Left is for errors by notational implementation

A code snippet from earlier – it’s a little funny since I was talking to dan chao who does a lot of scalaz, but no haskell, and we couldn’t pinpoint exactly why Left is used to pass errors. If we all wrote right-to-left I guess we’d be doing things the other way around.

module Sum where

data Sum a b = First a | Second b deriving (Show, Eq)

-- OH! _This_ is why Left is for Errors and Right is for values.
-- By the implementation of typeclasses, operating on the right values while ignoring
-- the left is the natural notation when you write out the instances!

instance Functor (Sum a) where
  fmap f (First e) = First e
  fmap f (Second b) = Second (f b)

instance Applicative (Sum a) where
  pure v = Second v
  (First e) <*> _ = First e
  _ <*> (First e) = First e
  (Second f) <*> (Second b) = Second (f b)

instance Monad (Sum a) where
  return = pure
  (First e) >>= _ = First e
  (Second b) >>= f = f b