Deriving instances with GADTs
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
Hi Tim,
On Thu, Aug 4, 2011 at 08:57, 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?
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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2011/8/4 José Pedro Magalhães
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.
Aah - this is very useful, thanks!
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.
Aah - this is something like what I suspected (The type signature for OrderListLevel doesn't preclude eg OrderListLevel Int which would need an instance of (Eq|Show) for Order Int Limit, which does not exist.
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
Oh, brilliant, thank you! I'll take a look now. Thanks, Tim
participants (2)
-
José Pedro Magalhães -
Tim Cowlishaw