[Haskell-cafe] How to store Fixed data type in the database with persistent ?

[s9gf4ult at gmail.com]

All modern databases has field type NUMERIC(x, y) with arbitrary precision.

I need to store financial data with absolute accuracy, and I decided to use 
Fixed.
How can I store Fixed data type as NUMERIC ? I decided to use Snoyman's 
persistent, bit persistent can not use it from the box and there is a problem 
with custom field declaration.

Here is the instance of PersistField for Fixed I wrote

instance (HasResolution a) => PersistField (Fixed a) where
  toPersistValue a = PersistText $ T.pack $ show a
  -- fromPersistValue (PersistDouble d) = Right $ fromRational $ toRational d
  fromPersistValue (PersistText d) = case reads dpt of
    [(a, "")] -> Right a
    _         -> Left $ T.pack $ "Could not read value " ++ dpt ++ " as fixed 
value"
    where dpt = T.unpack d

  fromPersistValue a = Left $ T.append "Unexpected data value can not be 
converted to Fixed: " $ T.pack $ show a

  sqlType a = SqlOther $ T.pack $ "NUMERIC(" ++ (show l) ++ "," ++ (show p) ++ 
")"
    where
      p = round $ (log $ fromIntegral $ resolution a) / (log 10)
      l = p + 15                --  FIXME: this is maybe not very good
  isNullable _ = False

I did not found any proper PersistValue to convert into Fixed from. As well as 
converting Fixed to PersistValue is just a converting to string. Anyway the 
saving works properly, but thre reading does not - it just reads Doubles with 
rounding error.

If you uncomment the commented string in instance you will see, that accuracy 
is not absolute.

Here is test project to demonstrate the problem.

https://github.com/s9gf4ult/xres

If you launch main you will see that precission is not very good because of 
converting database value to Double and then converting to Fixed.

How can i solve this with persistent or what other framework works well with 
NUMERIC database field type ?
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