-Note [Deriving context]
-~~~~~~~~~~~~~~~~~~~~~~~
-With -fglasgow-exts, we allow things like (C Int a) in the simplified
-context for a derived instance declaration, because at a use of this
-instance, we might know that a=Bool, and have an instance for (C Int
-Bool)
-
-We nevertheless insist that each predicate meets the termination
-conditions. If not, the deriving mechanism generates larger and larger
-constraints. Example:
- data Succ a = S a
- data Seq a = Cons a (Seq (Succ a)) | Nil deriving Show
-
-Note the lack of a Show instance for Succ. First we'll generate
- instance (Show (Succ a), Show a) => Show (Seq a)
-and then
- instance (Show (Succ (Succ a)), Show (Succ a), Show a) => Show (Seq a)
-and so on. Instead we want to complain of no instance for (Show (Succ a)).
-