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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