Hello, all:

I was composing a node for SOPW, and spent the better part of an hour on it, when all of a sudden, I knew the proper answer. So I thought I'd post it anyway, but in Meditations rather than SOPW, just to amuse my fellow monks. The text between the two lines is what I had at the moment I realized what the answer was:

Hello, all--

I'm trying to figure out the best way to tell if a variable is read-only or not. I can do it by using eval and trapping the error after the fact, but that doesn't quite feel like the correct solution. I could dig around in Devel::Peek or constant.pm and see how hard it would be to determine it a priori, but I haven't monkeyed around with XS yet. Or there could simply be a better method that I've not been able to find.

I've tried a couple Google searches, and Super Search as well (various combinations of "read-only", "readonly", "alias", "@_", "Modification of a read-only"). But if there's a good way to determine the readonly status of a variable, I've not stumbled across it.

Note: This question is XY problem related, as I'm simply trying to test a function. My original goal was to write a trim subroutine to trim leading and trailing blanks from a list of strings, as I get tired of typing the same old thing. So I created:

sub trim {
    for (@_) {
        s/^\s+//;
        s/\s+$//;
    }
}

[download]

It works nicely, but in my test cases, I get the "Modification of a read-only value attempted" error because the first test case is:

subtest 'trim' => sub {
    is("leading", trim("   leading"), "trim: leading");
    ... more test cases ...
};

[download]

Since I want to operate on strings in place, it's obviously a brain-dead test case

I know that it's due to aliasing of @_/$_, but that's intentional, as I want trim to operate on strings in place. Ideally, there would be an is_readonly() function that I could use like:

sub trim {
    for (@_) {
        next if is_readonly($_);
        s/^\s+//;
        s/\s+$//;
    }
}

[download]

For now, I simply changed my test case to:

subtest 'trim' => sub {
    my @test = ("  leading", "trailing  ", "  both  ");
    trim(my $tmp = "  leading");
    is("leading", $tmp, "trim: leading");
    trim($tmp = "trailing  ");
    is("trailing", $tmp, "trim: trailing");
    trim($tmp = "  both  ");
    is("both", $tmp, "trim: leading & trailing");
    trim(@test);
    my $out = "'".join("', '",@test)."'";
    is("'leading', 'trailing', 'both'", $out, "trim: list");
    done_testing();
};

[download]

So, does anyone have a suggestion for me? I expect that the correct answer is to use eval, but I'm not sure.

The reason it took the better part of an hour is that I wanted to be sure I didn't post too much or too little code, and it takes a little time to refine a question properly.

So, I was writing a (to be commented-out) test for using an in-line literal, when I had the revelation: The behavior I want is for it to die with that error message. Otherwise, I'm using the function incorrectly, and I don't want to hide that from myself. So rather than having my code bugs prominent, I was was heading towards a way to make my code fail silently. D'oh!

Also, if I find myself frequently encountering that error, then I probably ought to make trim not operate in place, but instead simply return a list of values.

So while I'm still (slightly) interested in the answer to the question I posed, I'm not sure that it would be good (for me) to have the is_readonly function, as I might be tempted to misuse it in some similar future situation.

...roboticus

In reply to Amusing note on my own stupidity (longish) by roboticus

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.