Functional Programming Course

answers

Progression

Tips after having started

1. Use -info or -i to answer WTF questions.

2. Use -type or -t.

3. GHCi has context-sensitive TAB-completion.

   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.

5. Do not use tab characters

Running the tests

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 <tests>. 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

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

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
fail	failed
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

References

• The Haskell error function

• Glasgow Haskell Compiler