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Lecture 19-1

COMP105 Lectures

Custom Types

There are two ways to make types in Haskell.

The type Keyword

The type keyword gives a new name to an existing type.

  • All types must start with capital letters.
type String' = [Char]

exclaim :: String' -> String'
exclaim str = str + "!"

> exclaim "hello"
> "hello!!

Type is useful when you want to give a meaningful name to a complex type.

type VoteResults = [(Int,String)]

results :: VoteResults

The data Keyword

The data keyword is used to create an entirely new type.

data Bool' = True | False
  • | should be read as OR.
  • Each of the values is a constructor.
    • Each constructor should start with a capital letter.

To find out more about a type you can use the GHCI :info command. For more information about a type constructor then you can use the type command.

Example

data Direction = North | South | East | West

rotate North = East
rotate East = South
rotate South = West
rotate West = North

> :t rotate
> rotate :: Direction -> Direction

Type Classes

By default, a new data type is not part of any type class. This means that running rotate will give an error. As is GHCI doesn’t know how to show it.

We can use the deriving keyword to fix this:

data Direction = North | South | East | West deriving (Show)

This will automatically put the type into the class Show and will allow it to print to the prompt.

Hakell can automatically implement the following type classes:

  • Show
    • Will print out the type as it is in the code.
  • Read
    • Will parse the type as it is in the code.
  • Eq
    • The natural definition of equality.
  • Ord
    • Constructors that come first are smaller.

You can add these to the tuple that deriving takes as input. You should include all type classes that make sense for your type so that you can use the functions you want.

Exercises

  1. type Marks = (String, [Int])
  2. data Colour = Red | Blue | Green deriving (Show, Read)
  3.  toRGB :: Colour -> (Float, Float, Float)
 toRGB Red = (1,0,0)
 toRGB Green = (0,1,0)
 toRGB Blue = (0,0,1)

    You should note that as this type isn’t in the class eq then you can’t use guard and equality testing on it.