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ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2003/functional-programming-data-structures-and-algorithms-procedural-programming-and-digital-hardware/1/a/solution

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Past exam of the year 1 of the computer science course of the University of Oxford / 2003 / Functional programming, data structures and algorithms, procedural programming, and digital hardware / 1 / a / Solution by

Codex  0 2026-10-31

Use pattern matching to separate the empty list from a head and tail. In Haskell, hide the conflicting Standard Prelude names; the examination's init returns all prefixes of a list, unlike the usual Standard Prelude function of that name.

import Prelude hiding (map, length, init, tails, foldr, compare)

map :: (a -> b) -> [a] -> [b]
map f [] = []
map f (x:xs) = f x : map f xs

length :: Num n => [a] -> n
length [] = 0
length (_:xs) = 1 + length xs

copy :: (Eq n, Num n) => n -> a -> [a]
copy 0 xs = []
copy n xs = xs : copy (n-1) xs

combine :: [[a]] -> [[a]] -> [[a]]
combine [] _ = []
combine _ [] = []
combine (xs:xss) (ys:yss) = (xs ++ ys) : combine xss yss

init :: [a] -> [[a]]
init [] = [[]]
init (x:xs) = [] : map (x:) (init xs)

tails :: [a] -> [[a]]
tails [] = [[]]
tails (x:xs) = (x:xs) : tails xs

The map function preserves the input list shape while transforming each entry. length counts the cons cells. copy repeats the whole value xs, so for the specified use its second argument is a list and the result is a list of lists. Its inferred type is more general: the repeated value can have any data type. If one restricts the interface to lists, the corresponding type is (Eq n, Num n) => n -> [a] -> [[a]]. The numeric argument has the precondition that it is a nonnegative integer; the most general inferred numeric constraints do not guarantee termination on arbitrary values of their instances. An explicitly integral interface may instead use Integral n => n -> [a] -> [[a]].

combine performs list concatenation head by head and stops as soon as either outer list ends. For init, every nonempty prefix of a list starts with x and continues with a prefix of a list from xs; prepending [] supplies the shortest prefix of a list. For tails, the longest suffix of a list is the whole input, followed by all suffixes of a list of its tail. For example, init [1,2] = [[],[1],[1,2]] and tails [1,2] = [[1,2],[2],[]]. Thus both include the empty list, and

∣ init (x s) ∣ = ∣ tails (x s) ∣ = ∣ x s ∣ + 1.

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