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Functional Programming Course
https://github.com/system-f/fp-course

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Functional Programming Course

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README 

        
# Functional Programming Course



![System-F](https://system-f.gitlab.io/logo/systemf-450x450.jpg)



### Written by Tony Morris & Mark Hibberd



### With contributions from individuals and organisations (thanks!)



#### Special note 1



If you have arrived here by https://github.com/system-f/fp-course and you are

looking for the *answers* (not the exercises), please go to https://github.com/tonymorris/fp-course



#### Special note 2



As of February 2017, this repository is taking the place of the repository hosted at

[https://github.com/NICTA/course](https://github.com/NICTA/course) which is deprecated.



The new repository is located at [https://github.com/system-f/fp-course](https://github.com/system-f/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.



### 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. [[nicta-fp]](https://groups.google.com/forum/#!forum/nicta-fp) is a Google

   Group for any queries related to functional programming. This mailing list is

   owned by System F and is open to the public. Questions relating to this course

   are most welcome here.



2. [[haskell-exercises]](https://groups.google.com/forum/#!forum/haskell-exercises)

   is a Google Group for queries related specifically to this System F functional

   programming course material. This mailing list is not owned by System F, but is

   run by others who are keen to share ideas relating to the course.



4. \#bfpg [on Libera.chat](ircs://irc.libera.chat:6697/#bfpg) is the IRC channel of the

   Queensland Functional Programming Lab - the team that runs the course in Brisbane.



5. \#scalaz [on Libera.chat](ircs://irc.libera.chat:6697/#scalaz) is an IRC channel that is operated

   by others who are keen to share ideas relating to functional programming in

   general. Most of the participants of this channel have completed the System F

   functional programming course to some extent. They are in various timezones

   and share a passion for functional programming, so may be able to provide

   relatively quick assistance with questions.



### Getting Started



1. Install the Glasgow Haskell Compiler (GHC) version 8.0 or higher. 

   [GHCup](https://www.haskell.org/ghcup/) is the recommended way to do that these days



2. Change to the directory containing this document.



3. Execute the command `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](#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:

  * What does ____ mean?

  * What does the ____ function mean?

  * What is a ____ ?

  * Where did ____ come from ?

  * What is the structure of ____ ?



  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.



2. 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.



3. 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.



4. 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.



   Where types fall short, use the tests written in comments above each exercise.

   They can be copied and pasted into GHCi. You should also take the first step

   of following the types. Do it.



5. 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 stored under the `src/Test/` directory. Each module from the course that

has tests has a corresponding `Test.hs` file. Within each test module,

tests for each function are grouped using the `testGroup` function. Within each

test group there are test cases (`testCase` function), and properties

(`testProperty` function).



Tests are run using a built-in test runner that has no requirements

beyond those of the course (a supported version of GHCi). By default,

the full test suite is loaded, and each module's tests are

exported. You can run the tests in GHCi like this:



    >> test test_List



#### Specific modules



For convenience, each test module also exports individual tests. To run tests

from a single module, load it, and then run `test `. For example, in

`GHCi`:



    >> :l src/Test/ListTest.hs

    >> test headOrTest

    >> test productTest



#### `:reload` and run tests



There is also a custom `:test` command defined in `.ghci` that will

invoke `:reload` and then `test` in a single action:



    >> :test test_List

    >> :test headOrTest



#### doctest



The doctest tests are a mirror of the tests that reside in comments alongside

the code. They are not executable, but examples can be copied into GHCI.

Examples begin with `>>>` while properties begin with `prop>`.



### Progression



We recommend you perform some exercises before others. The first step

is to inspect the introduction modules.



* `Course.ExactlyOne`

* `Course.Validation`



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, we recommend the following progression of modules:



* `Course.List`

* `Course.Functor`

* `Course.Applicative`

* `Course.Monad`

* `Course.FileIO`

* `Course.State`

* `Course.StateT`

* `Course.Extend`

* `Course.Comonad`

* `Course.Contravariant`

* `Course.Compose`

* `Course.Traversable`

* `Course.ListZipper`

* `Course.Parser` *(see also `Course.Person` for the parsing rules)*

* `Course.MoreParser`

* `Course.JsonParser`

* `Course.Interactive`

* `Course.Anagrams`

* `Course.FastAnagrams`

* `Course.Cheque`



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.



Answers for the exercises can be found here:

[https://github.com/tonymorris/fp-course](https://github.com/tonymorris/fp-course)



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



### 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.



* values, assignment

* type signatures `::` reads as *has the type*

  * The `->` in a type signature is *right-associative*

* functions are values

* functions take arguments

  * functions take *only one argument* but we approximate with spoken

    language

  * functions can be declared inline using *lambda expressions*

  * the `\` symbol in a lambda expression denotes a Greek lambda

* operators, beginning with non-alpha character, are in infix position by

  default

  * use in prefix position by surrounding with *(parentheses)*

* regular identifiers, beginning with alpha character, are in prefix position by

  default

  * use in infix position by surrounding with ``backticks``

* polymorphism

  * type variables *always* start with a lower-case character

* data types, declared using the `data` keyword

  * following the `data` keyword is the *data type name*

  * following the data type name are zero of more type variables

  * then `=` sign

  * data types have zero or more constructors

    * data type constructors start with an upper-case character, or colon `(:)`

  * following each constructor is a list of zero or more *constructor arguments*

  * between each constructor is a pipe symbol `(|)`

  * the `deriving` keyword gives us default implementations for some functions

    on that data type

  * when constructors appear on the left side of `=` we are *pattern-matching*

  * when constructors appear on the right side of `=` we are *constructing*

* type-classes



### Learning the tools



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

covered first.



* GHCi

  * `:type`

  * `:info`

* values

* type signatures

  * `x :: T` is read as *x is of the type T*

* functions are values

* functions take arguments

* functions take one argument

* lambda expressions

* operators (infix/prefix)

  * identifiers starting with `isAlpha` are prefix by default, infix surrounded in backticks (\`)

  * other identifiers are infix by default, prefix surrounded in parentheses

* data types

  * `data` keyword

  * recursive data types

* pattern matching

* `deriving` keyword

* type-classes

* type parameters

  * always lower-case 'a'..'z'

  * aka generics, templates C++, parametric polymorphism



### Parser grammar assistance



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



| English   | Parser library                    |

|-----------|-----------------------------------|

| and then  | `bindParser` `>>=`                |

| always    | `valueParser` `pure`              |

| or        | `\|\|\|`                             |

| 0 or many | `list`                            |

| 1 or many | `list1`                           |

| is        | `is`                              |

| exactly n | `thisMany n`                      |

| call it x | `\x ->`                           |



### Monad comprehension



##### do-notation



* insert the word `do`

* turn `>>=` into `<-`

* delete `->`

* delete `\`

* swap each side of `<-`



##### LINQ



* write `from` on each line

* turn `>>=` into in

* delete `->`

* delete `\`

* swap each side of `in`

* turn value into `select`



### 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:



* `Optional`

  * `mapOptional`

  * `bindOptional`

  * `(??)`

  * `(<+>)`

* `List`

  * `headOr`

  * `product`

  * `length`

  * `map`

  * `filter`

  * `(++)`

  * `flatMap`

  * `reverse`

* `Functor`

  * `instance Functor List`

  * `instance Functor Optional`

  * `instance Functor ((->) t)`

  * `instance Functor void`

* `Applicative`

  * `instance Applicative List`

  * `instance Applicative Optional`

  * `instance Applicative ((->) t)`

  * `lift2`

  * `sequence`

* `FileIO`



### What about cabal and stack?



This repository's primary purpose is to support in-person instruction

for people who have potentially not even used development tools at

all. We have therefore designed the course around `ghci` as the

primary tool.



If you are a more experienced developer with tooling set up, and you

need a cabal file, `shell.nix` or `stack.yaml` to have working

development tools, run the `support/copy-tool-files.sh` script from

the root of the repository.



(Windows users, try running `support\copy-tool-files.bat` from the

repository root.)



### References



* [The Haskell `error` function](http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.html#v:error)



* [Glasgow Haskell Compiler](http://haskell.org/ghc)