Displaying the parsing results (type classes)

We want to be able to print a textual representation of values of our Document type. There are a few ways to do that:

  1. Write our own function of type Document -> String which we could then print, or
  2. Have Haskell write one for us

Haskell provides us with a mechanism that can automatically generate the implementation of a type class function called show, that will convert our type to String.

The type of the function show looks like this:

show :: Show a => a -> String

This is something new we haven't seen before. Between :: and => you see what is called a type class constraint on the type a. What we say in this signature, is that the function show can work on any type that is a member of the type class Show.

Type classes is a feature in Haskell that allows us to declare a common interface for different types. In our case, Haskell's standard library defines the type class Show in the following way (this is a simplified version but good enough for our purposes):

class Show a where
  show :: a -> String

A type class declaration describes a common interface for Haskell types. show is an overloaded function that will work for any type that is an instance of the type class Show. We can define an instance of a type class manually like this:

instance Show Bool where
  show x =
    case x of
      True -> "True"
      False -> "False"

Defining an instance means providing an implementation for the interface for a specific type. When we call the function show on a data type, the compiler will search the instance of the type it inferred, and use the implementation provided in the instance declaration.

ghci> show True
"True"
ghci> show 187
"187"
ghci> show "Hello"
"\"Hello\""

As can be seen above, the show function converts a value to its textual representation. Which is why "Hello" includes the quotes as well. The Show type class is usually used for debugging purposes.

Deriving instances

It is also possible to automatically generate implementations of a few selected type classes. Fortunately, Show is one of them.

If all the types we use in the definition of our data type already implement an instance of Show, we can automatically derive it by adding deriving Show at the end of the data definition.

data Structure
  = Header Natural String
  | Paragraph String
  | UnorderedList [String]
  | OrderedList [String]
  | CodeBlock [String]
  deriving Show

Now we can use the function show :: Show a => a -> String for any type that implements an instance of the Show type class. For example, with print:

print :: Show a => a -> IO ()
print = putStrLn . show

We can first convert our type to String and then write it to the standard output.

And because lists also implement Show for any element type that has a Show instance, we can now print Documents, because they are just aliases for [Structure]. Try it!

There are many type classes Haskellers use everyday. A couple more are Eq for equality and Ord for ordering. These are also special type classes that can be derived automatically.

Laws

Type classes often come with "rules" or "laws" that instances should satisfy, the purpose of these laws is to provide predictable behaviour across instances, so that when we run into a new instance we can be confident that it will behave in a certain expected way, and we can write code that works generically for all instances of a type class while expecting them to adhere to these rules.

As an example, let's look at the Semigroup type class:

class Semigroup a where
  (<>) :: a -> a -> a

This type class provides a common interface for types with an operation <> that can combine two values into one in some way.

This type class also mentions that this <> operation should be associative, meaning that these two sides should evaluate to the same result:

x <> (y <> z) = (x <> y) <> z

An example of a lawful instance of Semigroup is lists with the append operation (++):

instance Semigroup [a] where
  (<>) = (++)

Unfortunately the Haskell type system cannot "prove" that instances satisfy these laws, but as a community we often shun unlawful instances.

Many data types (together with their respective operations) can form a Semigroup, and instances don't even have to look similar or have a common analogy/metaphor (and this is true for many other type classes as well).

Type classes are often just interfaces with laws (or expected behaviour if you will). Approaching them with this mindset can be very liberating!

To put it a differently, type classes can be used to create abstractions - interfaces with laws/expected behaviour where we don't actually care about the concrete details of the underlying type, just that it implements a certain API and behaves in a certain way.

Regarding Semigroup, we have previously created a function that looks like <> for our Html EDSL! We can add a Semigroup instance for our Structure data type and have a nicer to use API!


Exercise: Please do this and remove the append_ function from the API.

Solution

Replace this:

append_ :: Structure -> Structure -> Structure
append_ c1 c2 =
  Structure (getStructureString c1 <> getStructureString c2)

With this:

instance Semigroup Structure where
  (<>) c1 c2 =
    Structure (getStructureString c1 <> getStructureString c2)

And remove the export of append_ in Html.hs. You won't need to further export anything as type class instances are exported automatically.

You will also need replace the usage of append_ with <> in hello.hs.