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Type classes and its instances

A type class (keyword: class) is a set of functions declarations that
- can have generic default bindings, and
- have instances (keyword: instance) with type specific bindings.
module Prelude already consists of many type classes
- - type classes are in bold
  - below all types that have implemented instances
  - e.g. type class Num has implemented instances for the following types
    - Int
    - Integer
    - Float
    - Double

example, implementing a Functor:

import Prelude hiding (Maybe(..), Functor(..))
 
main :: IO ()
main = 
    do
        print x1
 
class Functor f where
    fmap :: (a -> b) -> f a -> f b
 
data Maybe a = Just a | Nothing
    deriving Show
 
instance Functor Maybe where
    fmap f (Just x) = Just (f x)
    fmap _ Nothing = Nothing
 
x1 = fmap (+3) (Just 2)

executes, with output:
```
Just 5
```

example, implementing a Functor:
```
...
```
- executes, with output:
```
...
```

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