Question

Having a following custom newtype:

newtype QueryM a = QueryM (Connection -> IO a) 

How can I declare an Alternative instance for it while constraining the a to have Alternative instance too? Or can I?

I mean something like this:

instance (Alternative a) => Alternative (QueryM a) where

Answer 1

Compiles fine here if I add an argument to a:

import Control.Applicative

newtype QueryM a b = QueryM (Connection -> IO (a b))

type Connection = ()

instance Functor (QueryM a)
instance Applicative (QueryM a)

instance (Alternative a) => Alternative (QueryM a) where