Type Families language pragma

The Type Families language pragma provides the developer the ability to attribute an association between two different types. This is going to allow us to write the same function for different types.

In this example, the type class Falsey is somewhat of a loose boolean test on every day values . . not just Bool values. By allowing the developer to specify type within the class and instance we establish the association between the types:

{-# LANGUAGE TypeFamilies #-}

class Falsey a where
  type Value a
  isFalsey :: a -> Bool

instance Falsey [a] where
  type Value [a] = a
  isFalsey [] = True
  isFalsey _  = False

instance Falsey Bool where
  type Value Bool = Bool
  isFalsey x  = x

main :: IO ()
main = do
  print $ isFalsey []
  print $ isFalsey [1, 2, 3]
  print $ isFalsey True
  print $ isFalsey False