[Haskell-cafe] generics and sql

[nadine.and.henry at pobox.com]

Dear Group,

Greetings.  I have a feeling that what I am trying to do is easy, I
just don't know how to say it in Haskell.  Let's start with:

> {-# LANGUAGE 
>    DeriveDataTypeable,  GeneralizedNewtypeDeriving  #-}
> 
> import Database.HDBC
> import Data.Typeable (Typeable)
> import Data.Data 
> data C = C { str :: String, dbl:: Double }
>   deriving (Eq, Ord, Typeable, Data)
> 
> a :: C
> a = C "twelve" 12.0
> 

Now I load this up in ghci and I can do the following:

 toSql . str $ a  -- result: SqlString "twelve"
 toSql . dbl $ a  -- result: SqlDouble 12.0

but what I would really like to do is something like:

gmapQ toSql $ a

which results in:
<interactive>:1:7:
    Could not deduce (Convertible d SqlValue)
      arising from a use of `toSql'
    from the context (Data d)
      bound by a type expected by the context: Data d => d -> SqlValue
      at <interactive>:1:1-11
    Possible fix:
      add (Convertible d SqlValue) to the context of
        a type expected by the context: Data d => d -> SqlValue
      or add an instance declaration for (Convertible d SqlValue)
    In the first argument of `gmapQ', namely `toSql'
    In the expression: gmapQ toSql

In other words, I'm looking for a function with a signature:

(Whatever Instances I neeed here) => a -> [SqlValue]

I have tried various incantations of type signatures, but thus far I
can't get it right.  Can someone point me in the right direction?  Thanks.

Henry Laxen

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