(gettext.info)General Problems

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15.5.18.1 General Problems Parsing Perl Code
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   It is often heard that only Perl can parse Perl.  This is not true.
Perl cannot be _parsed_ at all, it can only be _executed_.  Perl has
various built-in ambiguities that can only be resolved at runtime.

   The following example may illustrate one common problem:

     print gettext "Hello World!";

   Although this example looks like a bullet-proof case of a function
invocation, it is not:

     open gettext, ">testfile" or die;
     print gettext "Hello world!"

   In this context, the string ‘gettext’ looks more like a file handle.
But not necessarily:

     use Locale::Messages qw (:libintl_h);
     open gettext ">testfile" or die;
     print gettext "Hello world!";

   Now, the file is probably syntactically incorrect, provided that the
module ‘Locale::Messages’ found first in the Perl include path exports a
function ‘gettext’.  But what if the module ‘Locale::Messages’ really
looks like this?

     use vars qw (*gettext);

     1;

   In this case, the string ‘gettext’ will be interpreted as a file
handle again, and the above example will create a file ‘testfile’ and
write the string “Hello world!” into it.  Even advanced control flow
analysis will not really help:

     if (0.5 < rand) {
        eval "use Sane";
     } else {
        eval "use InSane";
     }
     print gettext "Hello world!";

   If the module ‘Sane’ exports a function ‘gettext’ that does what we
expect, and the module ‘InSane’ opens a file for writing and associates
the _handle_ ‘gettext’ with this output stream, we are clueless again
about what will happen at runtime.  It is completely unpredictable.  The
truth is that Perl has so many ways to fill its symbol table at runtime
that it is impossible to interpret a particular piece of code without
executing it.

   Of course, ‘xgettext’ will not execute your Perl sources while
scanning for translatable strings, but rather use heuristics in order to
guess what you meant.

   Another problem is the ambiguity of the slash and the question mark.
Their interpretation depends on the context:

     # A pattern match.
     print "OK\n" if /foobar/;

     # A division.
     print 1 / 2;

     # Another pattern match.
     print "OK\n" if ?foobar?;

     # Conditional.
     print $x ? "foo" : "bar";

   The slash may either act as the division operator or introduce a
pattern match, whereas the question mark may act as the ternary
conditional operator or as a pattern match, too.  Other programming
languages like ‘awk’ present similar problems, but the consequences of a
misinterpretation are particularly nasty with Perl sources.  In ‘awk’
for instance, a statement can never exceed one line and the parser can
recover from a parsing error at the next newline and interpret the rest
of the input stream correctly.  Perl is different, as a pattern match is
terminated by the next appearance of the delimiter (the slash or the
question mark) in the input stream, regardless of the semantic context.
If a slash is really a division sign but mis-interpreted as a pattern
match, the rest of the input file is most probably parsed incorrectly.

   There are certain cases, where the ambiguity cannot be resolved at
all:

     $x = wantarray ? 1 : 0;

   The Perl built-in function ‘wantarray’ does not accept any arguments.
The Perl parser therefore knows that the question mark does not start a
regular expression but is the ternary conditional operator.

     sub wantarrays {}
     $x = wantarrays ? 1 : 0;

   Now the situation is different.  The function ‘wantarrays’ takes a
variable number of arguments (like any non-prototyped Perl function).
The question mark is now the delimiter of a pattern match, and hence the
piece of code does not compile.

     sub wantarrays() {}
     $x = wantarrays ? 1 : 0;

   Now the function is prototyped, Perl knows that it does not accept
any arguments, and the question mark is therefore interpreted as the
ternaray operator again.  But that unfortunately outsmarts ‘xgettext’.

   The Perl parser in ‘xgettext’ cannot know whether a function has a
prototype and what that prototype would look like.  It therefore makes
an educated guess.  If a function is known to be a Perl built-in and
this function does not accept any arguments, a following question mark
or slash is treated as an operator, otherwise as the delimiter of a
following regular expression.  The Perl built-ins that do not accept
arguments are ‘wantarray’, ‘fork’, ‘time’, ‘times’, ‘getlogin’,
‘getppid’, ‘getpwent’, ‘getgrent’, ‘gethostent’, ‘getnetent’,
‘getprotoent’, ‘getservent’, ‘setpwent’, ‘setgrent’, ‘endpwent’,
‘endgrent’, ‘endhostent’, ‘endnetent’, ‘endprotoent’, and ‘endservent’.

   If you find that ‘xgettext’ fails to extract strings from portions of
your sources, you should therefore look out for slashes and/or question
marks preceding these sections.  You may have come across a bug in
‘xgettext’’s Perl parser (and of course you should report that bug).  In
the meantime you should consider to reformulate your code in a manner
less challenging to ‘xgettext’.

   In particular, if the parser is too dumb to see that a function does
not accept arguments, use parentheses:

     $x = somefunc() ? 1 : 0;
     $y = (somefunc) ? 1 : 0;

   In fact the Perl parser itself has similar problems and warns you
about such constructs.