Re: Syntax "||" before sub
> On Nov 24, 2018, at 12:49 PM, David Precious wrote: > > I went to perldoc perlop expecting to be able to find a > section to point the OP at as a "here's the documentation for it", and > couldn't find anything particularly useful. I was able to find this in “perldoc perlop”: Assignment Operators "=" is the ordinary assignment operator. Assignment operators work as in C. That is, $x += 2; is equivalent to $x = $x + 2; although without duplicating any side effects that dereferencing the lvalue might trigger, such as from "tie()". Other assignment operators work similarly. The following are recognized: **=+=*=&=&.=<<=&&= -=/= |=|.=>>=||= .=%=^=^.= //= x= -- To unsubscribe, e-mail: beginners-unsubscr...@perl.org For additional commands, e-mail: beginners-h...@perl.org http://learn.perl.org/
Re: Syntax "||" before sub
On Thu, 22 Nov 2018 12:33:25 -0800
"John W. Krahn" wrote:
> On 2018-11-22 8:08 a.m., David Precious wrote:
> >
> >
> > You'll often see these operators used to provide default values.
> >
> > e.g.
> >
> >sub hello {
> >my $name = shift;
> >$name ||= 'Anonymous Person';
>
> Which is usually written as:
>
> sub hello {
> my $name = shift || 'Anonymous Person';
Sure - but that doesn't provide a simple, useful example of ||= which
the OP was asking about.
> > I do notice that there isn't actually a very useful section on ||=
> > and //= - I may try to raise a pull requests to add more
> > documentation on them.
>
> $var ||= 'VALUE';
>
> Is just shorthand for:
>
> $var = $var || 'VALUE';
>
> The syntax is borrowed from the C programming language and it is
> slightly more efficient when compiled to machine code.
Indeed - *I* know that, but it strikes me that it could be better
documented, to make it easier for people new to Perl to find the
documentation. I went to perldoc perlop expecting to be able to find a
section to point the OP at as a "here's the documentation for it", and
couldn't find anything particularly useful.
That strikes me as sub-optimal :)
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Re: Syntax "||" before sub
On Thu, Nov 22, 2018 at 12:33:25PM -0800, John W. Krahn wrote:
> On 2018-11-22 8:08 a.m., David Precious wrote:
> >
> > You'll often see these operators used to provide default values.
> >
> > e.g.
> >
> >sub hello {
> >my $name = shift;
> >$name ||= 'Anonymous Person';
>
> Which is usually written as:
>
>sub hello {
>my $name = shift || 'Anonymous Person';
Or, nowadays, and if your perl version(s) support it, as:
sub hello ($name = "Anonymous Person") {
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Re: Syntax "||" before sub
On 2018-11-22 8:08 a.m., David Precious wrote:
You'll often see these operators used to provide default values.
e.g.
sub hello {
my $name = shift;
$name ||= 'Anonymous Person';
Which is usually written as:
sub hello {
my $name = shift || 'Anonymous Person';
I do notice that there isn't actually a very useful section on ||=
and //= - I may try to raise a pull requests to add more documentation
on them.
$var ||= 'VALUE';
Is just shorthand for:
$var = $var || 'VALUE';
The syntax is borrowed from the C programming language and it is
slightly more efficient when compiled to machine code.
John
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Re: Syntax "||" before sub
On Thu, 22 Nov 2018 13:48:18 +
James Kerwin wrote:
> Hi All,
>
> I'm looking through some Perl files for software we use and I noticed
> this in one of the config files:
>
> $c->{guess_doc_type} ||= sub {
>
> All other similar config files show a similar structure, but without
> the "||" before the equals sign:
>
> $c->{validate_document} = sub {
>
> My question is, what is the "||" doing in the first example?
"||=" is the conditional assignment operator - it assigns the value on
the right-hand side to the thing on the left hand side, *but* only if
the thing on the left hand side doesn't evaluate to a true value.
For instance,
my $foo;
$foo ||= "Bar";
# $foo is now "Bar" - because it wasn't true before
$foo ||= "Baz";
# $foo is still "Bar" - it wasn't changed
In your example, the value on the right hand side is a coderef = so, if
$c->{guess_doc_type} didn't already contain something truthy, then it
will be set to a coderef defined by the sub { ... } block.
See also "//=", the defined-or operator, which works in pretty much the
same way, but checking for definedness - the difference being that if
the left hand side contained something that wasn't a truthy value, ||=
would overwrite it, whereas //= wouldn't.
You'll often see these operators used to provide default values.
e.g.
sub hello {
my $name = shift;
$name ||= 'Anonymous Person';
return "Hi there, $name!";
}
> I've attempted to Google this, but found no solid answer.
Yeah, Googling for operators when you don't know their name is rarely
useful :) Perl's operators are all documented quite well at:
https://perldoc.perl.org/perlop.html
I do notice that there isn't actually a very useful section on ||=
and //= - I may try to raise a pull requests to add more documentation
on them.
Cheers
Dave P
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Re: Syntax "||" before sub
Hi Sebastian,
Thanks for that info! It's much more thorough than what I discovered about
this. I'll keep it in mind as I delve deeper...
Thanks,
James
On Thu, Nov 22, 2018 at 1:58 PM James Kerwin wrote:
> Ahhh sorrry to reply to my own post but I found this:
>
> https://stackoverflow.com/questions/29302181/what-is-in-perl-for
>
> I assume then that if " guess_doc_type" evaluates to false it moves on to
> the defined subroutine. My problem is that I can't find where "
> guess_doc_type" is defined... which is not a problem for this list to solve!
>
> Thanks,
> James
>
> On Thu, Nov 22, 2018 at 1:48 PM James Kerwin
> wrote:
>
>> Hi All,
>>
>> I'm looking through some Perl files for software we use and I noticed
>> this in one of the config files:
>>
>> $c->{guess_doc_type} ||= sub {
>>
>> All other similar config files show a similar structure, but without the
>> "||" before the equals sign:
>>
>> $c->{validate_document} = sub {
>>
>> My question is, what is the "||" doing in the first example?
>>
>> It would be useful to know before I go changing anything. My instinct is
>> that it's an "or" but that really doesn't make sense in this context.
>>
>> I've attempted to Google this, but found no solid answer.
>>
>> Thanks,
>> James
>>
>
Re: Syntax "||" before sub
Ahhh sorrry to reply to my own post but I found this:
https://stackoverflow.com/questions/29302181/what-is-in-perl-for
I assume then that if " guess_doc_type" evaluates to false it moves on to
the defined subroutine. My problem is that I can't find where "
guess_doc_type" is defined... which is not a problem for this list to solve!
Thanks,
James
On Thu, Nov 22, 2018 at 1:48 PM James Kerwin wrote:
> Hi All,
>
> I'm looking through some Perl files for software we use and I noticed this
> in one of the config files:
>
> $c->{guess_doc_type} ||= sub {
>
> All other similar config files show a similar structure, but without the
> "||" before the equals sign:
>
> $c->{validate_document} = sub {
>
> My question is, what is the "||" doing in the first example?
>
> It would be useful to know before I go changing anything. My instinct is
> that it's an "or" but that really doesn't make sense in this context.
>
> I've attempted to Google this, but found no solid answer.
>
> Thanks,
> James
>
Syntax "||" before sub
Hi All,
I'm looking through some Perl files for software we use and I noticed this
in one of the config files:
$c->{guess_doc_type} ||= sub {
All other similar config files show a similar structure, but without the
"||" before the equals sign:
$c->{validate_document} = sub {
My question is, what is the "||" doing in the first example?
It would be useful to know before I go changing anything. My instinct is
that it's an "or" but that really doesn't make sense in this context.
I've attempted to Google this, but found no solid answer.
Thanks,
James
