April 25, 2024
Answer to Q2
aToad position: aFrog position + 3 * 2 - aToad position
The answer returned is aToad
The position of aToad is -3
Explanation.
A Toad instance will always return itself when
it receives the keyword message position:
There are no brackets in the message expression above. It could have
been written
aToad position: (aFrogposition + 3 * 2 - aToad position)
or
aToad position: ((aFrogposition) + 3 * 2 - aToad position)
or
aToad position: ((aFrogposition) + 3 * 2 - (aToad position))
All these expressions would yield the same answer. Without any brackets
Smalltalk reads left to right and evaluates the first unary expression
it finds, in this case
aFrog position. This returns the number
object 1.
Smalltalk continues its search for unary expressions and finds aToad
position. This returns the number object 11.
In effect Smalltalk must now evaluate aToad position: 1 + 3 * 2 - 11
Reading left to right, binary before keyword, the number object
1 receives the binary message +
3 and returns the number object 4.
4 now becomes the receiver of the binary
message * 2 and returns the number object
8.
8 becomes the receiver of the binary message - 11 and returns
the number object -3.
-3 is passed as the argument to the keyword selector position:
and aToad finally receives the keyword message
position: -3
aToad returns itself as the message answer. Remember the textual
representation of this message answer object is not aToad but
An instance of class Toad (position -3, colour Brown)
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