8. Overview

Some examples in this chapter assume you are familiar with Data.Set and Data.Map. You should ignore them on a first reading if they are unfamiliar.

• Types must start with capital letters. A type can be called Dinosaur but not dinosaur.

• Type synonyms, also known as type aliases.

    type BasicPoint = (Float, Float)
  

• Use Data to create new data types. Here is a Point data type:

    data Point = Point Float Float
    getX (Point x y) = x
  

• Use deriving to have an automatic simple output format and equality operation.

    data Point = Point Float Float deriving (Show, Eq)
  

• choices using |

    data Point2D = Point2D Float Float deriving (Show, Eq)
    data Point3D = Point2D Float Float Float deriving (Show, Eq)
    data AbstractPoint = Point2D | Point3D
  

• Named fields, like a Racket struct or Python’s @dataclass or perhaps just namedtuple.

    data Circle =
        Circle { center :: Point2D, radius :: Float }
            deriving (Show, Eq)
  

• Named fields make it easy to update. Using {...} after the name of a variable makes a copy and only changes the fields listed.

    p = Point2D 20.0 50.0
    c = Circle {center=p, radius=16.0}
    q = p{x=30.0} -- new x, same y=50 that pp had
    d = c{center=q} -- new center, same radius as c
  

• Danger: if the same field names are used in different records, enable the extension DisambiguateRecordFields by adding the following code at the start of your file.

    {-# LANGUAGE DisambiguateRecordFields #-}
  

• Either a b = Left a | Right b. Read about it in the chapter.

Typeclasses 102

This part of the chapter contains some challenging material.

• Making your new data type an instance of existing typeclasses, like Eq, Show, or Ord.

• Creating a new typeclass using class (MyThing a) where ...

• Building on existing types using instance (Eq a) => Eq (Maybe a) where …

• (class YesNo and instance YesNo [a] where ...) Making your own typeclass.

• Functor: Yikes, start reading the text!! Memorize the typeclass rules for Functor.

• Discovering kinds using :k. Abstract! Read.

• The note at the end: “you don’t have to understand everything we did here…”. Take that seriously.

Explanations: type vs data

• type

  These are just alternative names for the type on the right side of the equals sign. If a function that has an output of a BasicPoint, that output can be used as an input that is declared to require (Float,Float). These types are considered the weakest to use.

• data

  These are stronger types because you cannot use a Point in a place where a type with a different name is required, even if both contain two Floats.