Re: [Haskell-cafe] Deriving instances with GADTs
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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Re: [Haskell-cafe] Deriving instances with GADTs
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Deriving instances with GADTs
Hi Tim,
On Thu, Aug 4, 2011 at 08:57, 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?
>
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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>
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[Haskell-cafe] 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
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