{-# LANGUAGE EmptyDataDecls #-}
{-# LANGUAGE GADTs #-}

data A
data B
data C
data D

data L

data AST t where
A :: AST A
B :: AST B
C :: [AST B] -> AST C
D :: [AST L] -> AST D
L :: AST t -> AST L

I didn’t think it was possible, but apparently a GADT ‘forall’ can be unpacked (as opposed to the existential which could not). Which makes this possible:

tostr :: AST L -> [Char]
tostr (L A) = “A”
tostr (L B) = “B”
tostr (L _) = “C or D”

> map tostr $ [L A, L B, L A, L $ C [B, B]]
["A","B","A","C or D"]

Inspired by this post about attribute grammars: http://pseudo.fixme.fi/~opqdonut/blog/Attribute_Grammars__GADTs_and_MonadFix__part_2_.html

type A = A
type B = B
type C = C [B]
type D = D [L]
type L = LA A | LB B | LC C | LD D

I find this solution to very directly and elegantly model the problem description, not ugly at all.