On Thu, Aug 4, 2011 at 08:57, Tim Cowlishaw <tim@timcowlishaw.co.uk> wrote:

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?

Here you seem to be using newtype deriving in particular, which behaves differently from standard deriving. Compiling with -ddump-deriv will show you the instances GHC is generating, which can help in debugging.

Note however that deriving instances for GADTs is not trivial, in general. In particular, you should not assume that GHC knows that `s` can only be instantiated with `Buy` and `Sell` since (because we lack a proper kind system) nothing prevents you from later using, say, `Order Int Limit` somewhere.

I describe the issue in more detail in the paper:

José Pedro Magalhães and Johan Jeuring. Generic Programming for Indexed Datatypes.
Color pdf: http://dreixel.net/research/pdf/gpid.pdf
Greyscale pdf: http://dreixel.net/research/pdf/gpid_nocolor.pdf

Cheers,
Pedro

Many thanks in advance,

Tim

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