Am Mittwoch, 27. August 2008 22:34 schrieb Aaron Tomb:
On Aug 27, 2008, at 12:23 PM, Dan Weston wrote:
Huh? Type safety buys you not having to worry about dereferencing stale nonnull pointers (lifetime of reference exceeding lifetime of referent), but nothing about dereferencing null pointers, which are the moral equivalent of Nothing.
What type safety buys you, in my mind, is that Nothing is only a valid value for explicit Maybe types. In cases where you don't use Maybe, the "null" situation just can't occur. In languages with null pointers, any pointer could possibly be null.
When you do use Maybe, you have to explicitly handle the Just and Nothing cases separately. Pattern matching reminds you that both are possible. I tend to view fromJust as a function you should only use if you're _very_, _very_ sure that the Nothing case is impossible. But, if so, why are you using a Maybe type in the first place?
Good question. Maybe because you use a function which returns a Maybe result but you know in your use case it's always a Just. But then still a case fun args of Just val -> use val Nothing -> error $ LOCATION ++ " Hey, that ought to be impossible! Go investigate!" would be cleaner. If fromJust is more efficient and this code is called many times in your programme, that could count as a reason.
Aaron
Cheers, Daniel