Getting started is easy.
All you need are ghc
, stack
, and ghcid
.
Setup instructions here.
To compile and run tests when a file changes, use this command (from the project root directory):
ghcid
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.
-
Install the Glasgow Haskell Compiler (GHC) version 8.0 or higher. GHCup is the recommended way to do that these days
-
Change to the directory containing this document.
-
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 ./
. -
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. -
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.
- 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.
-
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. -
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.
-
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.
-
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.
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.Compose
Course.StateT
Course.Parser
(see alsoCourse.Person
for the parsing rules)Course.MoreParser
Course.JsonParser
Answers for the exercises can be found here: https://github.com/tonymorris/fp-course
After these are completed, complete the exercises in the projects
directory.
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
- The
- 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
- use in infix position by surrounding with
- 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
(:)
- 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
- following the
- type-classes
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
- identifiers starting with
- 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
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 -> |
- insert the word
do
- turn
>>=
into<-
- delete
->
- delete
\
- swap each side of
<-