There may be some fun to be had with QuantifiedConstraints. I've thought about solving similar parser/formatter problems this way, extending parsers via functor composition. Consider:

> data F f = F (f (F f))

> -- Sadly the below does not work. It seems like maybe it should be able to work?

> instance (forall a . Show a => Show (f a)) => Show (F f) where show (F f) = "(F " ++ show f ++ ")"

<interactive>:23:10: error:

    • The constraint ‘Show (f a)’

        is no smaller than the instance head ‘Show (F f)’

      (Use UndecidableInstances to permit this)

    • In the instance declaration for ‘Show (F f)’

> -- We can get something almost as good

> class (forall a . Show a => Show (f a)) => Show1 f

> instance Show1 f => Show (F f) where show (F f) = "(F " ++ show f ++ ")"

> instance Show1 Maybe

> show (F $ Just $ F $ Nothing)

"(F Just (F Nothing))"

On Fri, Dec 14, 2018 at 10:52 PM MarLinn <monkleyon@gmail.com> wrote:

Hi Ducis,

you can parametrise over type variables of other kinds than just *.

If what you write is really what you want, the most straightforward answer is simply

	data ExprG f
	    = Var      VarName
	    | Enclosed VarName (f Expr) VarName
	    | Prefix   (f Expr) (f Expr)
	    | Ternary  (f Expr) VarName (f Expr) VarName (f Expr)
	
	type Expr  = ExprG Identity -- From Data.Functor.Identity
	type ExprL = ExprG []
	type ExprD = ExprG DList

There is no mention of the word "functor" because you will have to add that constraint to the usage sites.

Downside: notice that the deriving clauses are gone because the instances aren't as easy to derive any more. Even the simplest and most harmless way I know to get that possibility back involves two language extensions: StandaloneDeriving and FlexibleInstances. With those you can write

	deriving instance Show (ExprG Identity)
	deriving instance Show (ExprG [])
	deriving instance Show (ExprG DList)
	deriving instance Eq   (ExprG Identity)
	:

I suspect though that what you actually want, but didn't write, is more along the lines of

	data ExprL = … | EnclosedL VarName [ExprL]       VarName | …   -- using ExprL instead of Expr on the right side

	data ExprD = … | EnclosedD VarName (DList ExprD) VarName | …   -- using ExprD instead of Expr on the right side

The good news is that if you have the first solution, this step is rather simple. Because you can just use replace Expr with ExprG f again:

	data ExprG f
	    = Var      VarName
	    | Enclosed VarName (f (ExprG f)) VarName
	    | Prefix   (f (ExprG f)) (f (ExprG f))
	    | Ternary  (f (ExprG f)) VarName (f (ExprG f)) VarName (f (ExprG f))

The better news is that although this looks repetitive and hard to read, it's well on the way to discovering the magic of the Free Monad.

Hope this helps.

Cheers,
MarLinn

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