux/{hash}/8.8.4/pkgdb on Linux
  2. sr\snapshots\{hash}\pkgdb on Windows

Custom Configuration

If you have Stack setup in such a way that these items are stored somewhere else on your system, you can customize these by creating a simple YAML file at the project root. This file should be called config.yaml and it should have one or both of the keys ghc_path and stack_package_db_path:

# haskellings/config.yaml
ghc_path: /bin/ghc-8.8.4
stack_package_db_path: /lib/stack_snapshots/8.8.4/pkgdb

Note these paths should be absolute and not relative! You can also produce this file by running the haskellings configure command like so:

>> haskellings configure
Please enter GHC Path (or leave blank for default): /bin/ghc-8.8.4
Please enter Stack package DB path (or leave blank): /lib/stack_snapshots/8.8.4/pkgdb

Haskellings needs to be able to find the appropriate version of GHC to actually compile and run individual exercises. It also needs to find Stack package databases to enable dependencies. Right now, we only look for these in the default locations, such as ~/.stack on Posix systems and %LOCALAPPDATA%\\sr on Windows. In the future we will allow the program to be configured to use different paths if your installation is different. See this issue.

Other Beginner Material

At Monday Morning Haskell, one of our core principles is that Haskell shouldn't be as hard to learn as people think it is. Accordingly, we have a wealth of other materials to help you get started learning the language!

  1. Beginners Checklist - This guide will point you to some other resources that can help you kickstart your Haskell journey!
  2. Recursion Workbook - Haskellings has several different exercises to help you learn about recursion. But this workbook has 2 in-depth chapters on the topic and offers 10 more practice problems that will help you master this vital skill!
  3. Liftoff Series - This series is another good starting point for your learning experience. It covers a lot of the same ground you'll cover in the Haskellings exercises, but offers a bit more depth and continuity. Plus it will help you get familiar with GHCI!
  4. Other Beginners Series - There are also several other complete series aimed at beginners! Take a look at these to learn more about monads, unit testing, and data structures!
  5. Haskell From Scratch Course - If you're ready for a more in-depth approach to Haskell foundations, this is the place to go. Our beginners course will illumine a lot more details about Haskell's basics for you. Through hours of video content, dozens more exercises, and a final project, you'll leave this course with the confidence to start your own Haskell projects.