Final by default?

[Ola Fosheim Grøstad" <ola.fosheim.grostad+dlang at gmail.com>]

On Monday, 17 March 2014 at 06:26:09 UTC, Marco Leise wrote:
> About two years ago we had that discussion and my opinion
> remains that there are too many "if"s and "assume"s for the
> compiler.
> It is not so simple to trace back where an object originated
> from when you call a method on it.

It might not be easy, but in my view the language should be 
designed to support future advanced compilers. If D gains 
traction on the C++ level then the resources will become 
available iff the language has the right constructs or affords 
extensions that makes advanced optimizations tractable. What is 
possible today is less imoortant...

  >It could be created though
> the factory mechanism in Object using a runtime string or it

If it is random then you know that it is random. If you want 
speed you create separate paths for the dominant instance types. 
Whole program optimizations is guided by profiling data.


> There are plenty of situations where it is virtually
> impossible to know the instance type statically.

But you might know that it is either A and B or C and D in most 
cases. Then you inline those cases and create specialized 
execution paths where profitable.

> Whole program analysis only works on ... well, whole programs.
> If you split off a library or two it doesn't work. E.g. you
> have your math stuff in a library and in your main program
> you write:
>
>   Matrix m1, m2;
>   m1.crossProduct(m2);
>
> Inside crossProduct (which is in the math lib), the compiler
> could not statically verify if it is the Matrix class or a
> sub-class.

In my view you should avoid not having source access, but even 
then it is sufficient to know the effect of the function. E.g. 
you can have a high level specification language asserting pre 
and post conditions if you insist on closed source.

>> With a compiler switch or pragmas that tell the compiler what 
>> can be dynamically subclassed the compiler can assume all 
>> leaves in the compile time specialization hierarchies to be 
>> final.
>
> Can you explain, how this would work and where it is used?

You specify what plugins are allowed to do and access at whatever 
resolution is necessary to enable the optimizations your program 
needs?

Ola.