[Haskell-cafe] Overloading

[Peter Caspers]

Hi Daniel,

> 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

yes, I understand (Maybe seems the equivalent of c++'s boost::optional<T>).

>> -- 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 | ...

ok, I will try to change my code in that direction. The idea is clear.

> I would use the descriptive names but leave out the 'date', because you could still have:
>
> import qualified Date
>
> Date.fromSerialNumber
>

also clear, yes. I think I have a better starting point now. Not 
impossible that I will come back later with further questions :-)

Thank you for your help
Peter