Computational Effects across Generated Binders Maintaining future-stage lexical scope

Code generation is the leading approach to making high-performance software reusable. Effects are indispensable in code generators, whether to report failures or to insert let-statements and ifguards. Extensive painful experience shows that unrestricted effects interact with generated binders in undesirable ways to produce unexpectedly unbound variables, or worse, unexpectedly bound ones. These subtleties prevent experts in the application domain, not in programming languages, from using and extending the generator. A pressing problem is thus to express the desired effects while regulating them so that the generated code is correct, or at least correctly scoped, by construction. In an imminently practical code-generation framework, we show how to express arbitrary effects, including mutable references and delimited control, that move open code across generated binders. The static types of our generator expressions not only ensure that a well-typed generator produces well-typed and well-scoped code, but also express the lexical scopes of generated binders and prevent mixing up variables with different scopes. This precise notion of lexical scope subsumes the complaints about intuitively wrong example generators in the literature. For the first time, we demonstrate statically safe let-insertion across an arbitrary number of binders. Our framework is implemented as a Haskell library that embeds an extensible typed higher-order domain-specific language. It may be regarded as ‘staged Haskell.’ The library is convenient to use thanks to the maturity of Haskell, higher-order abstract syntax, and polymorphism over generated type environments.