[fpc-pascal] Default record fields

[Karoly Balogh (Charlie/SGR)]

Hi,

On Fri, 22 Jun 2018, Ryan Joseph wrote:

> I want to do a pivot away from the macro stuff to ask another question.
> Since I’ve wanted to contribute to the compiler for so long and I
> finally have a little understanding I’d like to know if there’s anything
> minor I could do, that isn’t offensive to the compiler team.

<rant>It's not about the compiler team. It's about the integrity of a
programming language, which doesn't matter these days a lot, when all
languages turned into a feature race to provide the same broken concepts,
but with slightly different syntax. Nevermind. I sometimes do not even
understand how people did stuff for 50 years without
TObjectClassAdvancedTemplateGenericRecordRTTI...</rant>

> What comes to mind first as being minor (or not, I don’t know) and
> important are default record fields. Is there are reason this never
> existed in Pascal? It’s in C++, C#, Swift etc…. and I use it often.

Because C# and Shift are managed languages, and C++ just includes
everything for no good reason. And this is actually major, with far
fetching implications. C++ is an utter mess for this, with the default
initializer sometimes called and sometimes not, when you declare a
variable of this type. Because if you pull the way computers work into the
equation, it complicates things a lot. It matters where you define a
variable of this type. On the heap, on the stack or on the global variable
list, you have to compile different code for this, and you are probably
won't be able to cover all corner cases. (Eg. what if you allocate this
type with GetMem(sizeof(type))?)

> The compiler added advanced records but making a static class method
> which is hidden down in the implementation is annoying boiler plate I’m
> sick of doing (plus calling it, or forgetting to). The new initialize
> operator is the best alternative but again it’s boiler plate and adds
> methods in the implementation.
>
> Is there any reason we can’t do that? Is it a design choice?

It's a design choice.

In general, could you stop trying turning Pascal into C/C++? What C++
provides is mostly what is provided by advanced records (the useful
subset). Except C++ allows writing the default initializer into the type
declaration which then may or may not get executed, because C++ is a
godforsaken mess.

What you can also do is, to declare the type, then declare a set of
defaults with "const", which you can also write into the interface
section. Then if you need a certain initialization for that type, just do
it with an assignment.

var
  myvar: mytype;

const
  myvardefault: mytype = <initializer>;

begin
  myvar:=myvardefault;
end;

In this case the compiler will even warn you, if you A., forgot to
initialize the variable B., forgot to initialize a field in the
initializer. I'm using similar pattern in my 3D engine, where I have
various set of default vertices and matrices defined as consts, which are
still the same type, and i can initialize them this way to whichever
default I need..

If you need huge amounts of this construct, I would advise you to generate
it as Michael said, which is what you would do with extended preprocessing
or similar syntax sugar, like this one. It's just implicit code
generation.

Charlie