Introduction, Haskell, and Type Classes

These notes serve as material for the Advanced Programming (AP) course at DIKU. They do not form a complete textbook and may not be comprehensible outside the context of the course. In particular, they assume that basic Haskell programming skills are acquired through other means, for example via textbooks such as Programming in Haskell. The notes serve to emphasize the course perspectives, as well as contain material that we could not find of sufficient quality and brevity elsewhere.

The text will contain many links to other resources online. Unless explicitly indicated, you can consider these to be supplementary, rather than required reading.

There is one chapter per week, containing material of relevance to that week's teaching activities and assignment.

Course Principles

Although the Course Description contains a rather dry list of learning goals, you may also benefit from keeping the following principles in mind when reading these notes. They reflect the philosophy behind the course, and our rationale for picking the material and constructing the assignments.

In AP, we study programming methodologies based on the following principles.

• Precision. Code should clearly specify what it does and does not do; largely accomplished through the use of advanced type systems and type-directed design. This is why we use Haskell as our language in the course.

• Separation of concerns. It is a fairly mainstream point of view that modular programs, separated into independent units with minimal functional overlap and interdependence, are easier to write and maintain. By making use of programming techniques that provide precision, we can ensure and verify that our programs are structured thus. Monadic programming is one particularly clear example of this principle, and the one that is our principal object of study, which separates the use of effects from the definitions of effects.

• Principled design. By structuring programs along rigorously defined abstractions, such as monads or similar effect boundaries, we can develop principles for systems design that are both simple and effective, and elegantly support features such as resilience, that can often become quite messy.

We demonstrate these principles through code written in the Haskell programming language, but they are language-agnostic, and can be applied in any language (although often in more awkward forms, for languages that do not provide the requisite flexibility of expression).

While AP is a course focused on practical programming, the time constraints of a seven week course means that we cannot directly study the large programs where these techniques are the most valuable. Instead our examples, exercises, and assignments will be small "toy programs" that cut any unnecessary detail or functionality in order to focus on the essential principles. This does not mean that the techniques we teach in AP to not scale up to large programs; merely that we do not have time for you to observe it for yourselves. You will just have to use your imagination.

Why so many interpreters?

Many of the examples, exercises, and assignments in AP will be in the context of writing interpreters, type checkers, or parsers for programming languages. This is not solely because the teachers happen to enjoy this aspect of computer science, but rather because these domains contain the essence of problems that occur frequently in real-world programming.

• Interpretation is about making operational decisions and changing state based on program input.

• Type checking is input validation.

• Parsing is recovering structured information from unstructured input.

For didactical reasons, AP mostly focuses on problems that exclusively contains these problems (and little "business logic"), but the ideas we study are applicable even outside the rather narrow niche of implementing programming languages.

Testing

In the assignments you will be required to write tests. You must use the Haskell package Tasty to write our tests. Tasty is a meta-framework that allows different testing frameworks to be combined - in particular, hunit for unit tests and QuickCheck for property-based testing. Although you will initially only be writing unit tests, later in the course you will be taught property-based testing. In order to avoid having to switch testing framework halfway through the course, we use Tasty from the start.

Generally the code handout will already contain dependencies on the relevant packages. The ones we will use in the following are tasty and tasty-hunit, which you can add to the build-depends field of your .cabal file.

The Tasty documentation is fairly good and you are encouraged to read it. However, this section will list everything you need to know for the first week of AP.

Structuring tests

There are many ways of structuring test suites. Because the programs you will write in AP are so small, we will use a particularly simple scheme. For any Haskell module Foo, the tests will be in a corresponding test module called Foo_Tests. Each test module defines a test suite named test, which in Tasty is of type TestTree. We will see below how to define these.

To run the tests for an entire project, we write a test runner, which we will normally call runtests.hs. This test runner will import the various TestTrees, combine them if necessary, and pass them to the defaultMain function provided by Tasty. When the program is run, Tasty will run the test suite and report any errors. If the test runner is registered as a test suite in the .cabal file, we can use the cabal test command to run the tests. This will be the case for all code handouts that come with a .cabal file.

A test runner can look like this, where we assume the tests are defined in a module Foo_Tests:

import qualified Foo_Tests
import Test.Tasty (defaultMain)

main :: IO ()
main = defaultMain Foo_Tests.tests

If we load this module into ghci, we can also simply execute main to run the test suite. This makes interactive development easy.

Writing tests

To write a unit test, we import the module Test.Tasty.HUnit. This gives us access to a variety of functions that produce TestTrees. For example, testCase:

testCase :: TestName -> Assertion -> TestTree

The TestName type is a synonym for String and is used to label failures. The Assertion type is a synonym for IO (), but in practice it is constructed using a variety of constructor functions. One of the simplest is assertBool:

assertBool :: String -> Bool -> Assertion

We give it a String that is shown when the test fails, and then a Bool that is True when the test succeeds. This is how we can write a test that fails:

failingTest :: TestTree
failingTest = TestCase "should not work" $ assertBool "1 is not 2" $ 1==2

And here is one that succeeds:

successfulTest :: TestTree
successfulTest = TestCase "should work" $ assertBool "1 is 1" $ 1==1

We can combine multiple TestTrees with testGroup:

tests :: TestTree
tests =
  testGroup
    "unit test suite"
    [ successfulTest,
      failingTest
    ]

The first argument is a descriptive string, and the second is a list of TestTrees. We can use this to define test suites with arbitrarily complicated nesting (and support for running only parts of the entire test suite), but this is not needed for the comparatively simple test suites we write in AP.

The Test.Tasty.HUnit module also provides other handy operators for certain common cases. For example, @?= can be used for testing equality in a more concise way than using assertBool:

failingTest2 :: TestTree
failingTest2 = testCase "should not work 2" $ 1 @?= 2

When run, this will produce output such as the following:

unit test suite
  should work:       OK
  should not work:   FAIL
    .../Week1/Tests.hs:10:
    1 is not 2
    Use -p '/should not work/' to rerun this test only.
  should not work 2: FAIL
    .../Week1/Tests.hs:13:
    expected: 2
     but got: 1
    Use -p '$0=="unit test suite.should not work 2"' to rerun this test only.

Timeouts

Tasty does not know how long a test is supposed to run for, but sometimes we do. We can ask Tasty to fail tests after a specified period via the mkTimeout and localOption functions, which are imported from Test.Tasty. For example, if we want to apply a one second timeout to all tests contained in a given testTree, we could write:

tests :: TestTree
tests =
  localOption (mkTimeout 1000000) $ testGroup
    "unit test suite"
    [ successfulTest,
      failingTest
    ]

The timeout applies to each individual test in the tree passed to localOption, not to the entire test suite.

Useful types

This section discusses various useful Haskell types that are available in the base library, as well as common functions on those types. We will make use of some of these types in the later chapters.

Maybe

The Maybe type is available in the Prelude (the module that is implicitly imported in every Haskell program), but has the following definition:

data Maybe a = Nothing
             | Just a

A value of type Maybe a is either Nothing, representing the absence of a value, or Just x for some value x of type a. It is often called the option type, and serves roughly the same role as the null value in poorly designed languages, but in contrast to null is visible in the type system.

It is often used to represent an operation that can fail. For example, we can imagine a function of type

integerFromString :: String -> Maybe Int

that tries to turn a String into an Integer, and either returns Nothing, indicating that the String is malformed, or Just x, where x is the corresponding integer. We will return to this in Week 3.

One useful function for operating on Maybe values is the maybe function:

maybe :: b -> (a -> b) -> Maybe a -> b

It accepts a value that is returned in the Nothing case, and otherwise a function that is applied to the value in the Just case. It is equivalent to pattern matching, but is more concise.

Another function, which we must import from the Data.Maybe module, is fromMaybe:

fromMaybe :: a -> Maybe a -> a

It turns a Maybe a into an a by providing a default value:

> fromMaybe 0 Nothing
0
> fromMaybe 0 (Just 1)
1

Again this is nothing we could not write ourselves using case, but using these functions can result in neater code.

Either

The Either type is available in the Prelude and has this definition:

data Either a b = Left a
                | Right b

It is used to represent two different possibilities, with different types. In practice, it is often used to represent functions that can fail, with the Left constructor used to represent information about the failure, and the Right constructor used to represent success.

A useful function for manipulating Either values is either:

either :: (a -> c) -> (b -> c) -> Either a b -> c

Void

The Void type must be imported from Data.Void and has the following definition:

data Void

This odd-looking type has no constructors, meaning there are no values of type Void. This is admittedly somewhat exotic, but it has some uses. For example, if we have a function of type Int -> Void, we know that this function cannot possibly return, as no value of type Void can be constructed. This is not really useful for a pure function, but if we have an impure function with side-effects, such as the infinite loops that are used in servers for reading incoming requests (later in the course), then it may be sensible to use a Void return type to clarify that the function will never terminate.

Another use of Void is to eliminate cases in polymorphic types. For example, if we have a type Either Void a, then we know that the Left case can never occur. This means we do not need to handle it when pattern matching the Either type.

> Left undefined :: Either Void Integer
Left *** Exception: Prelude.undefined

Type classes

Type classes are Haskell's main way of specifying abstract interfaces that can be implemented by concrete types. For example, the predefined Eq type class is the interface for any type a that support equality:

class Eq a where
  (==) :: a -> a -> Bool
  (/=) :: a -> a -> Bool

This interface defines two methods, (==) and (/=), of the specified type, which all type classes must implement. The enclosing parentheses denote that these are actually the infix operators == and /=.

We can write polymorhic functions that require that the polymorphic types implement a type class. This is done by adding a type class constraint to the type of the function. For example:

contains :: (Eq a) => a -> [a] -> Bool
contains _ [] = False
contains x (y : ys) = x == y || contains x ys

In the definition of contains, we are able to use the == method on values of type a. If we removed the (Eq a) constraint from the type signature, we would get a type error.

Implementing an instance

When implementing an instance for a type class, we must implement all the methods described in the interface.

For example, if we define our own type for representing a collection of programming languages:

data PL
  = Haskell
  | FSharp
  | Futhark
  | SML
  | OCaml

Then we can define an Eq instance for PL as follows:

instance Eq PL where
  Haskell == Haskell = True
  FSharp == FSharp = True
  Futhark == Futhark = True
  SML == SML = True
  OCaml == OCaml = True
  _ == _ = False

  x /= y = not $ x == y

In fact, the Eq class has a default method for /= expressed in terms of ==, so we can elide the definition of /= in our instance.

Haskell has a handful of built-in type classes. For us, the most important of these are Eq (equality), Ord (ordering), and Show (converting to text). The Haskell compiler is able to automatically derive instances for these when defining a datatype:

data PL
  = Haskell
  | FSharp
  | Futhark
  | SML
  | OCaml
  deriving (Eq, Ord, Show)

This is very convenient, as the definitions of such instances are usually very formulaic. You should add deriving (Eq, Ord, Show) to all datatypes you define.

Type class laws

Type classes are often associated with a set of laws that must hold for any instance of the type class. For example, instances of Eq must follow the usual laws we would expect for an equality relation:

• Reflexivity: x == x = True
• Symmetry: x == y = y == x
• Transitivity: if x == y && y == z = True, then x == z = True
• Extensionality: if x == y = True and f is a function whose return type is an instance of Eq, then f x == f y = True
• Negation: x /= y = not (x == y).

Unfortunately, these laws are not checked by the type system, so we must be careful verify that our instances behave correctly. An instance that follows the proscribed laws is called lawful. The instances that are automatically derived by the compiler will always be lawful (unless they depend on hand-written instances that are not lawful).

Functor

One of the important standard type classes is Functor, which abstracts the notion of a "container of values", where we can apply a function to transform the contained values. We should not carry this metaphor too far, however: some of the types that are instances of Functor are only "containers" in the loosest of senses. The somewhat exotic name Functor is inspired by a branch of mathematics called category theory (as are many other Haskell terms), but we do not need to understand category theory in order to understand the Functor type class:

class Functor f where
  fmap :: (a -> b) -> f a -> f b

The fmap method specified by Functor is essentially a generalisation of the map we are used to for lists. One interesting detail is that Functor instances are not defined for types, but for type constructors. See how the fmap method turns an f a into an f b, intuitively changing the a values to b values. That means f by itself is not a type - it must be applied to a type, and hence is a type constructor.

This is perhaps a bit easier to understand if we first define our own type of linked lists.

data List a
  = Nil
  | Cons a (List a)
  deriving (Eq, Ord, Show)

Our List type is equivalent to Haskell's built-in list type (which is already an instance of Functor), but without the syntactic sugar. We can define a Functor instance for List as follows:

instance Functor List where
  fmap _ Nil = Nil
  fmap f (Cons x xs) = Cons (f x) (fmap f xs)

Functor laws

Any Functor instance must obey these laws:

• Identity: fmap id == id.

• Composition: fmap (f . g) == fmap f . fmap g.

Intuitively, they say that fmap is not allowed to do anything beyond applying the provided function. For example, we cannot store a count of how many times fmap has been applied, or otherwise tweak the observable structure of the container that is being fmaped (e.g. by reversing the list or some such).

Foldable

Type classes allow us to write functions that are generic and reusable in varied contexts. An example of this is the standard class Foldable, which allows us to iterate across all elements of a "container". Its true definition looks more complicated than it really is, due to a large number of optional methods. The following is an abbreviated (but still correct) description of Foldable:

class Foldable t where
  foldr :: (a -> b -> b) -> b -> t a -> b

That is, we must provide a method foldr for iterating across the elements of type a, while updating an accumulator of type b, which yields a final accumulator b. An instance of Foldable for out List type looks like this:

instance Foldable List where
  foldr _ acc Nil = acc
  foldr f acc (Cons x xs) = f x (foldr f acc xs)

Once a type is an instance of Foldable, we can define a remarkable number of interesting functions in termss of foldr (all of which are already defined for you in the Prelude).

For example, we can turn any Foldable into a list:

toList :: (Foldable f) => f a -> [a]
toList = foldr op []
  where
    op x acc = x : acc

> toList (Cons 1 (Cons 2 (Cons 3 Nil)))
[1,2,3]

Or we can see whether a given element is contained in the collection:

elem :: (Eq a, Foldable f) => a -> f a -> Bool
elem needle = foldr op False
  where
    op x acc =
      acc || x == needle

> elem 1 (Cons 1 (Cons 2 (Cons 3 Nil)))
True
> elem 4 (Cons 1 (Cons 2 (Cons 3 Nil)))
False

Phantom Types

In this section we will briefly look at a programming technique that uses types to associate extra information with values and expressions, without any run-time overhead. As our example, we will consider writing a function that implements the kinetic energy formula:

\[ E = \frac{1}{2} m v^2 \]

Here m is the mass of the object, v is its velocity, and E is the resulting kinetic energy. We can represent this easily as a Haskell function:

energy :: Double -> Double -> Double
energy m v = 0.5 * m * (v ** 2)

However, it is quite easy to mix up which of the two arguments is the mass and which is the velocity, given that they have the same types (Double). To clarify matters, we can define some type synonyms:

type Mass = Double

type Velocity = Double

type Energy = Double

energy :: Mass -> Velocity -> Energy
energy m v = 0.5 * m * (v ** 2)

However, type synonyms are just that: synonyms. We can use a Mass whenever a Velocity is expected (or an Energy for that matter), so while the above makes the type of the energy function easier to read for a human, the type checker will not catch mistakes for us.

Instead, let us try to define actual types for representing the physical quantities of interest.

data Mass = Mass Double
  deriving (Show)

data Velocity = Velocity Double
  deriving (Show)

data Energy = Energy Double
  deriving (Show)

energy :: Mass -> Velocity -> Energy
energy (Mass m) (Velocity v) = Energy (0.5 * m * (v ** 2))

Now we are certainly prevented from screwing up.

> energy (Mass 1) (Velocity 2)
Energy 2.0
> energy (Velocity 2) (Mass 1)

<interactive>:54:9-18: error: [GHC-83865]
    • Couldn't match expected type ‘Mass’ with actual type ‘Velocity’
    • In the first argument of ‘energy’, namely ‘(Velocity 2)’
      In the expression: energy (Velocity 2) (Mass 1)
      In an equation for ‘it’: it = energy (Velocity 2) (Mass 1)

<interactive>:54:22-27: error: [GHC-83865]
    • Couldn't match expected type ‘Velocity’ with actual type ‘Mass’
    • In the second argument of ‘energy’, namely ‘(Mass 1)’
      In the expression: energy (Velocity 2) (Mass 1)
      In an equation for ‘it’: it = energy (Velocity 2) (Mass 1)

However, this solution is somewhat heavyweight if we want to also support operations on the Mass, Velocity, and Energy types. After all, from a mathematical (or physical) standpoint, these are all numbers with units, and we may want to support number-like operations on them. Unfortunately, we end up having to implement duplicate functions for every type:

doubleMass :: Mass -> Mass
doubleMass (Mass x) = Mass (2 * x)

doubleVelocity :: Velocity -> Velocity
doubleVelocity (Velocity x) = Velocity (2 * x)

doubleEnergy :: Energy -> Energy
doubleEnergy (Energy x) = Energy (2 * x)

This is not great, and becomes quite messy once we have many functions and types. Instead, let us take a step back and consider how physicists work with their formulae. Each number is associated with a unit (joules, kilograms, etc), and the units are tracked across calculations to ensure that the operations make sense (you cannot add joules and kilograms). This is very much like a type system, but they don't do it by inventing new kinds of numbers (well, they do that sometimes), but rather by adding a kind of unit tag to the numbers.

Haskell allows us to do a very similar thing. First, we define some types that do not have constructors:

data Joule

data Kilogram

data MetrePerSecond

Similar to Void, we cannot ever have a value of type Kilogram, but we can use it at the type level. Specifically, we now define a type constructor Q for representing a quantity of some unit:

data Q unit = Q Double
  deriving (Eq, Ord, Show)

Note that we do not actually use the type parameter unit in the right hand side of the definition. It is a phantom type, that exists only at compile-time, in order to constrain how Qs can be used. When constructing a value of type Q, we can instantiate that unit with anything we want. For example:

weightOfUnladenSwallow :: Q Kilogram
weightOfUnladenSwallow = Q 0.020

We can still make mistakes when we create these values from scratch, by providing a nonsense unit:

speedOfUnladenSwallow :: Q Joule
speedOfUnladenSwallow = Q 9

But at least we are prevented from mixing unrelated quantities:

> speedOfUnladenSwallow == weightOfUnladenSwallow

<interactive>:69:26-47: error: [GHC-83865]
    • Couldn't match type ‘Kilogram’ with ‘Joule’
      Expected: Q Joule
        Actual: Q Kilogram

Now we can define a safe energy function:

energy :: Q Kilogram -> Q MetrePerSecond -> Q Joule
energy (Q m) (Q v) = Q (0.5 * m * (v ** 2))

And in contrast to before, we can define unit-preserving utility functions that apply to any Q:

double :: Q unit -> Q unit
double (Q x) = Q (2 * x)

Phantom types is a convenient technique that requires only a small amount of boilerplate, and can be used to prevent incorrect use of APIs. The type errors are usually fairly simple as well. While phantom types do not guarantee the absence of errors - that requires techniques outside the scope of our course - they are a very practical programming technique, and one of our first examples of fancy use of types. We will return to these ideas later in the course.

Using newtype

Instead of using data, it is best practice to define Q with newtype:

newtype Q unit = Q Double

We can use newtype whenever we define a datatype with a single constructor that has a single-value - intuitively, whenever we simply "wrap" an underlying type. The difference between data and newtype are semantically almost nil (and the edge case does not matter for this course), but newtype is slightly more efficient, as the constructor does not exist when the program executes, meaning our use of newtype carries no performance overhead whatsoever. In contrast, a type declared with data must store the constructor.