IRIX Base Documentation 2002 November/SGI IRIX Base Documentation 2002 November.iso/usr/share/catman/u_man/cat1/perlstyle.z/perlstyle

Text File | 2002-10-03 | 10.4 KB | 331 lines

PPPPEEEERRRRLLLLSSSSTTTTYYYYLLLLEEEE((((1111))))                                                      PPPPEEEERRRRLLLLSSSSTTTTYYYYLLLLEEEE((((1111))))



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     perlstyle - Perl style guide

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     Each programmer will, of course, have his or her own preferences in
     regards to formatting, but there are some general guidelines that will
     make your programs easier to read, understand, and maintain.

     The most important thing is to run your programs under the ----wwww flag at all
     times.  You may turn it off explicitly for particular portions of code
     via the $^W variable if you must.  You should also always run under use
     strict or know the reason why not.  The use sigtrap and even use
     diagnostics pragmas may also prove useful.

     Regarding aesthetics of code lay out, about the only thing Larry cares
     strongly about is that the closing curly brace of a multi-line BLOCK
     should line up with the keyword that started the construct.  Beyond that,
     he has other preferences that aren't so strong:

     +o   4-column indent.

     +o   Opening curly on same line as keyword, if possible, otherwise line
         up.

     +o   Space before the opening curly of a multi-line BLOCK.

     +o   One-line BLOCK may be put on one line, including curlies.

     +o   No space before the semicolon.

     +o   Semicolon omitted in "short" one-line BLOCK.

     +o   Space around most operators.

     +o   Space around a "complex" subscript (inside brackets).

     +o   Blank lines between chunks that do different things.

     +o   Uncuddled elses.

     +o   No space between function name and its opening parenthesis.

     +o   Space after each comma.

     +o   Long lines broken after an operator (except "and" and "or").

     +o   Space after last parenthesis matching on current line.

     +o   Line up corresponding items vertically.






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PPPPEEEERRRRLLLLSSSSTTTTYYYYLLLLEEEE((((1111))))                                                      PPPPEEEERRRRLLLLSSSSTTTTYYYYLLLLEEEE((((1111))))



     +o   Omit redundant punctuation as long as clarity doesn't suffer.

     Larry has his reasons for each of these things, but he doesn't claim that
     everyone else's mind works the same as his does.

     Here are some other more substantive style issues to think about:

     +o   Just because you _C_A_N do something a particular way doesn't mean that
         you _S_H_O_U_L_D do it that way.  Perl is designed to give you several ways
         to do anything, so consider picking the most readable one.  For
         instance

             open(FOO,$foo) || die "Can't open $foo: $!";

         is better than

             die "Can't open $foo: $!" unless open(FOO,$foo);

         because the second way hides the main point of the statement in a
         modifier.  On the other hand

             print "Starting analysis\n" if $verbose;

         is better than

             $verbose && print "Starting analysis\n";

         because the main point isn't whether the user typed ----vvvv or not.

         Similarly, just because an operator lets you assume default arguments
         doesn't mean that you have to make use of the defaults.  The defaults
         are there for lazy systems programmers writing one-shot programs.  If
         you want your program to be readable, consider supplying the
         argument.

         Along the same lines, just because you _C_A_N omit parentheses in many
         places doesn't mean that you ought to:

             return print reverse sort num values %array;
             return print(reverse(sort num (values(%array))));

         When in doubt, parenthesize.  At the very least it will let some poor
         schmuck bounce on the % key in vvvviiii.

         Even if you aren't in doubt, consider the mental welfare of the
         person who has to maintain the code after you, and who will probably
         put parentheses in the wrong place.

     +o   Don't go through silly contortions to exit a loop at the top or the
         bottom, when Perl provides the last operator so you can exit in the
         middle.  Just "outdent" it a little to make it more visible:




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PPPPEEEERRRRLLLLSSSSTTTTYYYYLLLLEEEE((((1111))))                                                      PPPPEEEERRRRLLLLSSSSTTTTYYYYLLLLEEEE((((1111))))



             LINE:
                 for (;;) {
                     statements;
                   last LINE if $foo;
                     next LINE if /^#/;
                     statements;
                 }


     +o   Don't be afraid to use loop labels--they're there to enhance
         readability as well as to allow multilevel loop breaks.  See the
         previous example.

     +o   Avoid using _g_r_e_p() (or _m_a_p()) or `backticks` in a void context, that
         is, when you just throw away their return values.  Those functions
         all have return values, so use them.  Otherwise use a _f_o_r_e_a_c_h() loop
         or the _s_y_s_t_e_m() function instead.

     +o   For portability, when using features that may not be implemented on
         every machine, test the construct in an eval to see if it fails.  If
         you know what version or patchlevel a particular feature was
         implemented, you can test $] ($PERL_VERSION in English) to see if it
         will be there.  The Config module will also let you interrogate
         values determined by the CCCCoooonnnnffffiiiigggguuuurrrreeee program when Perl was installed.

     +o   Choose mnemonic identifiers.  If you can't remember what mnemonic
         means, you've got a problem.

     +o   While short identifiers like $gotit are probably ok, use underscores
         to separate words.  It is generally easier to read
         $var_names_like_this than $VarNamesLikeThis, especially for non-
         native speakers of English. It's also a simple rule that works
         consistently with VAR_NAMES_LIKE_THIS.

         Package names are sometimes an exception to this rule.  Perl
         informally reserves lowercase module names for "pragma" modules like
         integer and strict.  Other modules should begin with a capital letter
         and use mixed case, but probably without underscores due to
         limitations in primitive file systems' representations of module
         names as files that must fit into a few sparse bytes.

     +o   You may find it helpful to use letter case to indicate the scope or
         nature of a variable. For example:

             $ALL_CAPS_HERE   constants only (beware clashes with perl vars!)
             $Some_Caps_Here  package-wide global/static
             $no_caps_here    function scope my() or local() variables

         Function and method names seem to work best as all lowercase.  E.g.,
         $obj->_a_s__s_t_r_i_n_g().

         You can use a leading underscore to indicate that a variable or



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PPPPEEEERRRRLLLLSSSSTTTTYYYYLLLLEEEE((((1111))))                                                      PPPPEEEERRRRLLLLSSSSTTTTYYYYLLLLEEEE((((1111))))



         function should not be used outside the package that defined it.

     +o   If you have a really hairy regular expression, use the /x modifier
         and put in some whitespace to make it look a little less like line
         noise.  Don't use slash as a delimiter when your regexp has slashes
         or backslashes.

     +o   Use the new "and" and "or" operators to avoid having to parenthesize
         list operators so much, and to reduce the incidence of punctuation
         operators like && and ||.  Call your subroutines as if they were
         functions or list operators to avoid excessive ampersands and
         parentheses.

     +o   Use here documents instead of repeated _p_r_i_n_t() statements.

     +o   Line up corresponding things vertically, especially if it'd be too
         long to fit on one line anyway.

             $IDX = $ST_MTIME;
             $IDX = $ST_ATIME       if $opt_u;
             $IDX = $ST_CTIME       if $opt_c;
             $IDX = $ST_SIZE        if $opt_s;

             mkdir $tmpdir, 0700 or die "can't mkdir $tmpdir: $!";
             chdir($tmpdir)      or die "can't chdir $tmpdir: $!";
             mkdir 'tmp',   0777 or die "can't mkdir $tmpdir/tmp: $!";


     +o   Always check the return codes of system calls.  Good error messages
         should go to STDERR, include which program caused the problem, what
         the failed system call and arguments were, and (VERY IMPORTANT)
         should contain the standard system error message for what went wrong.
         Here's a simple but sufficient example:

             opendir(D, $dir)     or die "can't opendir $dir: $!";


     +o   Line up your transliterations when it makes sense:

             tr [abc]
                [xyz];


     +o   Think about reusability.  Why waste brainpower on a one-shot when you
         might want to do something like it again?  Consider generalizing your
         code.  Consider writing a module or object class.  Consider making
         your code run cleanly with use strict and ----wwww in effect.  Consider
         giving away your code.  Consider changing your whole world view.
         Consider... oh, never mind.






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PPPPEEEERRRRLLLLSSSSTTTTYYYYLLLLEEEE((((1111))))                                                      PPPPEEEERRRRLLLLSSSSTTTTYYYYLLLLEEEE((((1111))))



     +o   Be consistent.

     +o   Be nice.




















































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