5.3 Watch Your QuotesThere are some subtleties associated with the way quotes or blocks are interpreted by Perl. Consider the differences between the following statements: $str = '$c = 10'; # eval $str; # 1 eval "$str"; # 2 eval '$str'; # 3 eval { $str }; # 4 Cases 1 and 2 have identical results, and cases 3 and 4 behave identically. Can you see why? The trick is to know what the interpreter does before handing it over to eval .
In case 1, Perl gives the
contents
of
$str
to
eval
, just as it would for any other function. Hence
eval
sees the string In case 2, Perl does variable interpolation on the double-quoted string before handing it over to eval . Again, eval sees the contents of $str , compiles it, and executes it, assigning 10 to $c . In case 3, the argument to eval is a single-quoted string, which is not expanded during the variable interpolation stage. For this reason, eval sees a hardcoded string (with the characters "$", "s", "t", "r") and treats it like a little program as before. As a standalone program, it is quite useless, of course. Since eval returns the result of the last expression, it returns the value of $str (the string $c = 10 ). That is, if you say, $s = eval '$str'; $s will contain $c = 10 . Case 4 is identical to case 3, except that the code inside the block is checked for syntax errors at compile-time (at the same time as the rest of the script). That's all there is to know about eval . Now, let us see how to use it for expression evaluation, exception handling, and efficiency. Copyright © 2001 O'Reilly & Associates. All rights reserved. |
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