[Haskell-cafe] Deriving instances with GADTs

[Daniel Schüssler]

Hi,

you can declare an instance 

instance Show (OrderType s o) where ...

this makes sense since it means "for every type s and o, OrderType s o is 
showable", which is vacuously true if (s,o) isn't in {Buy,Sell} x {Market, 
Limit} (because in that case, there /is/ no non-bottom value of type 
'OrderType s o').

Obviously, this will make the newtype's 'deriving' clause work.

Furthermore, you can use {-# LANGUAGE StandaloneDeriving #-} and write

deriving instance Show (OrderType s o)

(Standalone deriving works differently from a 'deriving' clause; the former 
generates the code regardless of how weird the type is and lets the 
typechecker decide. For your GADT, this succeeds).

Cheers,
Daniel Schüssler

On 2011-August-04 Thursday 08:57:46 Tim Cowlishaw 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?
> 
> Many thanks in advance,
> 
> Tim
> 
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