I have index.pl and subs.pl. When I run the program, the user inserts the date of birth and then it is passed to the getage() subroutine in subs.pl, which has many subroutines.
getage() than implicitly calls another subroutine called validate() which validates the date entered by user.
When I run the index.pl and the user enters the date as 03-04-2005, the following error comes out:
can't modify non-lvalue subroutine call at subs.pl line 85, <> line 1
At 85th line of subs.pl I have:
list(my $val,my #value) = validate($dob);
validate() returns a message and the date($dob) which is sent from getage().
Some code from validate():
sub validate {
my $dob = shift;
my $error;
my #test;
#test = split("-",$dob);
if (!#test) {
$error = "date separator should be - ";
return ($error,#test);
}
...
The solution seems to be:
my ($val, #value) = validate($dob);
based on my intuitive understanding of what that code intended - but I can't be certain till you provide some more context (what does validate() return, what does list() mean?)
If you meant list() as a means to force $val and #value into a list, you merely need to enclose both in parenthesis: ($val, #value) to do that
An lvalue is a variable you can modify. (one that can be on the left side of an assignment, hence the name). In most circumstances, a value returned by a sub is not one you can modify.
In your example, you are attempting exactly that: assigning the return value of validate($dob) to the nonmodifiable return value of list($val, #value).
in line
list(my $val,my #value) = validate($dob);
remove 'list' and it works fine
ie
(my $val,my #value) = validate($dob);
thanks to Kayra and others who answered
Related
I have script:
#!/usr/bin/env perl
sub t0 {
return; # We return nothing for ERROR
}
sub t1 {
#z = ();
return #z; # We return array (which maybe empty) for no ERROR
}
In scalar context I can distinguish error and OK status:
my $x1 = t0(); # undef
my $x2 = t1(); # 0
Is there a way to distinguish error and empty array returned from subroutine when it is called in list context?
my #x1 = t0(); # empty list
my #x2 = t1(); # empty list
I am feeling that I need "0E0" (Zero but TRUE) but for list context.
A way to distingish between the empty list and an error in this case would be to return an array reference, e.g.
sub t0 {
return undef; # We return undef for ERROR
}
sub t1 {
#z = ();
return \#z; # We return array (which maybe empty) for no ERROR
}
So basically the functions do not behave differently in list context. They always return a single scalar, which may be undef, which indicates an error, or return a reference to an array.
No.
Return an array reference if successful (which the caller will have to dereference) and undef on failure.
Or throw an exception on failure (which the calling code could catch).
In list context a sub will return a list, even if it only has one element which is undef. If you must return data in a flat list – if it is not feasible to change the callers, say in a large codebase -- you can die on error and eval somewhere at a top level. (As die is an exception it "bubbles.")
The other option for the flat list is rather unappealing, and it still requires changes to callers. You can return an empty list or an undef and then test accordingly -- but see the comment below.
sub test {
# set #z (but not with undef!), $error
return undef if $error;
return #z;
}
my #ret = test();
if (not #ret) {
# empty list
}
else {
foreach my $elem (#ret) {
if (not defined $elem) {
warn "Undefined";
last
}
# ...
}
}
However, as pointed out by ikegami, if #z gets assigned an undef legitimately (without error), for example a variable that happens to be undefined, this code can't tell the difference. So the function would have to be changed further to deal with that.
I want to make a console and change the automatic completion function when I press tab but I want to differentiate between two cases:
If I press tab and the beginning of the command matches a list I supplied in an array, the auto complete will be according to this array.
If I press tab and the command isn't recognized from the list I supplied, I want the generic completion function to work, so t hat it will auto complete directories and file names in the current directory.
Is it possible?
Thanks a lot.
Edit: I'm trying to do it inside a perl script. I saw this example:
rl_attempted_completion_function
A reference to an alternative function to create matches.
The function is called with TEXT, LINE_BUFFER, START, and END. LINE_BUFFER is a current input buffer string. START and END are indices in LINE_BUFFER saying what the boundaries of TEXT are.
If this function exists and returns null list or undef, or if this variable is set to undef, then an internal function rl_complete() will call the value of $rl_completion_entry_function to generate matches, otherwise the array of strings returned will be used.
The default value of this variable is undef. You can use it as follows;
use Term::ReadLine;
...
my $term = new Term::ReadLine 'sample';
my $attribs = $term->Attribs;
...
sub sample_completion {
my ($text, $line, $start, $end) = #_;
# If first word then username completion, else filename completion
if (substr($line, 0, $start) =~ /^\s*$/) {
return $term->completion_matches($text,
$attribs->{'username_completion_function'});
} else {
return ();
}
}
...
$attribs->{attempted_completion_function} = \&sample_completion;
completion_matches(TEXT, ENTRY_FUNC)
What I want to do is that in case when tab is pressed it recognizes a substring from an array I provide, the auto completion will be from that array (if there are multiple matches it will give all of them like a regular unix console).
Otherwise, I want the auto completion to be file recognition.
The subroutine used internally by Term::ReadLine::Gnu to provide the default completion is filename_completion_function, which you can call directly from your custom subroutine:
use Term::ReadLine;
my $term = new Term::ReadLine 'MyTerm';
$term->Attribs->{'completion_entry_function'} = \&my_completion;
my $ans = $term->readline('How can I help you? ');
sub my_completion {
my ($text, $state) = #_;
if (my_test) {
return my_custom_stuff;
}
else {
return Term::ReadLine::Gnu->filename_completion_function($text, $state);
}
}
In my Perl Catalyst application, I get the value of a URL parameter like this, typically:
my $val = $c->request->params->{arg} || '';
But the URL could contain multiple arg=$Val. I only want to keep the first value of arg=. I could add this throughout my code:
my $val = $c->request->params->{arg} || '';
$val = $val->[0] if (ref($val) eq 'ARRAY');
That is rather ugly. Is there a way to pick up the first value or a url parameter in a better way?
Does your app actually expect multiple values for parameter arg? If not, all you need is
my $val = $c->request->params->{arg} || '';
Sure, it will be garbage if the user provides you with a garbage url, but there's nothing you can do to prevent the user from giving you garbage.
If it's actually valid to have more than one value for parameter arg, why would you want just the first value? You'd actually want all the values.
sub param_vals {
my ($params, $name) = #_;
return () if !exists($params->{name});
return $params->{$name} if !ref($params->{name});
return #{ $params->{$name} };
}
my #args = param_vals($c->request->{params}, 'arg');
I just read the code to Catalyst::Request but I don't see anything to always pull out a single value. Too bad Cat doesn't use something like Hash::MultiValue!
I have the code below:
my $content = $response->decoded_content((charset => 'UTF-8'));
my $feed = XML::Feed->parse(\$content) || $logger->error("When retrieving $URL: ", XML::Feed->errstr);
if (defined $feed) {
for my $entry ($feed->entries) {
#DO SOMETHING
}
}
For some site, XML::FEED saying that it can't detect the feed type. This is something I have to look at but this is not my question at the moment.
This sample code is inside a while loop has I'm retrieving different RSS and I would like to have the script running even when some URLs failed.
The defined function seems to not work as I get the error message:
Can't call method "entries" without a package or object reference
Can someone tell me what is the right way to handle the test?
You first have to check the value of $feed.
The error message you describe is obvious: $feed is not a package / object reference, but it can be a simple hash for instance. So it's defined.
Add my favourite debugging line right in front of if(defined):
warn Data::Dumper->new([ $feed ],[ '*feed' ])->Sortkeys(1)->Dump();use Data::Dumper;
and you'll see the value in a nice way.
Without testing I'd say that $feed contains the result of your logger, which might be 1 or 0 or something like that, because you set the value of $feed to XML::Feed->parse, and if this is not successful (undefined) it's the result of $logger->error.
You'd better write it like:
my $feed = XML::Feed->parse(\$content);
if (defined $feed) {
for my $entry ($feed->entries) {
#DO SOMETHING
}
}
else {
$logger->error("When retrieving $URL: ", XML::Feed->errstr);
}
because parse is said to return an object, and I guess it returns undef on error.
The error message means what it says: $feed is neither a package nor an object reference. It passes the defined test because there are many defined values which are neither packages nor object references.
In this particular case, you're seeing this error because you are misuing ||:
my $feed = XML::Feed->parse(\$content) || $logger->error("When retrieving $URL: ", XML::Feed->errstr);
If the parse call should fail and return undef, this evaluates to
my $feed = ( undef || $logger->error("When retrieving $URL: ", XML::Feed->errstr) );
which evaluates to
my $feed = $logger->error("When retrieving $URL: ", XML::Feed->errstr);
. The return value of $logger->error is unknown to me, but presumably it is neither a package nor an object reference. And if it were one, it probably would be the wrong one to put in a variable named $feed.
The documentation for XML::Feed mentions parsing with a construct like
my $feed = XML::Feed->parse(URI->new('http://example.com/atom.xml'))
or die XML::Feed->errstr;
This is not the same thing. Their respective precedence rules make || and or suitable for different applications; specifically, you should only use || when you want the value on the right-hand side for something. Do not use it only for the short-circuit side effect.
You can solve this by replacing the || with or to get the right evaluation order. While you are there, you probably should also eliminate the redundant defined test.
I have a Perl routine that manages error checking. There are about 10 different checks and some are nested, based on prior success. These are typically not exceptional cases where I would need to croak/die. Also, once an error occurs, there's no point in running through the rest of the checks.
However, I can't seem to think of a neat way to solve this issue except by using something analogous to the following horrid hack:
sub lots_of_checks
{
if(failcond)
{
goto failstate:
}
elsif(failcond2)
{
goto failstate;
}
#This continues on and on until...
return 1; #O happy day!
failstate:
return 0; #Dead...
}
What I would prefer to be able to do would be something like so:
do
{
if(failcond)
{
last;
}
#...
};
An empty return statement is a better way of returning false from a Perl sub than returning 0. The latter value will actually be true in list context:
sub lots_of_checks {
return if fail_condition_1;
return if fail_condition_2;
# ...
return 1;
}
Perhaps you want to have a look at the following articles about exception handling in perl5:
perl.com: Object Oriented Exception Handling in Perl
perlfoundation.com: Exception Handling in Perl
You absolutely can do what you prefer.
Check: {
last Check
if failcond1;
last Check
if failcond2;
success();
}
Why would you not use exceptions? Any case where the normal flow of the code should not be followed is an exception. Using "return" or "goto" is really the same thing, just more "not what you want".
(What you really want are continuations, which "return", "goto", "last", and "throw" are all special cases of. While Perl does not have full continuations, we do have escape continuations; see http://metacpan.org/pod/Continuation::Escape)
In your code example, you write:
do
{
if(failcond)
{
last;
}
#...
};
This is probably the same as:
eval {
if(failcond){
die 'failcond';
}
}
If you want to be tricky and ignore other exceptions:
my $magic = [];
eval {
if(failcond){
die $magic;
}
}
if ($# != $magic) {
die; # rethrow
}
Or, you can use the Continuation::Escape module mentioned above. But
there is no reason to ignore exceptions; it is perfectly acceptable
to use them this way.
Given your example, I'd write it this way:
sub lots_of_checks {
local $_ = shift; # You can use 'my' here in 5.10+
return if /condition1/;
return if /condition2/;
# etc.
return 1;
}
Note the bare return instead of return 0. This is usually better because it respects context; the value will be undef in scalar context and () (the empty list) in list context.
If you want to hold to a single-exit point (which is slightly un-Perlish), you can do it without resorting to goto. As the documentation for last states:
... a block by itself is semantically identical to a loop that executes once.
Thus "last" can be used to effect an early exit out of such a block.
sub lots_of_checks {
local $_ = shift;
my $all_clear;
{
last if /condition1/;
last if /condition2/;
# ...
$all_clear = 1; # only set if all checks pass
}
return unless $all_clear;
return 1;
}
If you want to keep your single in/single out structure, you can modify the other suggestions slightly to get:
sub lots_of_checks
{
goto failstate if failcond1;
goto failstate if failcond2;
# This continues on and on until...
return 1; # O happy day!
failstate:
# Any clean up code here.
return; # Dead...
}
IMO, Perl's use of the statement modifier form "return if EXPR" makes guard clauses more readable than they are in C. When you first see the line, you know that you have a guard clause. This feature is often denigrated, but in this case I am quite fond of it.
Using the goto with the statement modifier retains the clarity, and reduces clutter, while it preserves your single exit code style. I've used this form when I had complex clean up to do after failing validation for a routine.