Haskell Overlapping/Incoherent Instances

Question

I know this is code is a bit silly, but can someone explain why this isList [42] returns True whereas isList2 [42] prints False

Answer 1

The following code does the trick without requiring IncoherentInstances:

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE OverlappingInstances #-}

class IsList a where
  isList :: a -> Bool

instance IsList a where
  isList x = False

instance IsList [a] where
  isList x = True

isList2 :: (IsList a) => a -> Bool
isList2 = isList

main = do
  print (isList (42 :: Int))
  print (isList [42 :: Int])
  print (isList2 (42 :: Int))
  print (isList2 [42 :: Int])

I'd recommend not using IncoherentInstances, it seems to cause a lot of trouble, as you can silently call different overloads depending on context quite easily.