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Re^2: Importing a variable into a package

by frazap (Monk)
on May 16, 2018 at 09:35 UTC ( #1214621=note: print w/replies, xml ) Need Help??

in reply to Re: Importing a variable into a package (updated)
in thread Importing a variable into a package

Thanks for the detail explanation !

From my tests with Text::Template, I saw that I had to name variables without the package component to catch syntax errors. Having read the docs (sorry, can't find exactly where) this became clearer.

There is no begin bloc in the loop that gives values to the glob variables Re: Text::Template and delayed use vars as in your last working example

eval ' package T; print "B: $x\n"; 1 ' or warn "B: $@";

This seems to be what happen in the module (version 1.47), around line 309 (the print line is mine):

sub fill_in { ... my $fi_progtext = "package $fi_eval_package; $fi_prepend;\n$fi_lcomment\n$fi_tex +t;"; ... print "eval on \n", $fi_progtext,"\n"; $fi_res = eval $fi_progtext;

Which prints

eval on package T; use strict;; #line 1 dokpe.tmpl $annee ; eval on package T; use strict;; #line 4 dokpe.tmpl "\t";
For a template file starting with
Plan de fermeture { $annee } Bibliothèque de la Faculté des Sciences (Dokpe) Motif{"\t"}

So to be sure I understand correctly: since I use eval during the script runtime, I do not need the begin block to assign a value to the glob. In my example above, the begin was needed because the print statement was run during the execution time. Is that correct ?

Thanks again


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Re^3: Importing a variable into a package (updated)
by haukex (Bishop) on May 17, 2018 at 08:35 UTC

    Your understanding is <update> almost correct, just your reason for needing the BEGIN in the root node is a little off, the problem happens at the compile time of the print $y. It's probably best if you look at compile time vs. run time more in general. </update>

    Perl of course has to parse and compile a piece of code like *T::x=\"x"; when it encounters it. However, that piece of code isn't run until runtime, that is, the assignment to the glob doesn't happen until after the entire surrounding code is compiled and while it is being executed.

    So if the assignment is immediately followed by print $x;, when Perl tries to compile that statement, it does not yet know about $x!

    That's why in your example code you need to move the execution of the assignment into the compile time of the surrounding code via BEGIN.

    And in the case of the module, which does indeed appear to use eval: eval STRING needs to compile and run the code it is given, but it does not do so until the eval itself is executed, that is, the runtime of the eval statement. So in this case, if the assignment to the glob happens at run time instead of compile time, as long as it happens before the eval, this will still work, because the assignment is still happening before the compile time of the code that was given to eval.

    Note that this way, with BEGIN and eval in Perl it's possible to basically have as many compile and run times as one wants.

    Also a few updates for clarity.

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