Reflections on LMS: exploring front-end alternatives

Metaprogramming techniques to generate code at runtime in a general-purpose meta-language have seen a surge of interest in recent years, driven by the widening performance gap between high-level languages and emerging hardware platforms. In the context of Scala, the LMS (Lightweight Modular Staging) framework has contributed to ``abstraction without regret''--high-level programming without performance penalty--in a number of challenging domains, through runtime code generation and embedded compiler pipelines based on stacks of DSLs. Based on this experience, this paper crystallizes some of the design decisions of LMS and discusses potential alternatives, which maintain the underlying spirit but differ in implementation choices: specifically, strategies for realizing more flexible front-end embeddings using type classes instead of higher-kinded types, and strategies for type-safe metaprogramming with untyped intermediate representations.