Let's see. How many of these languages do you think use named parameters at the call site?:

I think that some time ago, someone found that they could do it. That a combination of Perl's syntax and hashes meant that it was possible. And kinda cute. And for complex constructors with lots of possible parameters, many optional, it makes a certain amount of sense. You mostly don't call heavy constructors in tight loops so there's no great harm in using it. For constructors.

Mostly, it is just as hard to look up the naming and spelling and casing conventions of named parameters when writing the calls,

and just as hard to interpret the meaning of those names when reading them.

Yeah, there's a much better and cheaper way - don't name them, name the indices into @_ via constant subs, if you need names instead of numbers for sake of code clarity:

sub FOO () { 0 }
sub BAR () { 1 }

sub routine {
    my $bar = $_[BAR];
    $bar   += munge( $_[FOO] );
}
[download]

But it is crucial for that discussion to identify when it is beneficial to use named parameters, and why. I can think of:

• frameworks - you write code that gets called, and there's a convention for what each call brings along. POE is a good example
• looking up a subroutine or method - you want to make use of some subroutines you use seldom, and a quick glance should suffice to know what it needs
• myriads of options - but mostly you need just a few of them. Tk is a good example for that

All other reasons seem to be based on gusto. But then, in early perl OO, objects were mostly blessed hashrefs (tutorials and perl pods are full of them), and much unreflected use of named parameters stems from there, I guess.

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

Yeah, there's a much better and cheaper way - don't name them, name the indices into @_ via constant subs,

Yeah! I demonstrated the potential for that back in Micro optimisations can pay off, and needn't be a maintenance problem & despite the jury's verdict, the performance gains attributable to minimising subroutine call overhead in that type of cpu intensive, heavily iterative (3d graphics with hidden line removal) are distinctly measurable.

I'd only use this for subs that are unavoidably called at the centre of several levels of loop. The 2D & 3D Vector classes in the code in Re: Re^2: Micro optimisations can pay off, and needn't be a maintenance problem (I don't believe it) are a perfect example of the sort of code that can benefit from this technique.

Any particular reason for using constant subs? It achieves the same thing, but to me use constant is just clearer of the intent and saves a little typing:

use constant { FOO => 0, BAR => 1 };
[download]

What would be really cool is for an alternative sub declaration syntax that declared scoped constants for subs. Eg.

sub Point3D::new( CLASS, X, Y, Z ) {
    return bless [ @_[ X, Y, Z ] ], $_[ CLASS ];
}
[download]

---

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

"Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."

Any particular reason for using constant subs?

Well, er... it's always the same reason for not using a module: I happily hack along, until... it gets boring.

"Hey, that's boring!", I begin thinking, "isn't there a module?" ;-)

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

But for most general purpose subroutines and method calls, the need for named parameters--ie. calls that take so many arguments that naming them is beneficial beyond an aid memoire for the casual tourist to the code--is strongly indicative of something seriously wrong in the design of the API.

I respectfully disagree.

Named parameters means I don't have to pass a string of undefs because one particular call doesn't use those parameters. APIs using positional parameters have a way of requiring difficult upgrade path.

It's also self-documenting -- instead of a list of variables, each variable is named, which can only help the future software forensic expert.

Many years ago, I wrote a User Interface program in C, and one of the things that I used was lots of parameter passing, knowing enough that global variables were not the answer. Eventually, I had a couple of routines that required a dozen or so parameters, and as the code matured into a lovely congealed mass of spaghetti, I began to dread getting in there to fiddle with calls to that code, precisely because I had to add 'just one more' parameter at the end.

The alternative could have been to pass in a pointer to a struct, which is more or less a hashref, but I wasn't secure enough in my abilities to do that. Too bad, because it would have been the right thing to do, just as using a hashref is the right thing to do.

Named parameters means I don't have to pass a string of undefs because one particular call doesn't use those parameters. APIs using positional parameters have a way of requiring difficult upgrade path.

I personally believe that named parameters can indeed be very useful. In this sense Perl 6 with its extremely complete and flexible sub signatures is fantastic. Perl 5 is also charming for the far reaching semantics it can get out of its very simple mechanism of parameter passing, allowing one to emulate named parameters.

However, as far as your remark about "a string of undefs" (I presume you really mean "list") is concerned, I would like to point out that while the fact that several commas "collapse" into one fits perfectly well into Perl's semantics, I have occasionally desired say $x,,,,$y to be a shortcut for $x,undef,undef,undef,$y.

--
If you can't understand the incipit, then please check the IPB Campaign.

I have occasionally desired say $x,,,,$y to be a shortcut for $x,undef,undef,undef,$y.

Be happy that you need the undef! Counting the nothings between the commas must be one of the most frustrating exercises ever devised.

CountZero

A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James

Brilliant. Yet another clever way that Perl takes care of things automagically.

I hadn't realized that, because I wasn't thinking Perlishly enough. Thank you.

Alex / talexb / Toronto

"Groklaw is the open-source mentality applied to legal research" ~ Linus Torvalds

Named parameters means I don't have to pass a string of undefs because one particular call doesn't use those parameters.

I don't suppose you have any concrete examples you'd care to share?

APIs using positional parameters have a way of requiring difficult upgrade path.

And yet, other than tcl, I can't find reference to a single other language that has felt the need to implement named parameters?

Don't take me wrongly. The are absolutely some calls in many APIs (from many languages) that would benefit from this kind of self documentation.

• CreateWindow() with its 11 parameters, some of which are themselves structs or bit-fileds is an obvious candidate.
• CreateFile() with its 7 parameters including 4 bit-fields is another.

  But by and large, most of them are constructors. And where APIs regulary require the user to supply a list of undefs in order to use the call, architypically select undef,undef,undef, 0.1; these are generally and widely acknowledged, even by their authors, as being "ones that got away".

  With most functions that sometimes require more than 3 parameters, there is a 'natural ordering' that means that any omitted parameters will come at the end. Eg. substr, splice, read. Even in a function rich API like Perl's there are suprisingly few calls that require more than 3 args, and almost none that require the use of placeholders for distinct functionality.

  And that's the clue for me. If an API (beyond constructors), cannot be designed such that any omitted arguments fall at the end, then it is really two (or more) apis that have been conflated. select is the prime example as noted above, and it isn't hard to see how to change that:

  • my $old = setStdout( $new );

    sub setStdout {
        my $new = shift;
        return select( $new );
    }
    [download]

  • usleep( 0.1 );

    sub usleep {
        my $time = shift;
        return select undef, undef, undef, $time;
    }
    [download]

  • select $read, $write, $error, $time );

    Of course, IO::Select does a much better job of dealing with this form.

  ---

  Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

  "Science is about questioning the status quo. Questioning authority".

  In the absence of evidence, opinion is indistinguishable from prejudice.

  "Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."

I don't suppose you have any concrete examples you'd care to share?

Nope -- none to hand. That doesn't lessen my assertion that named parameters are a fine alternative to positional parameters, for the reasons I've already listed. I will modify that by saying that if there are just a few parameters to a function, positional parameters will work fine, but if there's a chance that some of the parameters might be optional, a hashref of named parameters is the way to go.

Alex / talexb / Toronto

"Groklaw is the open-source mentality applied to legal research" ~ Linus Torvalds

And yet, other than tcl, I can't find reference to a single other language that has felt the need to implement named parameters?

I can think of Common Lisp, Ruby, and Python off the top of my head...