Given possible states A,B,C,D and the if-elsif-else structure 

      if( state equals A) { Take action1 }
   elsif( state equals B) { Take action2 }
   else                   { Take action3 }

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any given state will cause exactly 1 action to be taken no matter what the value of state.

However using this 

    unless( state equals A ) { Take action1 }
elsunless( state equals B ) { Take action2 }
else                        { Take action3 }

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If the state is C or D, by the logic of the statements, actions 1 and 2 would need to be taken; but as the entire block terminates once one condition succeeds, action2 would only ever be performed if the state was A, which is counter-intuative to say the least. Also, when would the else clause be performed?

Effectively, the structure above is equivalent to 

   unless( state equals A) { Take action1 }
   elsif ( state equals A) { Take action2 }
   else                    { Take action3 }

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which is already legal in Perl, but is actually the same as 

   unless( state equals A) { Take action1 }
   else                    { Take action2 }

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As action three will never be taken as every state except A will have caused action 1 and then the contruct terminates.

Nah! You're thinking of Simon Templar, originally played (on UKTV) by Roger Moore and later by Ian Ogilvy

In reply to Re: if/unless and chaining by BrowserUk
in thread if/unless and chaining by John M. Dlugosz

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