I'm reading Perl which is quite interesting. But while reading goto from here in Perl I got a doubt.
I know that goto statement has three types.
goto LABEL.
goto EXPR.
goto &NAME.
But in this three types, what is the use of third one goto &NAME?
This is also seems to be like a function call.
Then,
What is the real difference between goto &NAME and normal function call in Perl?
When we use goto &NAME?
Can anyone please explain with example.
Thanks in advance.
It says in the goto page
The goto &NAME form is quite different from the other forms of
goto. In fact, it isn't a goto in the normal sense at all, and
doesn't have the stigma associated with other gotos.
Then follows the answer to your question
Instead, it
exits the current subroutine (losing any changes set by local())
and immediately calls in its place the named subroutine using the
current value of #_.
With a normal function call the execution continues on the next line after the function exits.
The rest of that paragraph is well worth reading as well, and answers your second question
This is used by AUTOLOAD subroutines that wish to load another subroutine and then pretend that the other subroutine had been called in the first place (except that any modifications to #_ in the current subroutine are propagated to the other subroutine.) After the goto, not even caller will be able to tell that this routine was called first.
A basic example. With a subroutine deeper defined somewhere, compare
sub func_top {
deeper( #_ ); # pass its own arguments
# The rest of the code here runs after deeper() returns
}
with
sub func_top {
goto &deeper; # #_ is passed to it, as it is at this point
# Control never returns here
}
At the statement goto &deeper the sub func_top is exited. So after deeper completes, the control returns to after the func_top call.
In a sense, func_top is replaced by deeper.
Trying to pass arguments with goto &func results in errors, even just for goto &deeper().
Related
I have a Perl function, which does not return any value. It does not take any arguments also.
sub test {
#do my logic
}
Can I do as :
sub test() {
#do my logic
}
Will the subroutine test be inlined? Will this work? (meaning will the function call be replaced with the function definition. And will my program execute faster?)
The function test() is being called 5000 times. And my Perl program takes a longer time to execute than expected. So I want to make my program faster.
Thanks in advance!
This is answered in Constant Functions in perlsub
Functions with a prototype of () are potential candidates for inlining. If the result after optimization and constant folding is either a constant or a lexically-scoped scalar which has no other references, then it will be used in place of function calls made without &. Calls made using & are never inlined. (See constant.pm for an easy way to declare most constants.)
So your sub test() should be inlined if it satisfies the above conditions. Nobody can tell without seeing the function, so either show it or try.
This is most easily checked with B::Deparse, see further down the linked perlsub section.
I would urge you to profile the program to ensure that the function call overhead is the problem.
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I've created a subroutine named "MENU" with a label named "INIT_MENU" at the top of the subroutine, but when i call this label i got a error : Can't goto subroutine outside a subroutine at program.pl line 15
Here is an example:
sub MENU {INIT_MENU: print "blah blah";}
and here is the line 15:
goto &MENU, INIT_MENU;
Sorry if it is a duplicated question, i searched in all possible places and even in the official site of Perl
The goto expects a single argument, so this code first executes goto &MENU and then after the comma operator it evaluates the constant INIT_MENU in a void context (drawing a warning).
The purpose of goto &NAME is to replace the subroutine in which it is encountered with subroutine NAME; it doesn't make sense calling it outside of a subroutine and this is an error. From perldiag
Can't goto subroutine outside a subroutine
(F) The deeply magical "goto subroutine" call can only replace one subroutine call for another. It can't manufacture one out of whole cloth. In general you should be calling it out of only an AUTOLOAD routine anyway. See goto.
A comment on the purpose of this.
A lot has been written on the harmfulness of goto LABEL.† Not to mention that INIT_MENU: cannot be found when hidden inside a routine. The upshot is that there are always better ways.
A sampler
Call the function and pass the parameter
MENU(INIT_MENU);
sub MENU {
my $menu = shift;
if ($menu eq 'INIT_MENU') { ... }
...
}
If, for some reason, you want to 'hide' the call to MENU use goto &MENU as intended
sub top_level { # pass INIT_MENU to this sub
...
goto &MENU; # also passes its #_ as it is at this point
# the control doesn't return here; MENU took place of this sub
}
This altogether replaces top_level() with MENU() and passes on its #_ to it. Then process input in MENU as above (for example) to trigger the chosen block. After this "not even caller will be able to tell that this routine was called first," see goto. Normally unneeded
Why even have MENU()? Instead, have menu options in separate subs and their references can be values in a hash where keys are names of options. So you'd have a dispatch table and after the logic (or user choice) selects a menu item its code is run directly
my %do_menu_item = ( INIT_MENU => sub { code for INIT_MENU }, ... );
...
$do_menu_item{$item_name}->();
Menu systems tend to grow bigger and more complex with development. It would make sense to adopt OO from start, and then there are yet other approaches for this detail
If you find yourself considering goto it may be time to reconsider (some of) the design.
† See for instance this post and this post. It's (even) worse than that in Perl since there are specific constructs (and restrictions) for situations where goto can be argued acceptable, so there is even less room for it. One example is jumping out of nested loops: there are labels in Perl.
Please take a look at the following code:
use strict;
use warnings;
print "subroutine is defined\n" if defined &myf;
myf();
sub myf
{
print "called myf\n";
}
undef &myf;
#myf();
print "now subroutine is defined\n" if defined &myf;
The output is
subroutine is defined
called myf
The first print statement can print, does that mean the interpreter (or compiler?) looks further and sees the subroutine definition? If so, why it doesn't see the undef &myf; as the second print statement?
Thanks
That doesn't have to do with scope, but with compile time and run time. Here's a simplified explanation.
The Perl interpreter will scan your code initially, and follow any use statements or BEGIN blocks. At that point, it sees all the subs, and notes them down in their respective packages. So now you have a &::myf in your symbol table.
When compile time has reached the end of the program, it will switch into run time.
At that point, it actually runs the code. Your first print statement is executed if &myf is defined. We know it is, because it got set at compile time. Perl then calls that function. All is well. Now you undef that entry in the symbol table. That occurs at run time, too.
After that, defined &myf returns false, so it doesn't print.
You even have the second call to myf() there in the code, but commented out. If you remove the comment, it will complain about Undefined subroutine &main::myf called. That's a good hint at what happened.
So in fact it doesn't look forward or backward in the code. It is already finished scanning the code at that time.
The different stages are explained in perlmod.
Note that there are not a lot of use cases for actually undefing a function. I don't see why you would remove it, unless you wanted to clean up your namespace manually.
In Perl, is it better to use goto or local function, and why with an example?
For example, I am using a code
sub data {
data;
}
data();
or
goto L:
L: if ( $i == 0 )
print "Hello!!!!";
Programmers don't die. They just GOSUB without RETURN.
That said, do not use goto in Perl.
If you want a statement to be executed once, just write that statement and don't use any flow control
If you want your program to execute the stuff several times, put it in a loop
If you want them to be executed from different places, put them in a sub
If you want the sub to be available in different programs, put them in a module
Don't use goto
There is one place where a goto makes sense in Perl. Matt Trout is talking about that in his blog post No, not that goto, the other goto.
Conventional wisdom is that other constructs are better than goto.
If you want to do something repeatedly, use a loop.
If you want to do something conditionally, use a block with if.
If you want to go somewhere and come back, use a function.
If you want to bail out early and "jump straight to the end", this can often be better written using an exception. (Perl handily manages
I have heard that people shouldn't be using & to call Perl subs, i.e:
function($a,$b,...);
# opposed to
&function($a,$b,...);
I know for one the argument list becomes optional, but what are some cases where it is appropriate to use the & and the cases where you should absolutely not be using it?
Also how does the performace increase come into play here when omitting the &?
I'm a frequent abuser of &, but mostly because I'm doing weird interface stuff. If you don't need one of these situations, don't use the &. Most of these are just to access a subroutine definition, not call a subroutine. It's all in perlsub.
Taking a reference to a named subroutine. This is probably the only common situation for most Perlers:
my $sub = \&foo;
Similarly, assigning to a typeglob, which allows you to call the subroutine with a different name:
*bar = \&foo;
Checking that a subroutine is defined, as you might in test suites:
if( defined &foo ) { ... }
Removing a subroutine definition, which shouldn't be common:
undef &foo;
Providing a dispatcher subroutine whose only job is to choose the right subroutine to call. This is the only situation I use & to call a subroutine, and when I expect to call the dispatcher many, many times and need to squeeze a little performance out of the operation:
sub figure_it_out_for_me {
# all of these re-use the current #_
if( ...some condition... ) { &foo }
elsif( ...some other... ) { &bar }
else { &default }
}
To jump into another subroutine using the current argument stack (and replacing the current subroutine in the call stack), an unrare operation in dispatching, especially in AUTOLOAD:
goto ⊂
Call a subroutine that you've named after a Perl built-in. The & always gives you the user-defined one. That's why we teach it in Learning Perl. You don't really want to do that normally, but it's one of the features of &.
There are some places where you could use them, but there are better ways:
To call a subroutine with the same name as a Perl built-in. Just don't have subroutines with the same name as a Perl built-in. Check perlfunc to see the list of built-in names you shouldn't use.
To disable prototypes. If you don't know what that means or why you'd want it, don't use the &. Some black magic code might need it, but in those cases you probably know what you are doing.
To dereference and execute a subroutine reference. Just use the -> notation.
IMO, the only time there's any reason to use & is if you're obtaining or calling a coderef, like:
sub foo() {
print "hi\n";
}
my $x = \&foo;
&$x();
The main time that you can use it that you absolutely shouldn't in most circumstances is when calling a sub that has a prototype that specifies any non-default call behavior. What I mean by this is that some prototypes allow reinterpretation of the argument list, for example converting #array and %hash specifications to references. So the sub will be expecting those reinterpretations to have occurred, and unless you go to whatever lengths are necessary to mimic them by hand, the sub will get inputs wildly different from those it expects.
I think mainly people are trying to tell you that you're still writing in Perl 4 style, and we have a much cleaner, nicer thing called Perl 5 now.
Regarding performance, there are various ways that Perl optimizes sub calls which & defeats, with one of the main ones being inlining of constants.
There is also one circumstance where using & provides a performance benefit: if you're forwarding a sub call with foo(#_). Using &foo is infinitesimally faster than foo(#_). I wouldn't recommend it unless you've definitively found by profiling that you need that micro-optimization.
The &subroutine() form disables prototype checking. This may or may not be what you want.
http://www.perl.com/doc/manual/html/pod/perlsub.html#Prototypes
Prototypes allow you to specify the numbers and types of your subroutine arguments, and have them checked at compile time. This can provide useful diagnostic assistance.
Prototypes don't apply to method calls, or calls made in the old-fashioned style using the & prefix.
The & is necessary to reference or dereference a subroutine or code reference
e.g.
sub foo {
# a subroutine
}
my $subref = \&foo; # take a reference to the subroutine
&$subref(#args); # make a subroutine call using the reference.
my $anon_func = sub { ... }; # anonymous code reference
&$anon_func(); # called like this
Protypes aren't applicable to subroutine references either.
The &subroutine form is also used in the so-called magic goto form.
The expression goto &subroutine replaces the current calling context with a call to the named subroutine, using the current value of #_.
In essence, you can completely switch a call to one subroutine with a call to the named one. This is commonly seen in AUTOLOAD blocks, where a deferred subroutine call can be made, perhaps with some modification to #_ , but it looks to the program entirely as if it was a call to the named sub.
e.g.
sub AUTOLOAD {
...
push #_, #extra_args; # add more arguments onto the parameter list
goto &subroutine ; # change call another subroutine, as if we were never here
}
}
Potentially this could be useful for tail call elimination, I suppose.
see detailed explanation of this technique here
I've read the arguments against using '&', but I nearly always use it. It saves me too much time not to. I spend a very large fraction of my Perl coding time looking for what parts of the code call a particular function. With a leading &, I can search and find them instantly. Without a leading &, I get the function definition, comments, and debug statements, usually tripling the amount of code I have to inspect to find what I'm looking for.
The main thing not using '&' buys you is it lets you use function prototypes. But Perl function prototypes may create errors as often as they prevent them, because they will take your argument list and reinterpret it in ways you might not expect, so that your function call no longer passes the arguments that it literally says it does.