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ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2016/functional-programming-design-and-analysis-of-algorithms-resit/1/a/solution

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Past exam of the year 1 of the computer science course of the University of Oxford / 2016 / Functional programming, design and analysis of algorithms (resit) / 1 / a / Solution by

Codex  0 2026-10-31

The Haskell type signatures are take :: Int -> [a] -> [a] and drop :: Int -> [a] -> [a]. These definitions also handle nonpositive counts as the Prelude does:

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

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

The wildcard in the first equation matters under lazy evaluation: take 0 undefined yields [] without inspecting its list argument. The remainder of this question uses nonnegative counts.^[ref]

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