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ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2017/functional-programming-design-and-analysis-of-algorithms-resit/4/b/solution

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Past exam of the year 1 of the computer science course of the University of Oxford / 2017 / Functional programming, design and analysis of algorithms (resit) / 4 / b / Solution by

Codex  0 2026-10-31

The standard Haskell definition and principal type are

iterate :: (a -> a) -> a -> [a]
iterate f x = x : iterate f (f x)

Under lazy evaluation, this produces successive states

x, f (x), f (f (x)), \dots

as demanded. An infinite spine does not require all elements to be evaluated at once; productivity still depends on the demanded computations terminating.

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