Re: not null

[Harald Korneliussen <vintermann@gmail.com>]

On Mar 6, 4:21 pm, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
wrote:
> On Fri, 6 Mar 2009 04:01:45 -0800 (PST), Harald Korneliussen wrote:
> > On Mar 4, 5:09 pm, Adam Beneschan <a...@irvine.com> wrote:
>
> >> I'm having difficulty understanding the point.  You have to have the
> >> idea of the lack of a reference.  How else would you implement a
> >> linked list abstraction?
>
> > You can use algebraic data types to do that in a type safe manner.
>
> No, you cannot, if safe = static. The nodes of the list have contracts to
> support certain operations. The only problem with null is that the virtual
> object corresponding to null does not fulfill the contract:

I don't understand your code example. But kind of like some Java
functions won't compile unless they're surrounded by an appropriate
try/catch block, you can make in Haskell a linked list get_next
function that causes the compiler to protest - at compile time,
naturally - if used in a context where the possibility of returning
Nothing (null) isn't considered.

(Embarrassingly enough, the built-in linked list of Haskell does not
do this, for historical reasons. Instead it uses the more old-
fashioned and unsafe way of throwing an exception, and unlike in Java
you don't have to  catch or explicitly say that you won't. But it's
easy to write a list that does it right)