From: Michael G. Schwern Date: Tue, 7 Oct 2003 20:39:36 +0000 (-0700) Subject: Re: [PATCH] perlop.pod Revamp - revision 4 X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=commitdiff_plain;h=d042e63d2577050672e3a651fad824e0022e97f5;p=p5sagit%2Fp5-mst-13.2.git Re: [PATCH] perlop.pod Revamp - revision 4 Message-Id: <20031007203936.X4301@ttul.org> (Originally from Shlomi Fish. Applied with tweaks.) p4raw-id: //depot/perl@21421 --- diff --git a/pod/perlop.pod b/pod/perlop.pod index 1550660..a50dee9 100644 --- a/pod/perlop.pod +++ b/pod/perlop.pod @@ -2,7 +2,24 @@ perlop - Perl operators and precedence -=head1 SYNOPSIS +=head1 DESCRIPTION + +=head2 Operator Precedence and Associativity + +Operator precedence and associativity work in Perl more or less like +they do in mathematics. + +I means some operators are evaluated before +others. For example, in C<2 + 4 * 5>, the multiplication has higher +precedence so C<4 * 5> is evaluated first yielding C<2 + 20 == +22> and not C<6 * 5 == 30>. + +I defines what happens if a sequence of the +same operators is used one after another: whether the evaluator will +evaluate the left operations first or the right. For example, in C<8 +- 4 - 2>, subtraction is left associative so Perl evaluates the +expression left to right. C<8 - 4> is evaluated first making the +expression C<4 - 2 == 2> and not C<8 - 2 == 6>. Perl operators have the following associativity and precedence, listed from highest precedence to lowest. Operators borrowed from @@ -40,8 +57,6 @@ In the following sections, these operators are covered in precedence order. Many operators can be overloaded for objects. See L. -=head1 DESCRIPTION - =head2 Terms and List Operators (Leftward) A TERM has the highest precedence in Perl. They include variables, @@ -83,8 +98,18 @@ Also note that print ($foo & 255) + 1, "\n"; -probably doesn't do what you expect at first glance. See -L for more discussion of this. +probably doesn't do what you expect at first glance. The parentheses +enclose the argument list for C which is evaluated (printing +the result of C<$foo & 255>). Then one is added to the return value +of C (usually 1). The result is something like this: + + 1 + 1, "\n"; # Obviously not what you meant. + +To do what you meant properly, you must write: + + print(($foo & 255) + 1, "\n"); + +See L for more discussion of this. Also parsed as terms are the C and C constructs, as well as subroutine and method calls, and the anonymous @@ -110,9 +135,14 @@ or a class name (that is, a package name). See L. =head2 Auto-increment and Auto-decrement -"++" and "--" work as in C. That is, if placed before a variable, they -increment or decrement the variable before returning the value, and if -placed after, increment or decrement the variable after returning the value. +"++" and "--" work as in C. That is, if placed before a variable, +they increment or decrement the variable by one before returning the +value, and if placed after, increment or decrement after returning the +value. + + $i = 0; $j = 0; + print $i++; # prints 0 + print ++$j; # prints 1 The auto-increment operator has a little extra builtin magic to it. If you increment a variable that is numeric, or that has ever been used in @@ -157,7 +187,7 @@ example, C<0666 & ~027> is 0640. (See also L and L.) Note that the width of the result is platform-dependent: ~0 is 32 bits wide on a 32-bit platform, but 64 bits wide on a 64-bit platform, so if you are expecting a certain bit -width, remember use the & operator to mask off the excess bits. +width, remember to use the & operator to mask off the excess bits. Unary "+" has no effect whatsoever, even on strings. It is useful syntactically for separating a function name from a parenthesized expression @@ -382,12 +412,12 @@ tests the left hand side's definedness instead of its truth. Thus, C<$a // $b> is similar to C (except that it returns the value of C<$a> rather than the value of C) and is exactly equivalent to C. This is very useful for providing default values -for variables. If you actually want to test if at least one of C<$a> and C<$b> is -defined, use C. +for variables. If you actually want to test if at least one of C<$a> and +C<$b> is defined, use C. -The C<||>, C and C<&&> operators differ from C's in that, rather than returning -0 or 1, they return the last value evaluated. Thus, a reasonably portable -way to find out the home directory might be: +The C<||>, C and C<&&> operators return the last value evaluated +(unlike C's C<||> and C<&&>, which return 0 or 1). Thus, a reasonably +portable way to find out the home directory might be: $home = $ENV{'HOME'} // $ENV{'LOGDIR'} // (getpwuid($<))[7] // die "You're homeless!\n"; @@ -619,9 +649,10 @@ argument and returns that value. This is just like C's comma operator. In list context, it's just the list argument separator, and inserts both its arguments into the list. -The => digraph is mostly just a synonym for the comma operator. It's useful for -documenting arguments that come in pairs. As of release 5.001, it also forces -any word to the left of it to be interpreted as a string. +The C<< => >> operator is a synonym for the comma, but forces any word +to its left to be interpreted as a string (as of 5.001). It is helpful +in documenting the correspondence between keys and values in hashes, +and other paired elements in lists. =head2 List Operators (Rightward)