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.
Related
I read the Perl document about Changes to the * prototype
Changes to the * prototype
The * character in a subroutine's prototype used to allow barewords to take precedence over most, but not all, subroutine names. It was never consistent and exhibited buggy behavior.
Now it has been changed, so subroutines always take precedence over barewords, which brings it into conformity with similarly prototyped built-in functions
Code example:
sub splat(*) { ... }
sub foo { ... }
splat(foo); # now always splat(foo())
splat(bar); # still splat('bar') as before
close(foo); # close(foo())
close(bar); # close('bar')
Can anyone explain to me the difference? Thank for any help.
All that perldelta entry is stating is that when foo() is a predefined function then, prior to 5.22, the call splat(foo) might have been interpreted by the parser as either splat(foo()) or splat('foo'), but you couldn't easily tell which. Now it will always be seen as splat(foo()).
I was writing a code and had an error as Prototype after '%' for main::compareHashes. I don't know what is a prototype, and I'm still confused after viewing numerous documents online. Can someone please explain?
You have something like
sub compareHashes(%$) { ... }
(%$) is the prototype. Prototypes affects how the call to the sub is parsed.
As for your the error you received, perldiag provides the following explanation:
A character follows % or # in a prototype. This is useless, since % and # gobble the rest of the subroutine arguments.
Whatever prototype you used makes no sense, and this is Perl's way of telling you that. You need to fix or remove compareHashes's prototype.
Looking at your previous question, your prototype is this:
sub compareHashes(%$hash1, %$hash2) { ... }
As a prototype, this is nonsense. Perhaps you should read the documentation on prototypes.
Prototypes in Perl are completely unlike prototypes in pretty any other language. Which is why expert Perl programmers do not recommend their use (outside of a tiny number of use cases where they are essential). Recent versions of Perl have added a new feature called function signatures which are far more like what most people expect prototypes to be - but they are currently marked as an experimental feature so not many people use them.
But let's look at your prototype and see what's wrong with it.
sub compareHashes(%$hash1, %$hash2) { ... }
Firstly, it contains what look like variable names. And Perl prototypes don't contain variable names. They are just a string of symbols describing the types of the arguments you are going to pass to the subroutine.
But the parser isn't complaining about the variable names. It doesn't get to the first variable name. It finds a problem before that. It doesn't like the %$. And that's because %$ makes no sense as a prototype.
%$ means "this subroutine takes two arguments - a hash followed by a scalar. But we (should!) know that it makes no sense to pass a scalar to a subroutine after a hash. That's because hash assignments are greedy and will eat up all of the remaining arguments in #_ - leaving nothing to go into the scalar.
You are saying that you will call the subroutine like this:
some_sub(%hash, $scalar);
And within the subroutine, you'll do this:
my (%hash, $scalar) = #_;
And that just won't work. That's what the error is telling you. That your prototype is nonsense.
Don't use prototypes. Prototypes don't work the way you think they do. They are an advanced Perl feature and should only be used in specific circumstances.
Update: I've just noticed this in your question:
I don't know what is a prototype
The prototype is the bit in parentheses between the subroutine name and the subroutine block. In your subroutine definition:
sub compareHashes(%$hash1, %$hash2) { ... }
The prototype is (%$hash1, %$hash2). I know why people use them - they look a lot like how subroutines work in other languages, but in Perl, they are usually far more trouble than they are worth. It's best to just drop them and just define the subroutine without a prototype:
sub compareHashes { ... }
In my perl code I've previously used the following two styles of writing which I've later found are being discouraged in modern perl:
# Style #1: Using & before calling a user-defined subroutine
&name_of_subroutine($something, $something_else);
# Style #2: Using ($$) to show the number of arguments in a user-defined sub
sub name_of_subroutine($$) {
# the body of a subroutine taking two arguments.
}
Since learning that those styles are not recommended I've simply stopped using them.
However, out of curiosity I'd like to know the following:
What is the origin of those two styles of writing? (I'm sure I've not dreamt up the styles myself.)
Why are those two styles of writing discouraged in modern perl?
Have the styles been considered best practice at some point in time?
The & sigil is not commonly used with function calls in modern Perl for two reasons. First, it is largely redundant since Perl will consider anything that looks like a function (followed by parens) a function. Secondly, there is a major difference between the way &function() and &function are executed, which may be confusing to less experienced Perl programmers. In the first case, the function is called with no arguments. In the second case, the function is called with the current #_ (and it can even make changes to the argument list which will be seen by later statements in that scope:
sub print_and_remove_first_arg {print 'first arg: ', shift, "\n"}
sub test {
&print_and_remove_first_arg;
print "remaining args: #_\n";
}
test 1, 2, 3;
prints
first arg: 1
remaining args: 2 3
So ultimately, using & for every function call ends up hiding the few &function; calls which can lead to hard to find bugs. In addition, using the & sigil prevents the honoring of function prototypes, which can be useful in some cases (if you know what you are doing), but also may lead to hard to track down bugs. Ultimately, & is a powerful modifier to function behavior, and should only be used when that behavior is desired.
Prototypes are similar, and their use should be limited in modern Perl. What must be stated explicitly is that prototypes in Perl are NOT function signatures. They are hints to the compiler that tell it to parse calls to those functions in a similar way as the built in functions. That is, each of the symbols in the prototype tells the compiler to impose that type of context on the argument. This functionality can be very helpful when defining functions that behave like map or push or keys which all treat their first argument differently than a standard list operator would.
sub my_map (&#) {...} # first arg is either a block or explicit code reference
my #ret = my_map {some_function($_)} 1 .. 10;
The reason sub ($$) {...} and similar uses of prototypes are discouraged is because 9 times out of 10 the author means "I want two args" and not "I want two args each with scalar context imposed on the call site". The former assertion is better written:
use Carp;
sub needs2 {
#_ == 2 or croak 'needs2 takes 2 arguments';
...
}
which would then allow the following calling style to work as expected:
my #array = (2, 4);
needs2 #array;
To sum up, both the & sigil and function prototypes are useful and powerful tools, but they should only be used when that functionality is required. Their superfluous use (or misuse as argument validation) leads to unintended behavior and difficult to track down bugs.
The & in function-calls was mandatory in Perl 4, so maybe you have picked that up from Programming perl (1991) by Larry Wall and Randal L. Schwartz, as I did, or somewhere similar.
As for the function prototypes, my guess is less qualified. Maybe you have been mimicking languages where it makes sense and/or is mandatory to declare argument lists, and since function prototypes in Perl look a little like argument lists, you've started adding them?
&function is discouraged because it makes the code less readable and isn't necessary (the cases that &function is necessary are rare and often better avoided).
Function prototypes aren't argument lists, so most of the time they'll just confuse your reader or lull you into a false sense of rigidity, so no need to use those unless you know exactly why you are.
& was mandatory in Perl 4, so they have been best/necessary practise. I don't think function prototypes ever has been.
For style #1, the & before the subroutine is only necessary if you have a subroutine that shares a name with a builtin and you need to disambiguate which one you wish to call, so that the interpreter knows what's going on. Otherwise, it's equivalent to calling the subroutine without &.
Since that's the case, I'd say its use is discouraged since you shouldn't be naming your subroutines with the same names as builtins, and it's good practice to define all your subroutines before you call them, for the sake of reading comprehension. In addition to this, if you define your subroutines before you call them, you can omit the parentheses, like in a builtin. Plus, just speaking visually, sticking & in front of every subroutine unnecessarily clutters up the file.
As for function prototypes, they were stuck into Perl after the fact and don't really do what they were made to do. From an article on perl.com:
For the most part, prototypes are more trouble than they're worth. For one thing, Perl doesn't check prototypes for methods because that would require the ability to determine, at compile time, which class will handle the method. Because you can alter #ISA at runtime--you see the problem. The main reason, however, is that prototypes aren't very smart. If you specify sub foo ($$$), you cannot pass it an array of three scalars (this is the problem with vec()). Instead, you have to say foo( $x[0], $x[1], $x[2] ), and that's just a pain.
In the end, it's better to comment your code to indicate what you intend for a subroutine to accept and do parameter checking yourself. As the article states, this is actually necessary for class methods, since no parameter checking occurs for them.
For what it's worth, Perl 6 adds formal parameter lists to the language like this:
sub do_something(Str $thing, Int $other) {
...
}
As per my sample code below, there are two styles to call a subroutine: subname and subname().
#!C:\Perl\bin\perl.exe
use strict;
use warnings;
use 5.010;
&marine(); # style 1
&marine; # style 2
sub marine {
state $n = 0; # private, persistent variable $n
$n += 1;
print "Hello, sailor number $n!\n";
}
Which one, &marine(); or &marine;, is the better choice if there are no arguments in the call?
In Learning Perl, where this example comes from, we're at the very beginning of showing you subroutines. We only tell you to use the & so that you, as the beginning Perler, don't run into a problem where you define a subroutine with the same name as a Perl built-in then wonder why it doesn't work. The & in front always calls your defined subroutine. Beginning students often create their own subroutine log to print a message because they are used to doing that in other technologies they use. In Perl, that's the math function builtin.
After you get used to using Perl and you know about the Perl built-ins (scan through perlfunc), drop the &. There's some special magic with & that you hardly ever need:
marine();
You can leave off the () if you've pre-declared the subroutine, but I normally leave the () there even for an empty argument list. It's a bit more robust since you're giving Perl the hint that the marine is a subroutine name. To me, I recognize that more quickly as a subroutine.
The side effect of using & without parentheses is that the subroutine is invoked with #_. This program
sub g {
print "g: #_\n";
}
sub f {
&g(); # g()
&g; # g(#_)
g(); # g()
g; # g()
}
f(1,2,3);
produces this output:
g:
g: 1 2 3
g:
g:
It's good style to declare your subroutines first with the sub keyword, then call them. (Of course there are ways around it, but why make things more complicated than necessary?)
Do not use the & syntax unless you know what it does exactly to #_ and subroutines declared with prototypes. It is terribly obscure, rarely needed and a source of bugs through unintended behaviour. Just leave it away – Perl::Critic aptly says about it:
Since Perl 5, the ampersand sigil is completely optional when invoking subroutines.
Now, given following these style hints, I prefer to call subroutines that require no parameters in style 1, that is to say marine();. The reasons are
visual consistency with subroutines that do require parameters
it cannot be confused with a different keyword.
As a general rule I recommend the following:
Unless you need the & because you're over riding a built in function or you have no parameter list omit it.
Always include the () as in marine().
I do both of these for code readability. The first rule makes it clear when I'm overriding internal Perl functions by making them distinct. The second makes it clear when I'm invoking functions.
perl allows you to omit parenthesis in your function call.
So you can call your function with arguments in two different ways:
your_function( arg1,arg2,arg3);
or
your function arg1,arg2,arg3 ;
Its a matter of choice that which form do you prefer. With users from C background the former is more intuitive.
I personally use the former for functions defined by me and latter for built in functions like:
print "something" instead of print("something")
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.