Re: Recommended Perl 6 best practices?

[Jacinta Richardson]

Carl Mäsak wrote:

 Do not combine 'ne' and '|', like this:
 
 die Unrecognized directive: TMPL_$directive
if $directive ne 'VAR' | 'LOOP' | 'IF';

 One is tempted to assume that this means the same as $directive ne
 'VAR' || $directive ne 'LOOP' || $directive ne 'IF, but it doesn't.
 Instead, it's a negated string comparison against three values, the
 results of which are then OR-ed together. The condition will always be
 true, because there's always at least two values that $directive is
 not string-equal to.

$directive ne 'VAR' || $directive ne 'LOOP' || $directive ne 'IF'

is always true.  For example assuming $directive = cat;

cat ne 'VAR' (true) || cat ne 'LOOP' (true) || cat ne 'IF' (true)

or assuming $directive = VAR;   Then:

VAR ne 'VAR (false) || VAR ne 'LOOP' (true) || VAR ne 'IF' (true)

which is clearly not what you wanted.  This works if you write instead:

$directive ne 'VAR'  $directive ne 'LOOP'  $directive ne 'IF'

VAR ne 'VAR (false)  VAR ne 'LOOP' (true)  VAR ne 'IF' (true)

Result: false.   (if it is not one of these things then)

Of course now I'm confused as to what:   $directive ne 'VAR' | 'LOOP' | 'IF'
does.  If it was equivalent to:

not ($directive eq 'VAR' || $directive eq 'LOOP' || $directive eq 'IF')

then it would be doing the right thing.  So since it isn't doing the right
thing, I can only assume that your statement above is in error and that:

$directive ne 'VAR' | 'LOOP' | 'IF'

IS INDEED the same as:

$directive ne 'VAR' || $directive ne 'LOOP' || $directive ne 'IF'

and that your expectation of the code is wrong.

 The correct form using junctions would be this:
 
 die Unrecognized directive: TMPL_$directive
if $directive ne 'VAR'  'LOOP'  'IF';

which makes sense, because this does give us:

$directive ne 'VAR'  $directive ne 'LOOP'  $directive ne 'IF'

and that is provably correct.

 The more general advice, then, would be not to use junctions together
 with negated equality operators. Instead, use the non-negated equality
 operator, and negate the whole expression.

This is probably a good idea regardless,

J

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Re: CGI Session management (was Re: the CGI.pm in Perl 6)

[Jacinta Richardson]

Fagyal Csongor wrote:

  # imagine something like:
 $cgi = new CGI;
 $html = HTML::CGI-new($cgi);
 $html-popup_menu( ... );   # I won't do this, but others might... :)

My biggest gripe with CGI's html methods is the inconsistency in their
names.  I use them every now and then, but I always have to go and look
up the documentation.  It's textfield isn't it?  So that would make
this one passwordfield: nope, it's password_field.  popup_menu so
scrolling_menu... Ah, no: scrolling_list.  How do I make it
multi-select again?

I'd love the Perl 6 version to have a compatibility mode where it
returned the methods we're all used to, but encouraged something which
was more intuitive.  Perhaps that would be better in two modules which
essentially did the same thing though.

All the best,

J