Skip to main content

Make our own typeclass

In this chapter, we will create a toy typeclass to practice what we've learned so far for fun.

define our own typeclasses​

The typeclass has a behavior to always reverse anything:

class Reversible a where
  reverseIt :: a -> a

make instances of our typeclass​

Int​

instance Reversible Int where
  reverseIt = negate

Boolean​

instance Reversible Boolean  where
  reverseIt true = false
  reverseIt false = true

String​

import Data.String as String

instance Reversible String where
  reverseIt = String.fromCodePointArray <<< Array.reverse <<< String.toCodePointArray

Array a​

import Data.Array as Array

instance Reversible (Array a) where
  reverseIt = Array.reverse

custom ADT​

data Direction = North | East | South | West

instance Show Direction where
  show North = "North"
  show South = "South"
  show East  = "East"
  show West  = "West"

instance Reversible Direction where
  reverseIt North = South
  reverseIt South = North
  reverseIt East  = West
  reverseIt West  = East

Run it in REPL​

> reverseIt 23
-23

> reverseIt false
true

> reverseIt "Hello!"
"!olleH"

> reverseIt [12, 13, 14]
[14,13,12]

> reverseIt North
South

> reverseIt 25.0
Error found:
in module $PSCI
at :1:1 - 1:15 (line 1, column 1 - line 1, column 15)

No type class instance was found for

  Reversible.Reversible Number