Bottom-up computation using trees of sublists (Functional Pearl)

Abstract

Some top-down problem specifications, if executed directly, may compute sub-problems repeatedly. Instead, we may want a bottom-up algorithm that stores solutions of sub-problems in a table to be reused. It can be tricky, however, to figure out how the table can be represented and efficiently maintained. We study a special case: computing a function $h$ taking lists as inputs such that $h~xs$ is defined in terms of all immediate sublists of $xs$. Richard Bird studied this problem in 2008, and presented a concise but cryptic algorithm without much explanation. We give this algorithm a proper derivation, and discover a key property that allows it to work. The algorithm builds trees that have certain shapes -- the sizes along the left spine is a diagonal in Pascal's triangle. The crucial function we derive transforms one diagonal to the next.