Combinators for impure yet hygienic code generation

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 if-guards. 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 hinder domain experts in 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. We present a code-combinator framework that lets us express arbitrary monadic 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. They also express the lexical scopes of generated binders and prevent mixing up variables with different scopes. For the first time ever we demonstrate statically safe and well-scoped loop exchange and constant factoring from arbitrarily nested loops. 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.' To become practical, the library relies on higher-order abstract syntax and polymorphism over generated type environments, and is written in the mature language.