Prototypes serve two purposes in Perl: they give the parser hints on what to expect (for example the (&) prototype means that parser expects a block after the subroutine name, just like map does), and they provide context (a sub with a ($) prototype imposes scalar context onto the argument).

They are not signatures, you can't use them to validate your arguments, unpacking argument lists or anything like that. See perlsub and Far More Than Everything You've Ever Wanted to Know about Prototypes in Perl for more details.

type myfunc (type arg,...)
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int i = 42;
printf ("%s\n",i);
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See the recent thread USAGE OF Prototype for further discussion and some examples of why prototypes might make your life miserable. You may expect that prototypes will help prevent bugs, but unless you know exactly what you are doing and why, prototypes can actually create more bugs than they prevent.

If you still want to use prototypes after you have read that thread and moritz's links above, perlsub has a description of the prototype syntax and some examples.

Best, beth

As an example of the misery you may expect if you approach Perl prototypes with the mindset of a C or C++ programmer, consider the code below and see if you can explain what is happening – and then imagine it is embedded in the middle of thousands of lines of code. Note that the code has no errors and throws no warnings.

>perl -wMstrict -le
"sub S ($$) {
   my ($scalar, @array) = @_;
   return $scalar + $array[0];
   }
 my @ra = (9, 8, 7, 6);
 print S(3, @ra);
"
7
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A Perl prototype is only marginally useful.
There is no "type checking".

One thing that a prototype can do is to allow you to call a user sub without () around the args...sort of mimicking a Perl built in function. I personally think this is a bad idea and use of prototype. I always enclose args in () for my functions.

Play with the below code which just shows a simple scenario with $ representing a scalar value.

#!/usr/bin/perl -w
use strict;

some_sub(1,2);    #called too early warning error
                  #but note that the code still runs!

sub some_sub($$) #this sub requires 2 scalars
{
   my ($a,$b) = @_;
   print "$a,$b\n";
}

some_sub 6,8;      #ok,now this is just fine
some_sub 10,11,12; #too many argument error

__END__
main::some_sub() called too early to check prototype at C:\TEMP\perl33
+.pl line 6.
1,2

if you reverse order or add proto above call:
sub some_sub($$);
some_sub(1,2);

sub some_sub($$)
{
   my ($a,$b) = @_;
   print "$a,$b\n";
}
Perl will be happy, but to what effect?
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