Expressive and Strongly Type-Safe Code Generation

Meta-programs are programs that generate other programs, but in weakly type-safe systems, type-checking a meta-program only establishes its own type safety, and generated programs need additional type-checking after generation. Strong type safety of a meta-program implies type safety of any generated object program, a property with important engineering benefits. Current strongly type-safe systems suffer from expressivity limitations and cannot support many meta-programs found in practice, for example automatic generation of lenses. To overcome this, we move away from the idea of staged meta-programming. Instead, we use an off-the-shelf dependently-typed language as the meta-language and a relatively standard, intrinsically well-typed representation of the object language. We scale this approach to practical meta-programming, by choosing a high-level, explicitly typed intermediate representation as the object language, rather than a surface programming language. We implement our approach as a library for the Glasgow Haskell Compiler (GHC) and evaluate it on several meta-programs, including a deriveLenses meta-program taken from a real-world Haskell lens library. Our evaluation demonstrates expressivity beyond the state of the art and applicability to real settings, at little cost in terms of code size.