Semantics-preserving inlining for metaprogramming

Inlining is used in many different ways in programming languages: some languages use it as a compiler-directive solely for optimization, some use it as a metaprogramming feature, and others lay their design in-between. This paper presents inlining through the lens of metaprogramming and we describe a powerful set of metaprogramming constructs that help programmers to unfold domain-specific decisions at compile-time. In a multi-paradigm language like Scala, the concern for generality of inlining poses several interesting questions and the challenge we tackle is to offer inlining without changing the model seen by the programmer. In this paper, we explore these questions by explaining the rationale behind the design of Scala-3’s inlining capability and how it relates to its metaprogramming architecture.