[Haskell-cafe] Monadic vs "pure" style

[Jacques Carette]

Bulat Ziganshin wrote:

>Language.Haskell.TH.Lib is full of definitions like this:
>
>infixP p1 n p2 = do p1' <- p1
>                    p2' <- p2
>                    return (InfixP p1' n p2')
>
>btw, such definitions can be simplified by using liftM/ap operations:
>
>instance (Monad m, MyNum v) => MyNum (m v) where
>    (.+) = liftM2 (.+)
>  
>
Such simplified forms then occur often enough that, in a scrapping 
boilerplate kind of way, I would really like to be able to write 
something like

instance (Monad m, MyNum v) => MyNum (m v) via lifting
or
lifted instance (Monad m, MyNum v) => MyNum (m v)
or
derived instance (Monad m, MyNum v) => MyNum (m v)

where all the operations from MyNum are obtained via applying the correct arity liftM function from the Monad class.

Jacques