Chapter 6: Subroutines

6.4. Prototypes

Perl lets you define your own functions to be called like Perl's built-in functions. Consider push(@array, $item), which must tacitly receive a reference to @array, not just the list values held in @array, so that the array can be modified. Prototypes let you declare subroutines to take arguments just like many of the built-ins, that is, with certain constraints on the number and types of arguments. We call them "prototypes", but they work more like automatic templates for the calling context than like what C or Java programmers would think of as prototypes. With these templates, Perl will automatically add implicit backslashes, or calls to scalar, or whatever else it takes to get things to show up in a way that matches the template. For instance, if you declare:

sub mypush (\@@);

Since prototypes are taken into consideration only at compile time, it naturally falls out that they have no influence on subroutine references like \&foo or on indirect subroutine calls like &{$subref} or $subref->(). Method calls are not influenced by prototypes, either. That's because the actual function to be called is indeterminate at compile time, depending as it does on inheritance, which is dynamically determined in Perl.

Since the intent is primarily to let you define subroutines that work like built-in functions, here are some prototypes you might use to emulate the corresponding built-ins:

Any backslashed prototype character (shown between parentheses in the left column above) represents an actual argument (exemplified in the right column), which absolutely must start with that character. Just as the first argument to keys must start with %, so too must the first argument to mykeys.

A semicolon separates mandatory arguments from optional arguments. (It would be redundant before @ or %, since lists can be null.) Unbackslashed prototype characters have special meanings. Any unbackslashed @ or % eats all the rest of the actual arguments and forces list context. (It's equivalent to LIST in a syntax description.) An argument represented by $ has scalar context forced on it. An & requires a reference to a named or anonymous subroutine.

A * allows the subroutine to accept anything in that slot that would be accepted by a built-in as a filehandle: a bare name, a constant, a scalar expression, a typeglob, or a reference to a typeglob. The value will be available to the subroutine either as a simple scalar or (in the latter two cases) as a reference to the typeglob. If you wish to always convert such arguments to a typeglob reference, use Symbol::qualify_to_ref as follows:

use Symbol 'qualify_to_ref';

sub foo (*) {
    my $fh = qualify_to_ref(shift, caller);
    ...
}

That is, if you say:

mytime +2;

The mygrep example also illustrates how & is treated specially when it is the first argument. Ordinarily, an & prototype would demand an argument like \&foo or sub{}. When it is the first argument, however, you can leave off the sub of your anonymous subroutine, and just pass a bare block in the "indirect object" slot (with no comma after it). So one nifty thing about the & prototype is that you can generate new syntax with it, provided the & is in the initial position:

sub try (&$) {
    my ($try, $catch) = @_;
    eval { &$try };
    if ($@) {
        local $_ = $@;
        &$catch;
    }
}
sub catch (&) { $_[0] }

try {
    die "phooey";
}                   # not the end of the function call!
catch {
    /phooey/ and print "unphooey\n";
};

[3] Yes, there are still unresolved issues having to do with the visibility of @_. We're ignoring that question for the moment. But if we make @_ lexically scoped someday, as already occurs in the experimental threaded versions of Perl, those anonymous subroutines can act like closures.

Here's a reimplementation of the grep operator (the built-in one is more efficient, of course):

sub mygrep (&@) {
    my $coderef = shift;
    my @result;
    foreach $_ (@_) {
        push(@result, $_) if &$coderef;
    }
    return @result;
}

6.4.1. Inlining Constant Functions

Functions prototyped with (), meaning that they take no arguments at all, are parsed like the time built-in. More interestingly, the compiler treats such functions as potential candidates for inlining. If the result of that function, after Perl's optimization and constant-folding pass, is either a constant or a lexically scoped scalar with no other references, then that value will be used in place of calls to that function. Calls made using &NAME are never inlined, however, just as they are not subject to any other prototype effects. (See the use constant pragma in Chapter 31, "Pragmatic Modules", for an easy way to declare such constants.)

Both version of these functions to compute will be inlined by the compiler:

sub pi ()           { 3.14159 }             # Not exact, but close
sub PI ()           { 4 * atan2(1, 1) }     # As good as it gets

sub FLAG_FOO ()     { 1 << 8 }
sub FLAG_BAR ()     { 1 << 9 }
sub FLAG_MASK ()    { FLAG_FOO | FLAG_BAR }

sub OPT_GLARCH ()   { (0x1B58 & FLAG_MASK) == 0 }
sub GLARCH_VAL ()   {
    if (OPT_GLARCH) { return 23 }
    else            { return 42 }
}

sub N () { int(GLARCH_VAL) / 3 }
BEGIN {                 # compiler runs this block at compile time
    my $prod = 1;       # persistent, private variable
    for (1 .. N) { $prod *= $_ }
    sub NFACT () { $prod }
}

If you redefine a subroutine that was eligible for inlining, you'll get a mandatory warning. (You can use this warning to tell whether the compiler inlined a particular subroutine.) The warning is considered severe enough not to be optional, because previously compiled invocations of the function will still use the old value of the function. If you need to redefine the subroutine, ensure that it isn't inlined either by dropping the () prototype (which changes calling semantics, so beware) or by thwarting the inlining mechanism in some other way, such as:

sub not_inlined () {
    return 23 if $$;
}

6.4.2. Care with Prototypes

It's probably best to put prototypes on new functions, not retrofit prototypes onto older ones. These are context templates, not ANSI C prototypes, so you must be especially careful about silently imposing a different context. Suppose, for example, you decide that a function should take just one parameter, like this:

sub func ($) {
    my $n = shift;
    print "you gave me $n\n";
}

func @foo;                  # counts @foo elements
func split /:/;             # counts number of fields returned
func "a", "b", "c";         # passes "a" only, discards "b" and "c"
func("a", "b", "c");        # suddenly, a compiler error!

If you're writing new code and would like a unary operator that takes only a scalar variable, not any old scalar expression, you could prototype it to take a scalar reference:

sub func (\$) {
    my $nref = shift;
    print "you gave me $$nref\n";
}

func @foo;              # compiler error, saw @, want $
func split/:/;          # compiler error, saw function, want $
func $s;                # this one is ok -- got real $ symbol
func $a[3];             # and this one
func $h{stuff}[-1];     # or even this
func 2+5;               # scalar expr still a compiler error
func ${ \(2+5) };       # ok, but is the cure worse than the disease?

6.3. Passing References		6.5. Subroutine Attributes

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