in reply to Useless unless

What I was trying to get at was an 'else' for statement-modifier 'if'. The idea is to avoid the (quasi)equivalent 
foo() if $x;
bar() unless $x;

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because that has serious potential problems. Namely, the conditional expression ($x) could be non-idempotent, or it could be expensive to evaluate. (Note that simply being a scalar variable doesn't avoid this problem: it could be tied.)

The workaround is to use a temporary variable: 

{                  | for my $b ( $x )   | for $x -> my $b
  my $b = $x;      | {                  | {
  foo() if $b;     |   foo() if $b;     |   foo() if $b;
  bar() unless $b; |   bar() unless $b; |   bar() unless $b;
}                  | }                  | }

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but that's getting ugly. You may as well use the standard if/else and avoid a temporary.

As for the suggestion of using the trinary operatory, I would just point out that the intended purpose of the trinary operator is as a selector between two expressions rather than two statements — that is, between data rather than code. Of course you can use it to select between expressions which are there for their side-effects... but using the trinary operator in void context is — or should be, to seasoned Perl programmers — as distasteful as using map in void context: Sure, it works, and has no ill effects, but as a matter of style, it's just bad.

Another approach to 'otherwise' would be 
foo() else bar() if $x;

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but that has the disadvantage of putting the "false" action right next to the 'if', so that a human might (mistakenly) see "... bar() if $x;" which is completely wrong.

Anyway, I really like 'otherwise', though I'm not seriously proposing it as an addition to the language. It's too late for such things. :-)

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Re^2: Useless unless
by Anonymous Monk on Aug 09, 2005 at 09:06 UTC
    Sure, it works, and has no ill effects, but as a matter of style, it's just bad.
    That's just a matter of opinion. A piece of opinion that doesn't come with a rational. Knowing you find the style bad is interesting for census takers, programmers will find it more interesting why you find it bad style.

    I rank using the trinary in void context on the same level as using 'or' or 'and' in void context: 

        /^#/ and next;
    open ... or die ...;
    -d $_ ? handle_directory($_) : handle_file($_);

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    and those are almost on the same level as using statement modifiers: 
        next if /^#/;
    die ... unless open ...;
    parse_file($_) for @files;

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    Either they are all bad, or none of them are. And for me, none of them are.
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Re^2: Useless unless
by Anonymous Monk on Aug 09, 2005 at 09:17 UTC
    You may as well use the standard if/else and avoid a temporary.
    No temporary variable, and no if/else, replacing 
        EXPR1 if $x;
    EXPR2 unless $x;

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    can be done as: 
       ((sub {EXPR1}, sub {EXPR2})[!!$x || 0])->();

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    except that EXPR1 and EXPR2 in the latter have their own scopes - but you have that with an if/else construct as well.

    If EXPR1 and EXPR2 are function calls using the same arguments, as in: 

        foo($arg, @args) if $x;
    bar($arg, @args) unless $x;

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    one could write: 
        ((\&foo, \&bar)[!!$x || 0])->($arg, @args)

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