NULLs

[Hans-Dieter.Dreier@materna.de]

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(Forwarding a message from jason@injektilo.org)



> Keeping track of possible types is rather easy because the type set =

can be =3D
> easily handled. Keeping track of possible values is hard; so it =

seems sensi=3D
> ble to me to try to map as much as possible of the checking =

sketched above =3D
> to type checking.
> =


I agree. If you're going to support preconditions, you might as well =

let those do the checking for you. I think that it usually won't be =

too obvious what the value of a variable is. Unless, of course, it =

was a constant. Either way, it's a feature that could be optionally =

added to the compiler and not affect the language in any way.

> >I think that it'd be possible to offer a higher-level construct =

for =3D
> =

> >this than checking to see if a pointer equaled NULL, though. Maybe =

a =3D
> =

> >defined keyword? Would this fit with the static type system? Of =3D
> =

> >course, it would probably be implemented by checking to see if the =

=3D
> =

> >reference was set to NULL but we don't need to know that.
> =

> Could you show an example?
> =


void Foo(Bar* pBar)
{
  if(valid(pBar))
  {
    // pBar points to a valid Bar object.
  }
}

valid() could be keyword much like sizeof()in C++. The compiler would =

require you check to make sure that a pointer was valid before =

allowing you to use it unless of course you're going to support some =

sort of precondition clauses like eiffel. If a precondition was that =

the pointer had to be valid, then you wouldn't need to check it in =

your function body.

I'm actually starting to like your idea of union types. They might =

come in handy for functions that could possibly work on two disparate =

types. Should it be possible to define a union of types and use that =

union as the type?

For example:

union MyUnion : Foo, Bar, Baz;

void doSomething(MyUnion* xxx)
{
  // now you have to do some sort of switch on the type of xxx
}

Should you required that you declare all pointers as a union between =

the type you want and the NULL type or should that be implied?

By the way, did you know that all of your equal ("=3D") signs are being =

translated into "=3D3D" for some strange reason?

Bye,
Jason.



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