On Fri, Aug 19, 2011 at 07:23, Henry House
Does there exist any sample code or other resources on writing a custom SQL-to-Haskell datatype converter instance for use with HDBC that would be accessible to someone just starting with Haskell? The reason I need this is because of this problem (using PostgreSQL):
Prelude Database.HDBC.PostgreSQL Database.HDBC> res <- (quickQuery db "select 1::numeric(5,4);" []) Prelude Database.HDBC.PostgreSQL Database.HDBC> res [[SqlRational (1 % 1)]] Prelude Database.HDBC.PostgreSQL Database.HDBC> res <- (quickQuery db "select 1::numeric(5,0);" []) [[SqlRational (1 % 1)]]
where db is a database connection. The SQL values 1::numeric(5,4) and 1::numeric(5,0) are supposed to be fixed-precision numbers having 4 and zero significant decimal figures after the decimal point, respectively. Both are offered by HDBC as the same SqlValue, SqlRational (1 % 1) but they are not really the same at all. The precision information has been lost. The native outputs of PostgreSQL, before HDBC's type conversion, are 1.0000 and 1 for 'select 1::numeric(5,4);' and 'select 1::numeric(5,0);', respectively.
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 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. Erik