Past exam of the year 1 of the computer science course of the University of Oxford / 2021 / Functional programming, design and analysis of algorithms (resit) / 2 / d / Solution

toTree needs Ord a; fromTree does not. Construction compares each key against a pivot. Flattening merely traverses and concatenates the buckets, so its more general type is Tree a -> [a]. A manually constructed tree need not satisfy the search-tree invariant; fromTree still flattens it, although its output need not be sorted.