Past exam of the year 1 of the computer science course of the University of Oxford / 2013 / Imperative programming / 5 / c / Solution

The doubling mixin delegates with a transformed argument.For A, the relevant Scala trait linearization is A, DoublePlugin, SumPlugin, Stack. A push first doubles the argument, then SumPlugin sees and sums that doubled value. The physical values are $2,4,6$ and the total is $12$.

For B, the Scala trait linearization is B, SumPlugin, DoublePlugin, Stack. SumPlugin sums the original argument, while its super.push doubles the stored value. The physical values are still $2,4,6$, but the counter is $1+2+3=6$. Hence the two lines printed areEquivalently,This is stackable trait modification: the order determines which arguments an outer mixin observes. In particular, the running-total specification from part (b) holds for A but does not survive the transforming inner mixin in B. With the given implementation, popping from B would subtract the doubled stored value, demonstrating why mixing individually sensible traits can break a combined representation invariant.