PL/SQL difference between DEFAULT and assignment operator - oracle

Question

I'm newbie in PL/SQL and this question might seem to be 'childish' so I'm sorry in advance, but Google didn't help me at all...
Is there any difference between following procedures?
Procedure p1(a Number DEFAULT 0) Is
Begin
DBMS_OUTPUT.put_line(a);
End;
Procedure p2(a Number := 0) Is
Begin
DBMS_OUTPUT.put_line(a);
End;
I have checked them, so when I call both of them without args the output is 0, but I'm not sure if there are any side effects.

Answer 1

If you follow Oracle's documentation
You can use the keyword DEFAULT instead of the assignment operator to
initialize variables. You can also use DEFAULT to initialize
subprogram parameters, cursor parameters, and fields in a user-defined
record.
Use DEFAULT for variables that have a typical value. Use the
assignment operator for variables (such as counters and accumulators)
that have no typical value.
DECLARE
blood_type CHAR DEFAULT 'O'; -- Same as blood_type CHAR := 'O';
hours_worked INTEGER DEFAULT 40; -- Typical value
employee_count INTEGER := 0; -- No typical value
BEGIN
NULL;
END;
/
So I guess internally is the same.

Answer 2

Let's take a look to the documentation of the latest Oracle release (12c R1) at the moment of writing. You explicitly ask for subprogram parameters so let's take that first.
Default Values for IN Subprogram Parameters:
When you declare a formal IN parameter, you can specify a default value for it. A formal parameter with a default value is called an optional parameter, because its corresponding actual parameter is optional in a subprogram invocation. If the actual parameter is omitted, then the invocation assigns the default value to the formal parameter.
The documentation doesn't mention default keyword but it still works. Example (in 12c R1):
declare
function f1(a in number) return number is begin return a; end;
function f2(a in number := 2) return number is begin return a; end;
function f3(a in number default 3) return number is begin return a; end;
begin
dbms_output.put_line(f1(1));
dbms_output.put_line(f2);
dbms_output.put_line(f3);
end;
/
Prints 1, 2, 3 as expected.
However as it's not mentioned in the authorative documentation I discourage the use of default keyword in this context.
Other interesting context is Initial Values of Variables and Constants:
To specify the initial value, use either the assignment operator (:=) or the keyword DEFAULT, followed by an expression.
And that's the only time default keyword is mentioned. All documentation examples use assigment only.
Conclusion
Technically default keyword and assigment are the same and work in both contextes, but only assigment is promoted in the documentation. I think Oracle is effectively deprecating the default keyword in this context and thus I won't recommend the use of it in new PL/SQL code. Assigment operator makes the same job with zero fuss.