如何在函数体的操作部分不赋值的情况下从函数自动返回“PI”的值

Question

我的代码在下面，我将不胜感激任何建议

(* //const
//pi=3.1415926;
//uses
//mathh.inc; *)
var
r,pole_kola,obwod_kola: real;

function Pi: valreal;
begin
 Pi:=3.1415926;
end;

procedure dane();
begin
  read(r);
end;

procedure obliczenia();
begin
  pole_kola:= {pi}Pi*r*r;
  obwod_kola:= 2*{pi}Pi*r;
end;

procedure wyniki();
begin
  writeln('pole koła: ',pole_kola:4:8);
  writeln('obwód koła: ',obwod_kola:4:8);
end;

begin
  writeln('podaj promien r: ');
  dane();
  obliczenia();
  wyniki();
end.

我如何使用函数 Pi :
https://www.freepascal.org/docs-html/rtl/system/pi.html

如果我尝试修改函数返回，则从函数返回“PI”的自动值而不在函数体的操作部分赋值

Function result does not seem to be set

function Pi() :valreal;

begin

end;

begin
    WriteLn('pi = ', Pi():1:20);
end.

Compiling main.pas
main.pas(1,10) Warning: Function result does not seem to be set
Linking a.out
8 lines compiled, 0.1 sec
1 warning(s) issued
pi = 0.00000000000000000000

在节目中

 ./main
podaj promien r: 
6
pole koła: 0.00000000
obwód koła: 0.00000000
