[fpc-devel] case is

[Mattias Gaertner]

Hi,

I just stumbled over the new Java feature "Pattern Matching for switch".
https://openjdk.java.net/jeps/420
IMO it is a misnomer, but it has some interesting ideas.

Basically for Pascal it is a case block using the "is" operator.
Pseudo code:

procedure Fly(o: TObject);
begin
  case o is
    TButton: TButton(o).foo;
    TControl: TControl(o).bar;
    TComponent: TComponent(o).meh;
    nil: Msg;
  else Run;
  end;
end;

The gain versus "if o is then..." is that the compiler warns if the
case statements are not sorted and can optimize the checks.

For example the above code could be converted to:

procedure Fly(o: TObject);
var
  tmp: TClass;
begin
  if o<>nil then
  begin
    tmp:=o.ClassType;
    repeat
      if tmp=TButton then
      begin
        TButton(o).foo;
        break;
      end else if tmp=TControl then
      begin
        TControl(o).bar;
        break;
      end else if tmp=TComponent then
      begin
        TComponent(o).meh;
        break;
      end;
      tmp:=tmp.ClassParent;
      if tmp=nil then
      begin
        Run;
        break;
      end;
    until false;
  end else
    Msg;
end;

What do you think?


Mattias