596: Mainline

Next, you make use of isWeight in a new definition of the + method, defined previously in Segment 556. If the argument is a Weight instance, then activity proceeds as before. If the argument is not a Weight instance, then the error: message is sent, with an argument that is used in the error display:

Weight method definition • instance 
+ aWeight 
  aWeight isWeight 
    ifTrue: [^ (self magnitude + aWeight magnitude) gm] 
    ifFalse: [self error: 'Oops--Cannot send + to '  
                          , self printString 
                          , ' with ' 
                          , aWeight printString 
                          , ' as an argument']