Runtime and compiler support for HAMTs

Abstract

Many functional languages---including Racket, Clojure, and Scala---provide a persistent-map datatype with an implementation based on Hash Array Mapped Tries (HAMTs). HAMTs enable efficient functional lookup, insertion, and deletion operations with a small memory footprint, especially when taking advantage of implementation techniques that have been developed since the original HAMT implementation. Racket's latest HAMT implementation is based on an intermediate data structure, a stencil vector, that supports an especially compact representation of HAMTs with help from the compiler and memory manager. That is, stencil vectors provide an abstraction to improve HAMT performance without burdening the compiler with all of the complexity and design choices of a HAMT implementation. Benchmark measurements show that HAMTs in Racket have performance comparable to other state-of-the-art implementations, while stencil-vector HAMTs are more compact and run as fast as alternative representations in Racket. Although we only report on Racket, our experience suggests that a stencil-vector datatype in other dynamic-language implementations might improve HAMT performance in those implementations.