[Haskell-cafe] Overloading

[Daniel Trstenjak]

Hi Peter,

> -- smart constructor with serialNumber
> date serialNumber
>          | serialNumber > 0 = Date serialNumber
>          | otherwise = error ("invalid serialNumber " ++ show serialNumber)

Instead of raising an error it's more secure to return a Maybe value.

date :: Int -> Maybe Date
date serialNumber
   | serialNumber > 0 = Just $ Date serialNumber
   | otherwise        = Nothing

> -- smart constructor with day month year
> date2 day month year
>         | month >= 1 && month <=12 = undefined
>         | otherwise = error ("invalid month " ++ show month)

To increase type safety it's a good idea to use as much explicit data
types instead of Int values as possible:

data Month = January | ...

> If this is the case, what would be the natural Haskell way of
> organizing the smart constructors ? Just number them as above ? Or
> naming them
> dateFromSerialNumber, dateFromDayMonthYear ?

I would use the descriptive names but leave out the 'date', because you could still have:

import qualified Date

Date.fromSerialNumber



Greetings,
Daniel