How to read compiler error

Question

I wrote an implementation of Monad for custom type:

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

  instance Functor (Sum a) where
    fmap _ (First x) = First x
    fmap f (Second y) = Second (f y)

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

  instance Monad (Sum a) where
    return = pure
    (First a) >>= _ = First a
    (Second a) >>= k = Second (k a)

and I know it is wrong. The compiler complains:

D:\haskell\chapter18\src\SumMonad.hs:39:24: error:
    * Couldn't match type `b' with `Sum a b'
      `b' is a rigid type variable bound by
        the type signature for:
          (>>=) :: forall a1 b. Sum a a1 -> (a1 -> Sum a b) -> Sum a b
        at D:\haskell\chapter18\src\SumMonad.hs:38:15
      Expected type: Sum a b
        Actual type: Sum a (Sum a b)
    * In the expression: Second (k a)
      In an equation for `>>=': (Second a) >>= k = Second (k a)
      In the instance declaration for `Monad (Sum a)'
    * Relevant bindings include
        k :: a1 -> Sum a b
          (bound at D:\haskell\chapter18\src\SumMonad.hs:39:20)
        (>>=) :: Sum a a1 -> (a1 -> Sum a b) -> Sum a b
          (bound at D:\haskell\chapter18\src\SumMonad.hs:38:5)
Failed, modules loaded: none.

How to read the compiler error?

How to read compiler error

1 Answers