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

[Henry House]

On Friday, 19 August 2011, Erik Hesselink wrote:
> Do you really need the precision info about the column, or do you just
> need the values at the right precision? Because you get the last thing
> already:
> 
> Prelude Database.HDBC.PostgreSQL Database.HDBC Data.Ratio
> Control.Monad> (fromSql . head . head) `liftM` (quickQuery db "select
> 1.231 ::numeric(5,0);" []) :: IO Rational
> 1 % 1
> Prelude Database.HDBC.PostgreSQL Database.HDBC Data.Ratio
> Control.Monad> (fromSql . head . head) `liftM` (quickQuery db "select
> 1.231 ::numeric(5,4);" []) :: IO Rational
> 1231 % 1000

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.

Prelude Database.HDBC.PostgreSQL Database.HDBC Data.Ratio Control.Monad>
(fromSql . head . head) `liftM`
   (quickQuery db "select 1.231 ::numeric(10,4);" []) :: IO Rational
-- gives 1231 % 1000 == 1.231 in decimal notation
Prelude Database.HDBC.PostgreSQL Database.HDBC Data.Ratio Control.Monad>
(fromSql . head . head) `liftM`
   (quickQuery db "select 1.231 ::numeric(10,8);" []) :: IO Rational
-- still gives 1231 % 1000 but should be 1.21310000 in decimal notation
-- or 1231000 % 1000000 in rational notation

> If you need the precision information, perhaps 'describeResult' will
> give you what you need. I've never used it, but it looks like it
> might.

Thanks for the suggestion. That will work, but ideally i'd like to have
the normal usage of quickQuery, etc return correct data. Unless I am
missing something here, the method of getting precision information
using describeResult would be to fetch the result set, then find any
columns having a numeric(x,y) SQL type and transform them. Even just
getting the raw string response from the RDBMS would be better than that
since the raw response is actually already a correctly formatted string
representation of the data so I could use it for output without any
further transformation.

Is there a way to get raw response data from the RDBMS through HDBC?

-- 
Henry House
+1 530 848-1238