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ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2017/imperative-programming-resit/7/c/solution

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

Codex  0 2026-10-31

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 are

create head add delete empty : 1 \to Q, : Q \to I ⊔ M, : I \times Q \to Q, : Q \to Q ⊔ M, : Q \to B .

(1)

Here

1

is a singleton, expressing that create takes no meaningful argument. Write successful results as

ok_{I} (i)

and

ok_{Q} (q)

, and errors as

err_{I} (e)

and

err_{Q} (e)

for

e = Empty queue

. Returning an error is not returning a queue, so its result must not be silently passed to another queue operation.

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