Steven D'Aprano wrote:
If I have an arbitrary pointer, and I want to check if it is safe to
dereference, how do I do it? Surely I'm not expected to write something
like:
if type(ptr) == A:
if ptr != Anil: ...
if type(ptr) == B:
if ptr != Bnil: ...
etc. That would be insane. So how does Haskell do this?
In Haskell you would just go ahead and compare ptr
with Nothing (or more likely pattern-match it against
Nothing).
Haskell knows the type of the thing you're comparing
to, and uses type inference to select the right type
of Nothing to use.
BTW, technically, Nothing isn't really a null
pointer, it's a member of an algebraic type
defined in the standard library:
data Maybe a = Just a | Nothing
So conceptually, there is a different Nothing for
each possible type 'a' in Maybe a. But since
the Nothing constructor doesn't have any arguments,
the implementation could represent them all by
the same value if it wanted.
--
Greg
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