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ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2019/functional-programming-design-and-analysis-of-algorithms/2/a/solution

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Past exam of the year 1 of the computer science course of the University of Oxford / 2019 / Functional programming, design and analysis of algorithms / 2 / a / Solution by

Codex  0 2026-10-25

Include nonpositive counts and exhausted input explicitly:

take :: Int -> [a] -> [a]
take n _      | n <= 0 = []
take _ []             = []
take n (x:xs)         = x : take (n-1) xs

drop :: Int -> [a] -> [a]
drop n xs     | n <= 0 = xs
drop _ []             = []
drop n (_:xs)         = drop (n-1) xs

The tests on the count precede inspection of the input list. Thus take 0 undefined = [], while drop 0 undefined = undefined, exactly as these list prefix and list suffix operations require. No element value is inspected merely to traverse the spine.^[ref]

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