Past exam of the year 1 of the computer science course of the University of Oxford / 2017 / Imperative programming (resit) / 7 / c / Solution

Use tagged disjoint unions for results that can contain either a value or an error message. This avoids ambiguity even if the item set itself contains the string Empty queue. The operation signatures areHere $1$ is a singleton, expressing that create takes no meaningful argument. Write successful results as ${\mathrm{ok}}_I(i)$ and ${\mathrm{ok}}_Q(q)$, and errors as ${\mathrm{err}}_I(e)$ and ${\mathrm{err}}_Q(e)$ for $e=\text{Empty queue}$. Returning an error is not returning a queue, so its result must not be silently passed to another queue operation.