[Haskell-cafe] custom SQL-to-Haskell type conversion in HDBC

[Henry House]

On Friday, 19 August 2011, Brandon Allbery wrote:
> On Fri, Aug 19, 2011 at 10:09, Henry House <hajhouse at hajhouse.org> wrote:
> 
> > I'm not sure I understand the distinction --- to my way of thinking,
> > getting the value at the right precision means getting the correct
> > number of significant decimal digits, which both your example and mine
> > fail to provide.
> >
> 
> So you want display format data instead of something that you can do
> computations with?  HDBC is giving you the latter; the precision is correct
> for computation, although it could be argued that a Fixed type would be
> better if you intend to propagate exact precision through operations.  (That
> said, I believe Fixed obeys exact mathematical precision instead of
> replicating your SQL backend's limitations, so you'd be unhappy again in
> case of multiplication or division.)

No, I want a data type in Haskell that mirrors the data type in the
RDBMS, not conversion of RDBMS fixed-precision data into something else
(Rational). If the data in the RDBMS represented a rational number of
arbitrary precision, then they would have been stored in the RDBMS as a
rational data type (assuming that the database was designed sensibly, of
course). The limitations of the SQL backend are deliberately chosen (no
one is forced to use fixed-precision numeric data types in PostgreSQL;
arbitrary-precision numeric is available as well as a true rational with
a contrib module). I'm well aware that management of roundoff error in
computations is a tricky issue that needs appropriate management but
that's not really my issue here.

> I would suggest that if you want SQL formatted string output, you describe
> that in the query instead of expecting HDBC to convert numeric data to
> string while duplicating your SQL backend's formatting.

That is a reasonable suggestion. Even better would be to read the result
set returned by the SQL backend without any type conversion being done.
AFAIK all result sets are returned as formatted text. The representation
of result sets as booleans, integers, etc, is a result of conversion in
the client.

> Otherwise, perhaps you'd be better suited to a language with a looser
> type system, so you can pretend string and numeric values are the same
> thing and usually get something resembling the right result.

I apologize, I can see that I was not clear at all in my question. This
is quite the opposite of what I am looking for: I would like a stricter
interpretation of data types --- that does not treat fixed-precision and
rational as the same thing --- not a looser one.

Anyway, while I appreciate the discussion about significant figures on
computation, etc, I was asking whether anyone can point me in the right
direction for implementing a custom SQL-to-Haskell type conversion,
replacing the built-in behavior of HDBC (numeric fixed-precision to
SqlRational) with a conversion that better suits my needs (e.g., as
provided by the Decimal library). Thanks.

-- 
Henry House