Set a default parameter for a function??

[Ian Branch 121]

Hi Team,

D12.

I have the following function:

function GetWeekDates(const GivenDate: TDateTime; const SOWDay: string; out startDate, endDate: TDateTime): Boolean;

I want to modify it so that SOWDay has a default value of 'SU' if SOWDay isn't supplied.

As SOWDay must be By-Value or a Constant, I can't do this:

function GetWeekDates(const GivenDate: TDateTime; var SOWDay: string = 'SU'; out startDate, endDate: TDateTime): Boolean;

is what I am after possible?

 

Delete Reference:  I did it with an overloaded function.

 

Regards & TIA,

Ian

Edited November 29, 2023 by Ian Branch

[FPiette 367]

You can specify default parameter values in a procedure or function heading. Default values are allowed only for typed const and value parameters. To provide a default value, end the parameter declaration with the = symbol followed by a constant expression that is assignment-compatible with the parameter's type.  Parameters with default values must occur at the end of the parameter list. That is, all parameters following the first declared default value must also have default values.

function GetWeekDates(const GivenDate: TDateTime; out startDate, endDate: TDateTime; const SOWDay: string = 'SU'): Boolean;

 

[Ian Branch 121]

1 hour ago, FPiette said:

all parameters following the first declared default value must also have default values.

Ahhhhhhh.  That was what I was missing.  I was not aware of that.

Thank you FPiette.

 

I will just have to remember the order of parameters when I call the function.  😉

 

Regards,

Ian

[uligerhardt 18]

If you don't want to change the order of parameters you can use two overloaded  functions like this:

interface

function GetWeekDates(const GivenDate: TDateTime; const SOWDay: string; out startDate, endDate: TDateTime): Boolean; overload;
function GetWeekDates(const GivenDate: TDateTime; out startDate, endDate: TDateTime): Boolean; overload;

implementation

function GetWeekDates(const GivenDate: TDateTime; const SOWDay: string; out startDate, endDate: TDateTime): Boolean;
begin
  //...
end;

function GetWeekDates(const GivenDate: TDateTime; out startDate, endDate: TDateTime): Boolean;
begin
  Result := GetWeekDates(GivenDate, 'SU', startDate, endDate);
end;

Alternatively, drop the overload and use different names like GetWeekDates and GetWeekDatesEx.

[Ian Branch 121]

Tks uligerhart.  That's what I did initially to get it going.

I will adapt to the reordered parameters.

 

Ian

[pmcgee 10]

Not to be negatively critical, but can I suggest that there's a discussion to be had as to whether out parameters should be an out-dated usage style.

 

 

@Ian Branch Can I take it that what you want returned from the function is EITHER a failure condition/error result OR a tuple of (startdate, enddate) ?

 

If so, this is really a function with a single return type ...

 

function GetWeekDates(const GivenDate: TDateTime; var SOWDay: string = 'SU') -> FAIL | (startdate, enddate) 

This could be known as a Nullable type, or Option type ... in Rust it is the Result type.

When you return from this function, you either have (effectively) no result, or an inhabited tuple.

 

There are quite a few published examples in Delphi of Nullable types .. .from Allen Bauer through to Spring4D.  I'm not sure all of them capture the idea in the most functional way.

 

[Ian Branch 121]

Hi pmcgee,

Interesting, never come across them nor can I say I have seen them.

What does FAIL represent?

 

Ian

[pmcgee 10]

Hi Ian.

A nullable or option type basically adds a single possible "uninhabited" state to a return type of any kind.

You can think of a function that can throw an exception in almost exactly the same way ... it has two possible states ... the normal return or the exception.

 

I was operating on the assumption that the boolean you returned from your original function was indicating whether you successfully created a date range.

Edited December 22, 2023 by pmcgee

[Ian Branch 121]

Hi pmvgee,

Your assumption was correct.

This option has fascinated me a little but I can't help wondering if it is technology for the sake of technology...

I can't see 'out' parameters dissappearing in any forseeable future so what advantage do tuples bring to the table/keyboard?

As far as I can see, In order to implement them you need another code set to support them.  I found 'generics.tuples.pas' from Malcolm Groves, on the face of it, it seems a whole lot of code for little gain.

 

Ian