FMTEYEWTK on Prototypes in Perl

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Tom Christiansen

Jul 21, 1999, 3:00:00 AM7/21/99
The major issue with "prototypes" in Perl is that the experienced
programmer comes to Perl bearing a pre-conceived but nevertheless
completely rational notion of what a "prototype" is and how it works,
yet this notion has shockingly little in common with what they really
do in Perl.


Nearly any programmer you encounter will, when asked what function
prototypes are for, report the standard text-book answer that function
prototypes are mainly used to catch usage errors at compile time
in functions called with unexpected numbers or types of parameters.
This is what programmers are expecting of prototypes, and what Perl does
not give them. In some ways, it can't.

A few respondents might further observe that prototypes in some
circumstances may permit the compiler to generate better code, or even
more code that is more correct. The classic example of the latter
situation is C<float f = sqrt(2)>. Without the prototype of C<double
sqrt(double n)> in scope, the compiler not only thinks that sqrt()
returns an int, it also thinks that that "2" above should be passed
into the function as an int rather than as a double, thereby generating
incorrect machine code. The prototype quietly fixes this, and probably
forbids or at least complains about passing in anything other than a
single number, such as two strings or nothing at all.


With that in mind, let's look at Perl's "prototypes". One can
argue that rather less misunderstanding might have arisen had Larry
historically chosen to call these "parameter context templates" rather
than "prototypes".

These mostly do nothing more that provide an implicit context coercion
in order to spare the caller from having to sprinkle the code with
calls to scalar() or to supply backslashes in order to pass aggregates
by reference. They do comparatively little in the way of checking the
type or number of arguments. So just what good are they, anyway?

They're good for creating user-defined functions that behave in much
the same way that Perl's own built-in functions behave with respect to
their effects upon the parser and upon implicit contexts. This has two
benefits: one to allow you to omit parentheses; the other to allow you,
nay *require* you, to omit a backslash.

For example, the built-in function time() is one that, unlike most new
functions devised by members of this august body, brooks no argument.
That means that writing

$x = time + 20_000_000;

is really the same as writing

$x = time() + 20_000_000;

The parser itself knows not to look for arguments. Perl gained support
for "prototypes" for precisely this very situation. The results of
translating cpp code via h2ph was wont to take something like

#define NATALITY 31203691

and convert that C code into Perl as:

sub NATALITY { 31203691 }

The catastrophic problem is that this no longer behaves as simple
token-for-token replacement where one terminal is replaced by a different
one without any effect on the surrounding code. Instead what happens is
that AF_INET becomes a Perl function, which, like all user-defined Perl
functions in the absence of "prototypes", is by nature a variadic one.
This produces a significantly different parse:

$x = NATALITY + 1;

silently becomes not

$x = NATALITY() + 1;

but rather

$x = NATALITY(+1);

Another undesirable consequence is that it thrusts you fully into the
quandary of not knowing whether to expect a terminal or not. That means
that several tokens, such as "<", "<<", and "/", all become ambiguous.
The "<" could be the binary infix numeric less-than operator, or it could
be the left-hand component of the circumfix readline operator. The "<<"
could be the binary infix bitwise left-shift operator, or it could be
the start of here-document. The "/" could be the binary infix numeric
division operator, or it could be the left-hand component of pattern match
quote operation. When you had something like this, you couldn't do simple
things in simple ways, and it confused people. They aren't used to having

if (NATALITY < 10)

be a syntax error. (The fact that it remain such in innumerable
other situations such as print() or length() is little consolation.)

So that's why Larry introduced "prototypes" into Perl. In particular,
the void "prototype"

sub NATALITY() { 31203691 }

cures this particular unpleasantry.

You see, "prototypes" were really a bug fix. Since Larry had already
started down this path--or, if you would, slope--he kept on going
by permitting user-defined functions to have (some of) the sorts of
parameter context templates long enjoyed by built-in functions.

Besides functions of no parameters--I'd call them void functions but that
threatens confusion between the input values and the output values--the
other two main flavors of parameter context templates are those that
take one input and those that can take many. Built-in functions
that manifest these two different behaviours are rand() and unlink()
respectively. Sometimes these are called "named unary operators" and
"named list operators".

So now we can classify all user-defined functions and most built-ins
into one of three possible sorts, depending on their parameter context
templates. There is a certain elegance here. The subroutine can through
its "prototype" tell its callers (and the compiler) whether it wants
zero, one, or many input values. The caller can communicate its desire
to receive zero, one, or many output values back from the subroutine.
Since zero, one, and as-many-as-I-want are the three nice numbers in
programming, this holds substantial aesthetic appeal.

These parameter context templates have both compile-time effects and
run-time effects. The compile-time effect of "void input" functions
has already been shown using time(). The monadic functions--that is,
the named unary operators--also affect the parse. This code

@a = (rand 10, 20);

will put two elements into the array, because it implicitly parses as

@a = (rand(10), 20);

That's because somewhere (, ultimately) there's a parameter
context template for rand() that sets up the function to act like

sub rand($);

The parser knows that this function is expecting one and only one
argument. That means that

@nums = (rand 10, rand 10, rand 10);

is really

@nums = (rand(10), rand(10), rand(10));

rather than

@nums = (rand(10, rand(10, rand(10))));

which is what it would have been if rand() had been a variadic function
instead of a monadic one.

A scalar context template has another effect. It causes an expression
evaluated to supply the monadic function's input value to be evaluated
in scalar context. That means that at run-time, wantarray() will now
return false. This way this code:

$x = rand fn();

is really

$x = rand(scalar(fn()));

but only because of the scalar "prototype".

Is this kind of thing is of any practical use? Perhaps. One example
would be:

socket(Sock, PF_INET, SOCK_STREAM, getprotobyname('tcp'))

The built-in socket() function is not a variadic one. It has a
particular parameter context template (a.k.a. "prototype") that assures
that getprotobyname() *shall* be called in scalar not list context.
This makes getprotobyname() return a single value instead of a list
of values.

As with socket(), which takes four scalar (I'm fudging a bit; the first is
a handle) arguments, you yourself can create functions that take several
scalar arguments. For example, the built-in dyadic function atan2() (or
prefix named binary operator, I suppose) has an effective "prototype" of:

sub atan2($$);

However, unlike monadic functions where the parser only gobbles as many
arguments as the function wants, such a "prototype" here will *not*
cause the parser to only grab two arguments. That means that

@a = (atan2 1, 2, 3);

does not become

@a = (atan2(1, 2), 3);

as one would be likely to infer upon learning how rand() works.
Instead, it is a syntax error at compile time. However, there is a
run-time effect. Something like this:

$x = atan2(fy(), fx())

calls both those functions in scalar context, supplying their two single
return values as input to atan2().


I said that "prototypes" can do two things: one, to allow you to dispense
with parentheses, and two, to allow you on occasion to dispense with
a backslash. Let's now look at the second case.

When you specify a "prototype" of $, @, or %, you may also precede that
with a backslash. (There are also "prototype" possibilities of & or *,
but they are not necessary for this discussion.) This parameter context
template means that the programmer must use that exact symbol, and Perl
will then supply the backslash to pass that argument by reference.

For example, suppose you wanted a function that stuck key-value pairs
into a hash, somewhat reminiscent of the way push() places additional
elements into an array. Here's how you'd do that:

sub hpush(\%@) {
my $href = shift; # NB: not %
while ( my ($k, $v) = splice(@_, 0, 2) ) {
$href->{$k} = $v;
hpush(%pieces, "queen" => 9, "rook" => 5);

This works out rather nicely. As you see, here as in so many other areas,
Perl's "prototypes" work out well when they are used for what they were
designed to do--that is, emulate built-ins functions by allowing calls
to a user-defined subroutine to be subject to the same implicit parameter
context conversions as are built-ins.

| BUGS |

So what's the problem? It's not just one, actually. There are rather
more than you probably realize. There are definitely more than someone
who simply hears that Perl has "prototypes" is likely to imagine. I know
of a few bugs, which I'll get out of the way first. These can be fixed.
The design issues are the important matters, and those are discussed
in the next section..

One bug with "prototypes" is that if you call:

$x = fn(@a);
sub fn(\@) { ... }

You get no warning to the effect that Perl already assumed that fn() was
just a standard variadic function; that is, one whose parameter context
template is simply C<sub fn(@)>. This should be reported, much as when
C complains when it catches you using a function without declaring its
return type and thus making the compiler guess the function's return
type to be int, but then you go off and later on in the source declare
the function to be of some other return type.

Another bug is that you can at compile time declare "prototypes"
with multiple backslashes, such as fn(\\@). These are accepted at
compile-time, but at run-time, raise an exception.

That's not the only thing that is silently accepted but completely
useless. Consider

sub fn(@@) { ... }
sub fn(%%) { ... }
sub fn(%$) { ... }
sub fn($%) { ... }
sub fn(@$) { ... }
sub fn($@) { ... }
sub fn(%@) { ... }
sub fn(@%) { ... }

What do those do? They don't raise an exception, but neither will they do
anything useful for you. This will be explained more in the text below.

Finally, there have historically been bugs related to the * and &
"prototypes". I know that Sarathy has worked on at least some of them,
and I am unsure on their exact status.


This section, I imagine, is what you've all been waiting for, and I
commend you for having read everything up to this point. I know people
hate to read, but I felt that without the proper background that I
attempted to provide above, I would not reach as many of you as I would
with it when it came time to explaining the grave problems inherent in
Perl's implementation of "prototypes".

That time is now.

The problems that arise from "prototypes" are many. Some are due to
inappropriate expectations on the part of the users, who for quite obvious
reasons expect Perl's "prototypes" to work like prototypes instead of like
implicit context coercion templates for input parameters to the function.

Sometimes users ask for support of exact prototypes for strings or
numbers or integers or floats or booleans. These requests are reasonably
easy to fend off. You just tell them that a scalar can happily hold
any of these at any time. You can't know from one moment to the next
whether $x contains one or the other of these. Go on to point out
that some things just *have* to be done through run-time assertions or
contract-validations. Like what? Well, such as, oh, a dyadic function
whose arguments should be two integers representing two opposing sides of
a right triangle whose hypotenuse is also an integral number of units.
Or a function that requires a 47-digit prime number as input. :-) Some
things you just have to check at run-time.

You might find yourself on slightly shakier ground when they ask how
to ensure and enforce that arguments be of particular object types.
It's shakier because strict typing is often more important to those
whose first stab at any problem is to throw an object at it (and you
wouldn't want them to think you the problem). But you can still work
your way out of this squeeze if you just remind them that first of all,
Perl is dynamically typed (that's what we did for the previous paragraph)
and that secondly, you should be using the method-call dispatch to
get polymorphism. If the OO folk continue to object, try redirecting
them to the documentation for Damian Conway's Class::MultiMethod module.
This should suffice to give you enough breathing room to make your escape.
Plus it might even solve their problem directly or inspire them to
approach the problem from a completely different direction.

But you can only dodge the more horrific issues for so long. These are
the ones that just seem broken as designed, at least if you're coming
from certain cultural prejudices. And there may not really be much
we can do about these matters, either, because they may be inevitable
consequences of how the Perl language works. These "surprises", or
brokennesses if you would, are side-effects of the Perl's design and
the initial purpose of "prototypes". This puts them somewhere between
difficult and impossible to "fix".

I suppose you could class all these problems into two groups, one
comprising those cases where Perl doesn't do what you want it to, and the
other comprising those cases where Perl does what you don't want it to,
Both are highly annoying.

The first surprise is that when Perl programmers see "$", "@", and
"%", they usually think "scalar", "array", and "hash" respectively.
This isn't completely accurate in all cases, but it is, nevertheless,
what they often think.

So when the user sees a "prototype" of "$", the primrose programming path
leads them to believe, lamentably, that the compiler will complain if
they pass something in that's not a scalar. Nothing could be further
from the truth!

The built-in function length() has a "$" prototype. That doesn't mean
that you can hope for an error if you don't pass in a single scalar value.
It means that whatsoever you pass in *shall* be subtly converted into a
scalar behind your back and under your nose (yes, these sorts of sordid
shenanigans get you both coming and going). This isn't what could even
charitably be referred to as error checking. This is implicit casting
between incompatible types.

@array = (1 .. 10);
print length(@array);

You might think that would be an error, but it's not, because there exists
an implicit coercion rule for arrays taken in scalar context: it's the
number of elements in that array. That number, in this case, is 10.
Now then, what do you imagine the length of 10 to be? That question
doesn't really make much sense as stated (neither did the last one,
though), but it just so happens that you've lucked out again: there's
another implicit coercion rule for treating a number like a string.
That yields "10", a string which you will note is two bytes long.
Thus the answer is 2.

Nifty, eh?

If you think that's bizarre, consider this:

print length(%ENV)

Surely that's a compilation error? Silly programmer, of course it's not!
This is Perl. You'd be astonished at how much sturm und drang the
Perl compiler will put up with--and probably are, on a regular basis.
Since you gave Perl a ridiculous request, PErl dutifully provides you
in return a ridiculous response--but not an error; oh no, not that!
The scalar sense of a hash is string representing its internal fullness.
This might be, for example, "5/32". Noting that this is a string of
five bytes, you can probably by now surmise the printed value: 5.

Nifty, eh?

That means that a function with a scalar prototype does not complain
if something is passed to it that's not a scalar. It simply silently
coerces into something it never was, and in all likelihood, was never
meant to be in the first place.

Now, there are a few rare places where the "$" prototype will actually
catch you making a mistake. Not many, but some. One is when you pass
it a list. Remember that lists and arrays aren't the same; this is
critical in later examples. This

print length("fred", "barney")

will raise a compile-time exception, because you've passed two arguments
to length(), but it wanted only one.

However, not all lists are so fortunate.

sub fn1 { return ("fred", "barney") }
print length fn1();

The answer there is 6. Why is that? Because you returned a list,
which in scalar context ended up being just the last element, "barney",
whose length was 6.

But now try this:

@names = ("fred", "barney")
sub fn2 { return @names }

print length fn2();

This time the answer is 1. Why? Because fn2() was called in scalar
context, and thus @names is in scalar context when it's returned.
There are two elements. The length of "2" is, of course, 1 byte.

So although a literal list can't be brazenly given to length() directly,
placing a list in a function call whose result is passed to length is,
unavoidably, "ok"--or surprising values of "ok". And notice also how
putting an array in the return is completely different from putting a
list there.

But even some literal lists are permitted if supplied as arguments
to length(). Here's one:

print length(@names[1,0])

This is not a compiler error. What's the answer? It's 4, because a
slice is just a list, and a list in scalar context is the last thing,
which in this case is "fred", whose length is 4. You might think that
the compiler would catch this, but it doesn't. And it certainly wouldn't
know what to do with

print length(@names[@indices])

Because there's no way--at least at compile time--to know whether @indices
might contain just one thing, the compiler isn't completely certain
that you're doing something nutty. The maintainers of your code
might be, but the compiler is more, well, permissive.

As you see, this scalar "prototype" is in no way useful for checking
the types or number of arguments, which is the thing virtually everyone
expects a prototype to be useful for. And if you think "$" is bad,
there's no silver lining in the clouds coming over the horizon. The rest
of the "prototypes" aren't any better.

Next let's examine the "@" "prototype". What's that? Is it an array?
No, it's not. It just looks like that. It's merely a list. Is it a
required list? Why no, it's not.

sub fn(@) { ... }

Can be called not only as


but also as

fn($scalar1, $scalar2)

and so on and so forth. The "@" really just says that this is a normal
Perl function, which means it's variadic. It pretty much means nothing
at all.

You could, if you were careful, use this in conjunction with "$", and
then it might have a tiny bit of meaning. For example:

sub fn($@) {
my ($scalar, @array) = @_;
print "Got $scalar and @array\n";

This isn't really much fun, either. It doesn't help you with the
number or types of arguments very much. Oh, calling it with nothing at
all is flagged at compile time, but that's it. These crazinesses are
all permitted, despite the "$@" prototype:

fn( xyz() )
fn( xyz(), xyz() )
fn( xyz(), xyz(), xyz() )
fn($scalar, @array)
fn(@array, @array)
fn(@array, @array, @array)
fn($scalar, %hash)
fn(%hash, %hash)
fn($scalar, @array, %hash)

It sure looks like that "$@" signature there is more trouble than it's
worth, now doesn't it? There's also the issue "@@" is accepted as a
"prototype", but actually means nothing. Or the one that "@$" is
accepted, and again, means nothing--you aren't going to get anything
evaluated in a scalar context that way.

Since we're having so much fun, let's move on to "%". This "prototype"
means what? That we're expecting a hash? Not at all! In fact, it is
completely identical to a "prototype" of just "@". Everything I said
about "@" is true for "%", because they are the same! You can't get
any type checking here. It doesn't even bother to check whether you
have an even number of arguments. Given a "prototype" of

sub fn(%) { }

these are all still licentiously permitted:

fn($scalar1, $scalar2)
fn(@array1, @array2)
fn(%hash1, %hash2)

You get the same issues with "%%", "%$", and "$%" as we saw earlier
with "@" instead of "%".

So you see, just like "$" and "@", "%" cannot be used for checking the
type or number of arguments, since it doesn't care about these matters.
In fact, it really doesn't care about anything at all. It's even worse
than the already useless "@". The "%" is just sitting there as though
it had no other purpose in life but to confuse you. I suspect it may
have succeeded. This is not your fault, though.

What about the reference "prototypes"? At some level, they're more
predictable than "$", "@", "%", which please remember meant scalar, list,
and um, well, list, respectively. You can also use "\$", "\@", and "\%"
to indicate references to scalars, arrays, and hashes, respectively.
(As to why "&" really means reference to function instead of using "\&"
for that, I leave as a meditation for the reader.) Well, I'm afraid
that these, too, may be more trouble than they're worth.

You see, those symbols don't actually say that you must pass in a
scalar reference, an array reference, and a hash reference. Rather,
they say you must pass in a scalar *variable*, an array *variable*,
and a hash *variable*. That means that the compiler insists upon
seeing a properly notated variable of the given type, complete with "$",
"@", or "%" in that slot. You must not use a backslash. The compiler
silently supplies the backslash for you. The hpush() function shown
above demonstrates this kind of thing in action.

To see how this works when you use "\@" in the "prototype", you haven't
declared a function as taking a reference to an array. Rather, you've
declared one that takes an array, which the compiler will pass by
(implicit) reference to you.

There are times when this is annoying. Consider the good old push()
function. Its "prototype" is "\@@", which means that it takes one array
and an optional list as arguments, and that that array shall be passed
by reference. Think of how often you've been forced to do something like

push @{ $hash{$string} }, $value;

Why can't you just do this:

push $hash{$string}, $value;

It's because of the "prototype". You *must* use the "@" sign. Yes,
I know there's probably a reference to an array there, but that's not
what the prototype says. The compiler doesn't want a reference to an
array (contrary to popular misunderstanding). It wants an array, and
you haven't given it one with a real "@" sign.

Passing in more than one aggregate into a Perl function is a problem,
because aggregates interpolate into parameter lists. For example,
C<add_vecpair(@this, @that)> will not normally be able to distinguish
between the first array and the second one.

Let's make a function that takes two arrays of numbers and returns a new
list where each element is the sum of the corresponding elements of the
two input lists. That subroutine definition would look like this:

sub add_vecpair( \@ \@ ) { ....

Make sure that that definition is seen by the compiler before it
compiles any calls to the function. Once this is done, the function can
(and *must*) then be called this way:

@c = add_vecpair(@a, @b);

Technically, once any function's definition has been seen by the compiler,
you don't need to use the parentheses on the call. This is the same

@c = add_vecpair @a, @b;

Neither of these calls looks as though it's passing array references
in, but because of the prototype, they are. The compiler adds the
backslashes for you. This can be annoying when one of the elements
isn't a literal array. For example, under the prototype, this call is
technically illegal, even though it would appear fine:

@c = add_vecpair(@a, [ values %hash ]); # prototype conflict

This is where you find yourself fighting with the fastidious prototype.
Here's how to make it shut up:

@c = add_vecpair(@a, @{ [ values %hash ] } );

Nifty, eh?

This is the same kind of thing you have to do when you use a prototyped
built-in in ways it's not expecting. For example,

if ($x > 10) {
push @a, $value;
} else {
push @b, $value;

Cannot be written as

push $x > 10 ? @a : @b , $value;

But instead requires a rather less obvious indirect approach. The extra
backslash and "@{}" dereferencing are there to keep the push() function's
formal prototype from complaining unnecessarily.

push @{ $x > 10 ? \@a : \@b }, $value;

If the function in question is user-defined instead of built-in, you can
disable the compiler's meddlesome prototype checking just by prefixing
the function call with an ampersand. You'll just have to make sure the
types on the call are right yourself then. For example:

@c = &add_vecpair(\@a, [ values %hash ]); # `&' ignores prototype

If the preceding sequence isn't enough to convince you to avoid
prototypes in most if not all situations. think about this: by enforcing
prototypes, you've broken the beautiful model of functions built to
take or return any number of arguments. It would have been more robust
to have written the function to accept any number of array references,
and sum up the corresponding elements of each. The extra backslash and
"@{}" dereferencing are there to keep the persnickety prototype checks
from carping unnecessarily. But what will you do without prototypes?
You'll just have to make sure the types on the call are right yourself
then, just as you always hae.

For example:

sub add_vecs {
my($vec, @result);
foreach $vec (@_) {
for (my $i = 0; $i < @$vec; $i++) {
$result[$i] += $vec->[$i];
return @result;
@sumvec = add_vecs \@a, \@b, \@c, \@d;
@sumvec = add_vecs \(@a, @b, @c, @d); # same thing

Now you can pass in C<\@foo>, C<[ whatever ]>, or $aref, where that
scalar variable contains a reference to an array. What happens if you
pass in the wrong thing? You take an exception at run time. But this
is the same situation if you were forced to pass in C<@$aref>
instead under a prototype.


So, have Perl's "prototypes" worked out ok? If the goal is to provide
something like what other languages call prototypes, something to let
the compiler catch errors of type and number occurring in calls to
subroutines, then the answer is certainly that they have not.

Of course, you could try to argue that that's not a fair question, since
"prototypes" were really supposed to be context coercion templates,
something to let you emulate a built-in function. This dodges the
fact that Perl's "prototypes" violate the principle of least surprise,
but so be it. Even in this limited capacity, their success has been no
more than limited.

That's because there are still a lot of built-in functions that cannot
be emulated, even given prototypes. There are odd-ball functions, like
defined() and exists() and undef(), all of which impose a type of context
on their arguments that you cannot begin to emulate with existing Perl
"prototypes". You also cannot use these to prototype new pseudo-quoting
functions like m//, s///, tr///, y///, q//, qq//, qx//, qw//, and qr//.

Of greater importance is the large body of functions that you cannot
use Perl to prototype because they include indirect objects in their
signatures. printf() and printf() are a bit annoying, although not for
the insurmountable reason. For example, consider this famous pair's
true prototype definitions from

sprintf sprintf ck_fun_locale mfst@ S L
prtf printf ck_listiob ims@ F? L

The first nastiness is that while

printf @args

is ok, that

sprintf @args

is not. Why? Because the sprintf() function has the compiler enforcing
a scalar context, so it gets passed the number of elements in @args
as the format, leaving the list empty. But printf() doesn't do that.
It takes the format from the first element of the list.

In the case of printf(), the compiler is busy doing something else,
anyway. It's considering whether you supplied the optional filehandle.
You can in theory (modulo bugs) specify a filehandle in a "prototype"
(or at least a typeglob) using the "*" symbol. And you can also specify
optional trailing arguments. But you cannot specify an optional leading
argument, the way print(), printf(), sort(), system(), and exec()
all tolerate.

Even if an optional leading argument were permitted, this will itself just
increase the potential for confusion. That's because this comma-less
argument is really the one that falls in the indirect object slot.
Indirect objects are pretty wicked. They are restricted to BAREWORDS,
simple scalar variables, or {BLOCKS}. That's why you can't say:

printf $FH{$some_name} $some_fmt, @some_data;

And no, the bloated and silly IO::Handle module doesn't "fix" this
in anyway.

If you encourage a prototype for the indirect object, you'll get more
people who will be writing code that uses indirect objects, and more
of them whom this will confuse. And let's not even begin to talk about
the problems of stacking them. It's not a pretty picture.


In summary, it should be no surprise to you who've read this complete note
that I myself do not use prototypes. I hope that now, you'll realize why.

A larger and more pressing question is whether we should
create named parameters. That is, something like

sub func ($this, $that) { ... }

or perhaps even

sub func (@these, @those) { ... }

or more likely

sub func (\@these, \@those) { ... }

My conclusion is that just adding names to Perl's existing "prototypes",
which are really mostly just parameter context templates for implicit
coercions, would be a mistake. It would encourage the use of something
that's extremely confusing at best, and at worst, fundamentally broken
by design.

186,282 miles per second: not just a good idea, it's the LAW!

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