[Haskell-cafe] generics and sql

[Edward Z. Yang]

Hmm, this is a bit peculiar.  The problem is you don't get
control over how gmapQ invokes the function toSql: it will
only ever be done with the type signature Data d => d -> u.
There is good reason for this too: imagined you tried to run
gmapQ toSql on a data-type that contained a member that was
not convertible to a SqlValue: then it ought to fail with a type
error!

You may be able to work around this with more generics madness:
use Typeable to check if the types of all the fields are kosher,
and then do an appropriate casts before invoking toSql.  But you
won't get particularly good static guarantees doing it this way.

So... what are you really trying to do? :-)

Edward

Excerpts from nadine.and.henry's message of Sun Apr 24 10:21:03 -0400 2011:
> 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
>