I'm trying to create a very simple monad in Haskell. The monad does nothing special but holding a counter as state.
module EmptyMonad
( EmptyMonad
) where
import Control.Monad
data EmptyMonad a = EmptyMonad
{ myValue :: a
, myState :: Int
} deriving (Show)
instance (Eq a) => Eq (EmptyMonad a) where
EmptyMonad x1 y1 == EmptyMonad x2 y2 = x1 == x2 && y1 == y2
instance Monad (EmptyMonad a) where
return x = EmptyMonad x 0
(EmptyMonad x y) >>= f = EmptyMonad x (y + 1)
After spending few hours on Monads, I cannot get my head around the error from the compiler:
EmptyMonad.hs:16:10: error:
• Expecting one fewer argument to ‘Monad EmptyMonad’
Expected kind ‘k0 -> Constraint’,
but ‘Monad EmptyMonad’ has kind ‘Constraint’
• In the instance declaration for ‘Monad EmptyMonad a’
Failed, modules loaded: none.
There are two main problems here:
* -> *
. So for instance []
, not [a]
; and >>=
expect an EmptyMonad a
, and a function a -> EmptyMonad b
and returns an EmptyMonad b
element. So we can fix the problems with the following solution:
instance Monad EmptyMonad where -- no a after EmptyMonad
return x = EmptyMonad x 0
(EmptyMonad x y) >>= f = fx {myState = y+1}
where fx = f x
So here we specify instance Monad EmptyMonad
since EmptyMonad
has kind * -> *
. Furthermore the bind operator will calculate fx
and then alter the myState
of that instance with y+1
.
That being said, nowadays you need to make EmptyMonad
an instance of Applicative
and Functor
as well.
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