[Haskell-cafe] Deriving instances with GADTs

[Tim Cowlishaw]

Hi all,

I've been writing a DSL to describe securities orders, and after a lot
of help from the kind folk of this list and #haskell have come up with
the following implementation, using generalised algebraic data types:

https://gist.github.com/1124621

Elsewhere in my application, I make use of the order type defined
therein in the following newtype declaration:

 newtype OrderListLevel s = OrderListLevel {orders :: [Order s Limit]}
deriving (Eq, Show)

However, the 'deriving' clause here fails:

src/Simulation/OrderList.hs:9:82:
    No instance for (Eq (Order s Limit))
      arising from the 'deriving' clause of a data type declaration
                   at src/Simulation/OrderList.hs:9:82-83

src/Simulation/OrderList.hs:9:86:
    No instance for (Show (Order s Limit))
      arising from the 'deriving' clause of a data type declaration
                   at src/Simulation/OrderList.hs:9:86-89



I don't fully understand this - the error is correct that there is no
instance of either Eq or Show for (Order s Limit), however, instances
are defined for Order Buy Limit and Order Sell Limit, and since these
are the only possible types that a value can be constructed with (the
type constructor is 'closed' over these types in some sense I guess),
it seems to me that this should provide enough information to derive
the Eq and Show instances. Am I making unreasonable expectations of
ghci's instance-deriving mechanism here, or missing something obvious?

Many thanks in advance,

Tim