SPUG: Scoping Guidelines; comments?

[Phillip Neal]

Tim,

My rules of thumb have always been:

1. Suspect everything is global

2. Use clever names in main for all variables. After all, it's
   not Fortran IV anymore.

3. Use even more clever names for variables in subroutines.

4. Avoid using $_ for all but the most trivial cases,especially
   in subroutines.

5. Use lot's of print statements when debugging so you can
   see exactly what you're playing with.

6. Suspect everything is global.

7. Suspect everything is global.

And most of all, suspect everything is global.

Phil


>
>SPUG-ticians,
>
>The less experienced students in our Perl Programming classes tend to
>have lots of trouble dealing with my vs. local, and hate nasty surprises
>like File::Find trashing the $_ that's acting as their loop variable.
>
>So I've been trying to come up with a "Cookbook" approach to
>providing "Scoping Guidance", and would be interested in the comments
>of you professional Perl programmers out there on what I've come up
>with so far.  Have I left anything important out?  Have I made
>recommendations that you disagree with?
>
>There's at least one recommendation here that I've never seen in any
>Perl documentation; enclosing your main program in {}.  This either
>means I've been amazingly clever in figuring out this useful technique
>n my own, or else I'm stupid because everybody else achieves the same
>result in some other way; which is it? 8-}
>
>There's also a sample program to drive home some of the points, which
>must be somewhat cryptic to fit on the same page.
>
>TIA,
>
>-Tim
>
>	The Consultix "Perl Scoping Guidelines", v0.8
>
>FOR SMALL OR SIMPLE PROGRAMS
>   Relax and enjoy the freedom of Perl !
>
>FOR ALL NON-TRIVIAL PROGRAMS (including LARGE/COMPLEX ones)
>* limit scope of variables
>   > use my for user variables, local for pre-defineds ($_, $/, etc.)
>* pass data to subs via the argument list (i.e., avoid globals)
>* enclose main program in { }
>   > so my vars won't have file scope, and leak into subs
>	  defined below
>
>IN LARGE OR COMPLEX PROGRAMS
>* avoid using $_ ; widespread use creates many problems!
>* localize temporary changes to built-in vars with new block
>	{ local $,=' '; print; }
>* put "use strict" at top of program, to force use of my()
>   > if you need a global variable, qualify with package name;
>	$count becomes $main::count
>
>SPECIAL CASES
>* $_ automatically localized to { } when provided as default
>	loop or function variable
>*  foreach (LIST) { },  grep { } LIST,  map { }	LIST,  etc.
>
>				EXAMPLE
>	(somewhat nonsensical, but fits on page with above!)
>
>{ 	# these parens confine MAIN variables declared with my()
>	my  @names=('Joe', 'Nancy');
>	local  $_='(303) 456-7890';
>	foreach (@names) { print  "$_\n"; }  # different, local $_
>	print  "$_\n";		# print the phone number
>	my  $result = compute(@names);
>	{ $\='\cL'; print  $result; }
>}	# end of MAIN
>
>sub  compute  {
>	local  $_='Test Data';	# won't collide with main's $_
>	foreach my $var (@_) {	#  $var unavailable to s2()
>		s2( )  and  print  $var ;   }	#  $_ available to s2()
>	my  $result=s2();  return  $result;	# not main's $result
>}
>

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