• Denis Koroskin (5/10) Jan 14 2009 I think I have found one more reason for Nullable concept to appear in D...
• Jarrett Billingsley (6/19) Jan 15 2009 How do you distinguish between the key being missing and there being a
• Steven Schveighoffer (23/36) Jan 15 2009 Because you can change the value, not what the value poitns to. For

"Denis Koroskin" <2korden gmail.com> writes:

I think I have found one more reason for Nullable concept to appear in D:

Since SafeD disallows pointers, it needs something safe to replace them.

For example, it is impossible to use AA in SafeD, because opIn_r returns a
pointer:

 Foo[Bar] aa;
 
 Bar bar = new Bar();
 Foo* fooPtr = bar in aa; // this line is disallowed in SafeD

I wonder why it returns a pointer, and shouldn't it return Nullable!(Foo) (or
just Foo? as an alias) instead?

 Foo? foo = bar in aa; // null, if item is missing

Note that now it became slightly more efficient, because we don't need an
additional pointer dereference, which was present when we returned Foo*!

"Jarrett Billingsley" <jarrett.billingsley gmail.com> writes:

On Thu, Jan 15, 2009 at 2:38 AM, Denis Koroskin <2korden gmail.com> wrote:
 I think I have found one more reason for Nullable concept to appear in D:

 Since SafeD disallows pointers, it needs something safe to replace them.

 For example, it is impossible to use AA in SafeD, because opIn_r returns a
 pointer:

 Foo[Bar] aa;

 Bar bar = new Bar();
 Foo* fooPtr = bar in aa; // this line is disallowed in SafeD

 I wonder why it returns a pointer, and shouldn't it return Nullable!(Foo)
 (or just Foo? as an alias) instead?

 Foo? foo = bar in aa; // null, if item is missing

 Note that now it became slightly more efficient, because we don't need an
 additional pointer dereference, which was present when we returned Foo*!

How do you distinguish between the key being missing and there being a
null reference in the AA?

Also, if you have an AA like:

Foo?[Bar] aa;

What is the return type of in?  "Foo??" ?

"Steven Schveighoffer" <schveiguy yahoo.com> writes:

"Denis Koroskin" wrote
I think I have found one more reason for Nullable concept to appear in D:

 Since SafeD disallows pointers, it needs something safe to replace them.

 For example, it is impossible to use AA in SafeD, because opIn_r returns a 
 pointer:

 Foo[Bar] aa;

 Bar bar = new Bar();
 Foo* fooPtr = bar in aa; // this line is disallowed in SafeD

 I wonder why it returns a pointer, and shouldn't it return Nullable!(Foo) 
 (or just Foo? as an alias) instead?

Because you can change the value, not what the value poitns to.  For 
example, in an AA, if I want to replace the object found at a specific key, 
I need a reference to the class reference, i.e. a pointer.

I think what might be useful is to make references nullable (and have some 
operator to determine if that is the case), and use references instead of 
pointers wherever a null return value is a possibility.  The danger of 
pointers is that you can willy-nilly point them to any memory location. 
References stick to where they were set (and in SafeD, cannot be created 
with pointer arithmetic).

so something like:

ref Foo fooPtr = bar in aa;
if(fooPtr !is nullref) // new keyword
{
 ...
}

or

if(fooPtr !refis null)

or with no new keywords:

if(ref fooPtr !is null)

 Foo? foo = bar in aa; // null, if item is missing

 Note that now it became slightly more efficient, because we don't need an 
 additional pointer dereference, which was present when we returned Foo*!

Then how do you change the value at that location?  To further demonstrate 
the point, imagine Foo is a struct, not a class.

-Steve