In [1]:

    

{-# LANGUAGE RebindableSyntax #-}
import ClassyPrelude()
import qualified ClassyPrelude as P


class Boolable bool where
    toBool :: bool -> Bool

instance Boolable Int where
    toBool 0 = False
    toBool _ = True

instance Boolable Integer where
    toBool 0 = False
    toBool _ = True

instance Boolable Float where
    toBool 0 = False
    toBool _ = True

instance Boolable Double where
    toBool 0 = False
    toBool _ = True

instance Boolable (Maybe a) where
    toBool Nothing = False
    toBool _ = True

instance Boolable [a] where
    toBool [] = False
    toBool _ = True

-- instance (Num a, Eq a) => Boolable a where
--     toBool num = negate num == num

ifThenElse :: Boolable bool => bool -> a -> a -> a 
ifThenElse boolable true false = case toBool boolable of
                        True -> true
                        False -> false


    

In [3]:

    

if 0 then undefined else "falsy"
if 2 then "truthy" else undefined
if [] then undefined else "truthy"


    

    






"falsy"






    






"truthy"






    






"truthy"

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