I have been trying to write a DSL for Povray (see www.povray.org) in
Haskell, using the technique of:
http://okmij.org/ftp/papers/tagless-final-APLAS.pdf
with some inspiration taken from
http://okmij.org/ftp/Haskell/DSLSharing.hs

The Povray Scene Description Language is a very declarative language,
with few high level constructs (even loops take a bit of work) --
which is why I'm putting it in Haskell.

At one point, I needed a "varargs" function for the DSL, a function f
:: b -> a -> b dressed up to take a variable number of 'a's, known at
compile time.  This was easy enough:

> data Nil a = Nil
> data Cons b a = a ::: b a
> infixr 1 :::
>
> class VarArgs v where
>   apply_args :: (s -> a -> s) -> s -> v a -> s
>
> instance VarArgs Nil where
>   apply_args _ start _ = start
>
> instance VarArgs b => VarArgs (Cons b) where
>   apply_args f start (a ::: b) = apply_args f (f start a) b

The solution is quite workable:  I can simply write the following, and
I believe the summation is expanded out at compile-time:

> apply_args (+) 0 (2 ::: 3 ::: 8 ::: 1 ::: (-3) ::: Nil)

But I found I also needed a function to take a union type -- that is,
the function would either take an argument of type T1, or of type T2,
known at compile time.  I tried a similar technique as I tried with
varargs, and unfortunately ended up with this:

> data LeftOf a b = L a
> data RightOf a b = R b
>
> class Union u where
>   apply_union :: (a -> c) -> (b -> c) -> (u a b) -> c
>
> instance Union LeftOf where
>   apply_union f _ (L a) = f a
>
> instance Union RightOf where
>   apply_union _ g (R b) = g b
>
> type A = Integer
> type B = String
> type C = ()
>
> type Union_ABC u1 u2 = u1 A (u2 B C)
>
> f_A = show . (+ 3)
> f_B = reverse
> f_C = const "unit"
>
> f :: (Union u1, Union u2) => Union_ABC u1 u2 -> String
> f = apply_union f_A (apply_union f_B f_C)
>
> main = do
>   putStrLn $ f $ (L 6 :: Union_ABC LeftOne LeftOne)
>   putStrLn $ f $ R (L "hello, world")
>   putStrLn $ f $ R (R ())

Notice a lot of ugliness in my example:  e.g., the definition of f,
the type signature of f (I can't move the context into the
type-synonym Union_ABC), creating objects of the union type, and the
unpleasant surprise that I needed to provide the type of 'L 6'.  This
solution is very not-scalable:  the Povray SDL is a "messy" language,
and for my DSL I would need approximately 20 or 30 such unions, each a
union of about 20 types (admittedly with a lot of overlap from union
to union).

I think the solution is to abandon the lofty ideal of statically
determining argument types;  instead have a universal type with tags
to distinguish types dynamically:

> data Universal = UA A | UB B | UC C
> f :: Universal -> String
> f (UA a) = f_A a
> f (UB b) = f_B b
> f (UC c) = f_C c
>
> main2 = do
>   putStrLn $ f $ UA 6
>   putStrLn $ f $ UB "hello, world"
>   putStrLn $ f $ UC ()

...but I'm not ready to give up hope yet.  Suggestions please?

Eric
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