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Functional Programming Course

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Written by Tony Morris & Mark Hibberd for Data61

With contributions from individuals (thanks!)

This version is maintained by Chris Allen at https://github.com/bitemyapp/fp-course

Introduction

The course is structured according to a linear progression and uses the Haskell programming language to learn programming concepts pertaining to functional programming.

Exercises are annotated with a comment containing the word "Exercise." The existing code compiles, however answers have been replaced with a call to the Haskell error function and so the code will throw an exception if it is run. Some exercises contain tips, which are annotated with a preceding "Tip:". It is not necessary to adhere to tips. Tips are provided for potential guidance, which may be discarded if you prefer a different path to a solution.

The exercises are designed in a way that requires personal guidance, so if you attempt it on your own and feel a little lost, this is normal. All the instructions are not contained herein.

Differences from the Original

The original course is located here.

Getting Help

There are two mailing lists for asking questions. All questions are welcome, however, your first post might be moderated. This is simply to prevent spam.

  1. #haskell-beginners on Freenode is an IRC channel that is operated by Chris Allen. The participants are volunteers and fellow learners who help others on their own time.

  2. #haskell or #haskell-beginners on the FPChat Slack. FPChat is a Slack community of fellow travelers. The #haskell and #haskell-beginners channels are good places to seek assistance with this course.

Getting Started

  1. Install Stack which will manage your compiler versions automatically and enable you to build this project.

  2. Change to the directory containing this document.

  3. Execute the command make ghci, which will compile and load all the source code. You may need to set permissions on the root directory and the ghci configuration file, chmod go-w .ghci ./.

  4. Inspect the introductory modules to get a feel for Haskell's syntax, then move on to the exercises starting with Course.Optional. The Progression section of this document lists the recommended order in which to attempt the exercises.

  5. Edit a source file to a proposed solution to an exercise. At the ghci prompt, issue the command :reload. This will compile your solution and reload it in the GHC interpreter. You may use :r for short.

Tips after having started

  1. Some questions take a particular form. These are called WTF questions. WTF questions are those of this form or similar:

They are all answerable with the :info command. For example, suppose you have the question, "What does the swiggletwoop function mean?" You may answer this at GHCi with:

> :info swiggletwoop

You may also use :i for short.

  1. Functional Programming techniques rely heavily on types. This reliance may feel foreign at first, however, it is an important part of this course. If you wish to know the type of an expression or value, use :type. For example,

    > :type reverse

    List t -> List t

    This tells you that the reverse function takes a list of elements of some arbitrary type (t) and returns a list of elements of that same type. Try it.

    You may also use :t for short.

  2. GHCi has TAB-completion. For example you might type the following:

    > :type rev

    Now hit the TAB key. If there is only one function in scope that begins with the characters rev, then that name will auto-complete. Try it. This completion is context-sensitive. For example, it doesn't make sense to ask for the type of a data type itself, so data type names will not auto-complete in that context, however, if you ask for :info, then they are included in that context. Be aware of this when you use auto-complete.

    This also works for file names:

    > readFile "/etc/pas"

    Now hit the TAB key. If there is only one existing filename on a path that begins with /etc/pas, then that name will auto-complete. Try it.

    If there is more than one identifier that can complete, hit TAB twice quickly. This will present you with your options to complete.

  3. Follow the types.

    You may find yourself in a position of being unsure how to proceed for a given exercise. You are encouraged to adopt a different perspective. Instead of asking how to proceed, ask how you might proceed while adhering to the guideline provided by the types for the exercise at hand.

    It is possible to follow the types without achieving the desired goal, however, this is reasonably unlikely at the start. As you become more reliant on following the types, you will develop more trust in the potential paths that they can take you, including identification of false paths.

    Your instructor must guide you where types fall short, but you should also take the first step. Do it.

  4. Do not use tab characters

    Set up your text editor to use space characters rather than tabs. Using tab characters in Haskell can lead to confusing error messages. GHC will give you a warning if your program contains a tab character.

Running the tests

Tests are available as a tasty test suite.

Hspec

Hspec tests are stored under the test/ directory. Each module from the course that has tests has a corresponding <MODULE>Spec.hs file. Within each test module, tests for each function are grouped using the describe function. Within each test group there are test cases (it function), and properties (prop function).

To run the full test suite, build the project as follows:

> stack test

Tasty will also allow you to run only those tests whose description match a pattern. Tests are organised in nested groups named after the relevant module and function, so pattern matching should be intuitive. For example, to run the tests for the List module you could run:

> stack test --ta "--match=Course.List/"

Likewise, to run only the tests for the headOr function in the List module, you could use:

> stack test --ta "--match=Course.List/headOr"

In addition, GHCi may be used to run tasty tests. Assuming you have run ghci from the root of the project, you may do the following:

# Ensure you've run `make test` at least once!
make test-ghci

From there you can run :main at your GHCi prompt to run the entire test suite. If you want to do the equivalent of stack test --ta "--match=ListSpec" in GHCi do this:

:main --match=Course.List/

Similarly you can run:

:main --match=Course.List/headOr

To target just the Course.List/headOr tests. If you've changed your code, run :r to reload and then run your :main invocation again.

Progression

It is recommended to perform some exercises before others. The first step is to inspect the introduction modules.

They contain examples of data structures and Haskell syntax. They do not contain exercises and exist to provide a cursory examination of Haskell syntax. The next step is to complete the exercises in Course.Optional.

After this, the following progression of modules is recommended:

During this progression, it is often the case that some exercises are abandoned due to time constraints and the benefit of completing some exercises over others. For example, in the progression, Course.Functor to Course.Monad, the exercises repeat a similar theme. Instead, a participant may wish to do different exercises, such as Course.Parser. In this case, the remaining answers are filled out, so that progress on to Course.Parser can begin (which depends on correct answers up to Course.Monad). It is recommended to take this deviation if it is felt that there is more reward in doing so.

After these are completed, complete the exercises in the projects directory.

IDEs

The use of IDEs (integrated development environments) is not recommended as many Haskell beginners will spin their wheels trying to get the tools installed and working.

Instead, if you'd like maximum interactivity and fast feedback the following is recommended:

All good (27 modules)

From there you should be able to edit a source file, save, and see ghcid reload and type-check your code automatically.

Introducing Haskell

This section is a guide for the instructor to introduce Haskell syntax. Each of these points should be covered before attempting the exercises.

Learning the tools

When this course is run in-person, some tools, particularly within Haskell, are covered first.

Parser grammar assistance

The exercises in Parser.hs can be assisted by stating problems in a specific way, with a conversion to code.

EnglishParser library
and thenbindParser >>=
alwaysvalueParser pure
or|||
0 or manylist
1 or manylist1
isis
exactly nthisMany n
failfailed
call it x\x ->

Monad comprehension

do-notation
LINQ

Demonstrate IO maintains referential transparency

Are these two programs, the same program?

p1 ::
  IO ()
p1 =
  let file = "/tmp/file"
  in  do  _ <- writeFile file "abcdef"
          x <- readFile file
          _ <- putStrLn x
          _ <- writeFile file "ghijkl"
          y <- readFile file
          putStrLn (show (x, y))

p2 ::
  IO ()
p2 =
  let file = "/tmp/file"
      expr = readFile file
  in  do  _ <- writeFile file "abcdef"
          x <- expr
          _ <- putStrLn x
          _ <- writeFile file "ghijkl"
          y <- expr
          putStrLn (show (x, y))

What about these two programs?

def writeFile(filename, contents):
    with open(filename, "w") as f:
    f.write(contents)

def readFile(filename):
    contents = ""
    with open(filename, "r") as f:
    contents = f.read()
    return contents

def p1():
    file = "/tmp/file"

    writeFile(file, "abcdef")
    x = readFile(file)
    print(x)
    writeFile(file, "ghijkl")
    y = readFile(file)
    print (x + y)

def p2():
    file = "/tmp/file"
    expr = readFile(file)

    writeFile(file, "abcdef")
    x = expr
    print(x)
    writeFile(file, "ghijkl")
    y = expr
    print (x + y)

One-day

Sometimes this course material is condensed into one-day. In these cases, the following exercises are recommended:

References