First, results of a search for read-only.

Read-only variable in foreach loop? References, Prototypes, and read-only values Modification of a read-only

Next, a first for me 

F:\dev\vladb>perl -MO=Deparse readonly.pl
Can't call method "PADLIST" on an undefined value at C:/Perl/lib/B/Dep
+arse.pm li
ne 1039.
CHECK failed--call queue aborted.

[download]
And now an attempt an an explanation.

perlsyn explains this well, but is going on when $path_list is the empty string (""), what your *ugly* code breaks down to is

foreach my $some_path ( (defined $path_list &&
                         $#$path_list >= 0)
                         ? @{$path_list} : "" ) {

clean_path($some_path); # $some_path is now ""

## i'd rewrite it as
foreach my $some_path ( (defined $path_list and
                         ref($path_list) eq 'ARRAY'
                         and scalar @{$path_list} )
                         ? @{$path_list} : '') {

[download]
anyway, as I was saying, you ought to check if $path_list is a reference to an ARRAY, and if it's not, you ought to return one([] what I meant was, undef, or (), or maybe even @{[]}, but definetly not '', hopefullly you get the picture) instead of the empty string(""), because what happens is the same thing I demonstrate with the following one line perl -e"for my $orK(1,2,3,4) { $orK =~ s/.//g; } " which would also throw the read-only error. Why, because (1,2,3,$foo,$4) is not a data structure (an array). It is a list. The for loop implies list context. And since it is not an array, $orK, which becomes the alias for each element of the list, points to hard coded members of a list, which are read only. The following perl -e"@f=qw,1 2 3 4,;for my $orK(@f){ $orK =~ s/.//g; }" gets no read-only error. Why? cause $orK becomes an alias for each member of the list, which in this case is a reference to $f[0] which is a data structure. Its pretty much the same as perl -e"@f=qw,1 2 3 4,;for my $orK($f[0],$f[1],$f[2],$f[3]){ $orK =~ s/.//g; }" That's it. Try it out, read perlsyn again, read japhys excellent context tutorial (List is a 4 letter word, it's on his website, link to which is on his homenode).

I'm done.

 
______crazyinsomniac_____________________________
Of all the things I've lost, I miss my mind the most.
perl -e "$q=$_;map({chr unpack qq;H*;,$_}split(q;;,q*H*));print;$q/$q;"

In reply to (crazyinsomniac) Re: How could I check if a variable is 'read-only'? by crazyinsomniac
in thread How could I check if a variable is 'read-only'? by vladb

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