I want to write a function whose first parameter is a description, and the second parameter is a code block. I want the finished code to read like:
verify "description" { boolean-assertion-block };
I'm specifically looking to avoid the sub keyword.
I can put the description AFTER the code block, no problem:
sub verify (&$) { ... }
But when I reverse the prototype symbol order:
sub verify ($&) { ... }
Then I get an error message:
Type of arg 2 to main::verify must be sub {} (not anonymous hash ({})) at ...
Clearly, Perl has special handling for the first argument being a code block.
So, maybe I can make it a curried function?
sub verify ($) {
my $message = shift;
return sub (&) { . . . }
}
But then I get a syntax error between the description and the code block:
syntax error at ... near ""..." { "
I tried altering the calling syntax to try to help out the compiler:
test "...", { BLOCK };
test("..."){ BLOCK };
test("...")({ BLOCK });
( test "..." )({ BLOCK });
No joy. Can Perl even do what I want to do?
The (&) prototype only has such niceness when used for the first argument in a sub. From perldoc perlsub:
The interesting thing about "&" is that you can generate new syntax with it, provided it's in the initial position
One way to provide a similar level of niceness would be:
sub verify ($%) {
my ( $message, %opts ) = #_;
my $coderef = $opts{using};
...;
}
sub using (&) {
my ( $coderef ) = #_;
return ( using => $coderef );
}
# The `verify` sub accepts a name followed by a hash of options:
#
verify(
"your message here",
"using" => sub { ... },
);
# The `using` sub returns a two-element list that will satisfy
# that hash of options:
#
verify "your message here", using {
...;
};
If you desperately want to allow a syntax exactly like:
verify "description" { boolean-assertion-block };
... then it is still possible, but requires the dark arts. Keyword::Simple is probably your best bet, but Devel::Declare and Filter::Simple are options.
You can only use the block syntax if the & is the first thing in your prototype. From perlsub:
An & requires an anonymous subroutine, which, if passed as the first argument, does not require the sub keyword or a subsequent comma
Other custom DSL such as in Dancer2 or Mojolicious typically use the sub keyword.
get '/foo' => sub {
...
};
Plack::Builder and Web::Scraper use blocks that return objects, which then can be nested.
Related
I have defined several Perl constants like use constant LVL_FATAL => 1; (perl 5.18.2).
When trying to create a function that needs the name and the value of the constant, I tried:
my $level = eval { 'LVL_' . $_ };
When $_ is FATAL, then $level contains LVL_FATAL, but not the value of LVL_FATAL.
I also tried the variants eval { $level }, eval { ($level) }, eval { my x = $level }, and eval { print $level }.
They all use LVL_FATAL.
However when I use eval "$level", then I get 1.
Also when i use eval { LVL_FATAL } I get 1, too.
Can I use the block variant instead of the string variant for robustness and performance reasons?
Remark
Remembering that Perl constants are basically functions, I tried eval ${level}(), but that did not work; eval { $level->() } seemed to do the job in the debugger, but when I used it in my program code, it did not work, however.
(When not using eval, LVL_FATAL->() will give an error ("Undefined subroutine &main::1 ..."), but LVL_FATAL() is OK.)
eval BLOCK and eval EXPR are very different.
eval BLOCK catches exceptions thrown by the code in the block. It is known as try in some other languages.
eval EXPR compiles and executes the string returned by EXPR. (Exceptions are also caught.)
You want the latter.
my $level = eval("LVL_$_");
die $# if $#;
But since constants can be used as subs, you could also use
my $level = do { no strict qw( refs ); "LVL_$_"->() };
You could also call this "sub" as a method.
my $name = "LVL_$_";
my $level = __PACKAGE__->$name();
I prefer the second-last version because it makes it obvious that you are doing something weird and dangerous (and it doesn't pretend that a sub that isn't a method is a method).
The following seems to work:
my $level = eval { my $name = "LVL_$_"; __PACKAGE__->$name() };
According to the documentation :
Constants belong to the package they are defined in. [...]
Constants may be exported by modules, and
may also be called as either class or instance methods, that is, as
Some::Package->CONSTANT or as $obj->CONSTANT where $obj is an instance
of Some::Package. Subclasses may define their own constants to
override those in their base class.
I'm getting this error and cannot understand why this happens. It happens when I jump to another subroutine. Perhaps there is something I need to understand about Mojolicious on why this happens.
Here is the source code of my program:
#!/usr/bin/perl
use Mojolicious::Lite;
get '/' => sub { &start_home; };
app->start;
sub start_home {
my $d = shift;
my $something = $d->param('something');
### Do things with $something.... etc.. etc..
&go_somewhere_else; ### Go somewhere else
}
sub go_somewhere_else {
my $c = shift;
$c->render(text => "Hello World!");
### End of program
}
I am passing a value on to the renderer and there is a value - Why would it say it is undefined? My understanding is that this only happens if you jump to a subroutine and try to render output.
My operating system is Windows and I am using Strawberry Perl.
You need to pass the context object $c/$d to your second function. The undefined value is your $c in go_somewhere_else, because you call it without a parameter.
Initially, to make it work, do this.
sub start_home {
my $d = shift;
my $something = $d->param('something');
go_somewhere_else($d);
}
You are now passing the context, which you named $d (that's not the conventional name), to the other function, and the warning will go away.
That's because the form &subname; without parenthesis () makes #_ (that's the list of arguments to the function) available inside of go_somewhere_else, but because you shifted $d off, #_ is now empty, and hence your $c inside go_somewhere_else is undef.
Alternatively, you could also change the shift to an assignment with #_. But please, don't do that!
sub start_home {
my ( $d ) = #_;
my $something = $d->param('something');
&go_somewhere_else;
}
There are more things odd to the point of almost wrong here.
get '/' => sub { &start_home; };
You are currying the the start_home function, but you are not actually adding another parameter. I explained above why this works. But it's not great. In fact, it's confusing and complicated.
Instead, you should use a code reference for the route.
get '/' => \&start_home;
Inside of start_home, you should call your context $c as is the convention. You should also not use the ampersand & notation for calling functions. That changes the behavior in a way you most certainly do not want.
sub start_home {
my $c = shift;
my $something = $c->param('something');
# ...
go_somewhere_else($c);
}
To learn more about how function calls work in Perl, refer to perlsub.
I'm writing some code where I am using a subroutine as both an lvalue and an rvalue to read and write database values. The problem is, I want it to react differently based on whether it is being used as an lvalue or an rvalue.
I want the subroutine to write to the database when it is used as an lvalue, and read from the database when it is used as an rvalue.
Example:
# Write some data
$database->record_name($subscript) = $value;
# Read some data
my $value = $database->record_name($subscript);
The only way I can think of the make this work is to find a way for the subroutine to recognize whether it is being used as an lvalue or an rvalue and react differently for each case.
Is there a way to do this?
Deciding how to behave on whether it was called as an lvalue or not is a bad idea since foo(record_name(...)) would call it as an lvalue.
Instead, you should decide how to behave on whether it is used as an lvalue or not.
You can do that by returning a magical value.
use Variable::Magic qw( cast wizard );
my $wiz = wizard(
data => sub { shift; \#_ },
get => sub { my ($ref, $args) = #_; $$ref = get_record_name(#$args); },
set => sub { my ($ref, $args) = #_; set_record_name(#$args, $$ref); },
);
sub record_name :lvalue {
cast(my $rv, $wiz, #_);
return $rv;
}
A little test:
use Data::Dumper;
sub get_record_name { print("get: #_\n"); return "val"; }
sub set_record_name { print("set: #_\n"); }
my $x = record_name("abc", "def"); # Called as rvalue
record_name("abc", "def") = "xyz"; # Called as lvalue. Used as lvalue.
my $y_ref = \record_name("abc", "def"); # Called as lvalue.
my $y = $$y_ref; # Used as rvalue.
$$y_ref = "xyz"; # Used as lvalue.
Output:
get: abc def
set: abc def xyz
get: abc def
set: abc def xyz
After seeing this, you've surely learned that you should abandon the idea of using an lvalue sub. It's possible to hide all that complexity (such as by using sentinel), but the complexity remains. The fanciness is not worth all the complexity. Use separate setters and getters or use an accessor whose role is based on the number of parameters passed to it ($s=acc(); vs acc($s)) instead.
For this situation you might like to try my Sentinel module.
It provides a function you can use in the accessor, to turn it into a more get/set style approach. E.g. you could
use Sentinel qw( sentinel );
sub get_record_name { ... }
sub set_record_name { ... }
sub record_name
{
sentinel get => \&get_record_name,
set => \&set_record_name,
obj => shift;
}
At this point, the following pairs of lines of code are equivalent
$name = $record->record_name;
$name = $record->get_record_name;
$record->record_name = $new_name;
$record->set_record_name( $new_name );
Of course, if you're not needing to provide the specific get_ and set_ prefixed versions of the methods as well, you could inline them as closures.
See the module docs also for further ideas.
In my opinion, lvalue subroutines in Perl were a dumb idea. Just support ->record_name($subscript, $value) as a setter and ->record_name($subscript) as a getter.
That said, you can use the Want module, like this
use Want;
sub record_name:lvalue {
if ( want('LVALUE') ) {
...
}
else {
...
}
}
though that will also treat this as an LVALUE:
foo( $database->record_name($subscript) );
If you want only assignment statements to be treated specially, use want('ASSIGN') instead.
I'm a bit messed up with the following:
I have a function that calls subroutines in the following way:
sub someFunction {
my $self = shift;
my $type = $self->{'type'};
if($type eq 'one_subroutine') {
$self->updateOneSubroutine();
}
elsif($type eq 'another_one_sub') {
$self->updateAnotherOneSub();
}
(...)
else {
die "Unsupported '$type'";
}
I have to change this to let each subroutine be coded in its own file, include all available files, and automagically call the subroutine inside.
I did this in a test file with the following code:
# Assume a routines subdir with one_subroutine.pm file with
sub updateOneSubroutine(){
$self = shift;
$self->doSomeThings();
(...) #my code
}
1;
test.pl
# Saves in routines hash_ref a pair of file_name => subRoutineName for each file in routines subdir.
# This will be used later to call subroutine.
opendir(DIR,"lib/routines") or die "routines directory not found";
for my $filename (readdir(DIR)) {
if($filename=~m/\.pm$/){
# includes file
require "lib/routines/$filename";
# get rid of file extension
$filename=~s/(.*)\.pm/$1/g;
my $subroutine = "update_${file}";
# camelizes the subroutine name
$subroutine=~s/_([a-z0-9])/\u$1/g;
$routine->{ $filename } = $subroutine;
}
}
{
no strict "refs";
$routine->{$param}();
}
where param is something like "one_subroutine", that matches with a filename available.
Since each subroutine receives $self in the call, I should call the routine by $self->something();
I've tried $self->$routine->{$param}() , $self->${routine->${param}}() and many other things without success. I've checked chapter 9 "dynamic subroutines" of mastering perl, and a similar question to perl monks, but I can't still figure out how to reference the subroutine in a way that represents $self->updateAnotherOneSub() , or something similar that lets $self be read as a param in those subroutines.
Thanks in advance, Keber.
This seems a bit like an X/Y problem. What exactly are you trying to do? If it is to reduce loading time, then modules like AutoSplit/AutoLoader might be of interest to you.
If it is to create some sort of data structure of subroutines, you should be installing anonymous subs into a hash, rather than giving them all names.
Given a subroutine reference:
my $code = sub {...};
you would call it as:
$self->$code(...);
If instead you have a subroutine name, you can lookup the coderef:
my $code = 'Package::With::The::Subroutines'->can('method_name');
and if that succeeds (check it), then you can use $self->$code(...) to call it.
Given this code:
{
no strict "refs";
$routine->{$param}();
}
You would pass $self to the routine with:
{
no strict "refs";
$routine->{$param}($self);
}
Or you could approach it the way I did above with can:
'package'->can($routine->{$param})->($self)
if you don't want to turn off strict 'refs'
Try to extract the method name first, then it should work. I did a small test script that may do something like you want to, so:
my $method = $routine->{$param};
$self->$method->();
You can and of course should check, if the desired method exists like Eric said:
if ($self->can($method)) {
$self->$method->();
}
The important part here is, that you extract the method name so you have it in a single variable; otherwise perl won't figure that out for you - and as far as I know there is no way of setting parens or braces to do so.
I have a perl script (simplified) like so:
my $dh = Stats::Datahandler->new(); ### homebrew module
my %url_map = (
'/(article|blog)/' => \$dh->articleDataHandler,
'/video/' => \$dh->nullDataHandler,
);
Essentially, I'm going to loop through %url_map, and if the current URL matches a key, I want to call the function pointed to by the value of that key:
foreach my $key (keys %url_map) {
if ($url =~ m{$key}) {
$url_map{$key}($url, $visits, $idsite);
$mapped = 1;
last;
}
}
But I'm getting the message:
Can't use string ("/article/") as a subroutine ref while "strict refs" in use at ./test.pl line 236.
Line 236 happens to be the line $url_map{$key}($url, $visits, $idsite);.
I've done similar things in the past, but I'm usually doing it without parameters to the function, and without using a module.
Since this is being answered here despite being a dup, I may as well post the right answer:
What you need to do is store a code reference as the values in your hash. To get a code reference to a method, you can use the UNIVERSAL::can method of all objects. However, this is not enough as the method needs to be passed an invocant. So it is clearest to skip ->can and just write it this way:
my %url_map = (
'/(article|blog)/' => sub {$dh->articleDataHandler(#_)},
'/video/' => sub {$dh->nullDataHandler(#_)},
);
This technique will store code references in the hash that when called with arguments, will in turn call the appropriate methods with those arguments.
This answer omits an important consideration, and that is making sure that caller works correctly in the methods. If you need this, please see the question I linked to above:
How to take code reference to constructor?
You're overthinking the problem. Figure out the string between the two forward slashes, then look up the method name (not reference) in a hash. You can use a scalar variable as a method name in Perl; the value becomes the method you actually call:
%url_map = (
'foo' => 'foo_method',
);
my( $type ) = $url =~ m|\A/(.*?)/|;
my $method = $url_map{$type} or die '...';
$dh->$method( #args );
Try to get rid of any loops where most of the iterations are useless to you. :)
my previous answer, which I don't like even though it's closer to the problem
You can get a reference to a method on a particular object with can (unless you've implemented it yourself to do otherwise):
my $dh = Stats::Datahandler->new(); ### homebrew module
my %url_map = (
'/(article|blog)/' => $dh->can( 'articleDataHandler' ),
'/video/' => $dh->can( 'nullDataHandler' ),
);
The way you have calls the method and takes a reference to the result. That's not what you want for deferred action.
Now, once you have that, you call it as a normal subroutine dereference, not a method call. It already knows its object:
BEGIN {
package Foo;
sub new { bless {}, $_[0] }
sub cat { print "cat is $_[0]!\n"; }
sub dog { print "dog is $_[0]!\n"; }
}
my $foo = Foo->new;
my %hash = (
'cat' => $foo->can( 'cat' ),
'dog' => $foo->can( 'dog' ),
);
my #tries = qw( cat dog catbird dogberg dogberry );
foreach my $try ( #tries ) {
print "Trying $try\n";
foreach my $key ( keys %hash ) {
print "\tTrying $key\n";
if ($try =~ m{$key}) {
$hash{$key}->($try);
last;
}
}
}
The best way to handle this is to wrap your method calls in an anonymous subroutine, which you can invoke later. You can also use the qr operator to store proper regexes to avoid the awkwardness of interpolating patterns into things. For example,
my #url_map = (
{ regex => qr{/(article|blog)/},
method => sub { $dh->articleDataHandler }
},
{ regex => qr{/video/},
method => sub { $dh->nullDataHandler }
}
);
Then run through it like this:
foreach my $map( #url_map ) {
if ( $url =~ $map->{regex} ) {
$map->{method}->();
$mapped = 1;
last;
}
}
This approach uses an array of hashes rather than a flat hash, so each regex can be associated with an anonymous sub ref that contains the code to execute. The ->() syntax dereferences the sub ref and invokes it. You can also pass parameters to the sub ref and they'll be visible in #_ within the sub's block. You can use this to invoke the method with parameters if you want.