[Haskell-cafe] Deriving instances with GADTs

[José Pedro Magalhães]

Hi Tim,

On Thu, Aug 4, 2011 at 08:57, Tim Cowlishaw <tim at 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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