I said I'd use unless( complex expression ) over if( ! ( complex expression ) ) so your argument doesn't apply since the if( ! ( complex expression ) ) would require the same translation under the same circumstances.

Whether it makes more sense to distribute the "not" inside of the complex expression depends on a lot of things.

    # unless object is valid and either isn't busy or can wait:
    unless(  $a  &&  ref($a)  &&  isa($a,'Foo')
         &&  (  ! $a->IsBusy()  ||  $a->CanWait()  )  ) {
        return;
    }
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is clearer to me than:

    # if object is not valid or both is busy and can't wait:
    if(  ! $a  ||  ! ref($a)  ||  ! isa($a,'Foo')
     ||  $a->IsBusy()  &&  ! $a->CanWait()  ) {
        return;
    }
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and I don't understand why you (seem to) think that unless can't be understood without translating it into if. I'd only use unless in a case where it makes the meaning clearer.

For simple cases, I find that the difference in clarity is minimal and so prefer the consistancy of always using if. As a statement modifier, unless sometimes reads more naturally. As a conditional block, the benefit of unless only becomes worthwhile to me when the expression is complex enough that factoring out the negation can make a relatively big difference in the clarity of the code.

        - tye (but my friends call me "Tye")