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

[Erik Hesselink]

On Fri, Aug 19, 2011 at 16:09, Henry House <hajhouse at hajhouse.org> wrote:
> 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

The % notation is a rational, so 'infinite' precision. So '1 % 1' and
'1000 % 1000' are exactly the same, semantically. It's like fractions
instead of decimal digits.

Why exactly do you need the precision information?

Erik