How to automatic return value of "PI" from function without assign in operation part of function body - function

Question

My code is under , i will be gratefull for any suggestion
(* //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.
How i can use function Pi :
https://www.freepascal.org/docs-html/rtl/system/pi.html
to return automatic value of "PI" from function without assign in operation part of function body if i try modify function get back
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
in program
 ./main
podaj promien r:
6
pole koła: 0.00000000
obwód koła: 0.00000000


Answer 1

In the task, I wanted to automatically use a ready value for PI (3.14…) without using my function. My function didn’t returned a value because I didn’t assigned one. Like we see here:
function Pi() :valreal;
begin
//here is nothing but must be returned a value
end;
begin
WriteLn('pi = ', Pi():1:20);
end.
Going by #derpirscher’s comment, the function written by hand always needs to return something. So I commented part of my syntax, and used the built-in function named PI. (Pascal includes that function.)
(* function pi: valreal; // If I define my own function, it must return a value
begin
pi:=3.1415926; // So in the body of the function, we must assign value
end; *)
We see that here
procedure obliczenia();
begin
pole_kola:= {pi}Pi*r*r; // using build in function
obwod_kola:= 2*{pi}Pi*r; // as above
end;
If we need to use the value of PI in our task/homework, we can use predefinied built-in functions because it is easier; it is good practice to use less syntax in our code.
Must remember: If we define a function i.e., named Pi ourselves, it has to return a value.
Under the comment, the entire syntax of my code with corrections:
var
r,pole_kola,obwod_kola: real;
(* function pi: valreal; // If I define my own function, it must return a value
begin
pi:=3.1415926; // So in the body of the function, we must assign value
end; *)
procedure dane();
begin
read(r);
end;
procedure obliczenia();
begin
pole_kola:= {pi}Pi*r*r; // using build in function
obwod_kola:= 2*{pi}Pi*r; // as above
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.