First-class open and closed code framgents

Staged languages that allow “evaluation under lambdas” are excellent implementation languages for programs that manipulate, specialize, and execute code at runtime. In statically typed staged languages, the existence of staging primitives demands a sound distinction between open code (that may be manipulated under lambdas) and closed code (that may be executed). We present λ [ ], a monomorphic type-safe staged language for manipulating code with free identifiers. It differs from most existing staged languages (such as, for example, derivatives of MetaML) in that its dynamic fragment is not hygienic; in other words, dynamic identifiers are not renamed during substitution. λ[ ] contains a first-class run operation, supports mutable cells (and other computational effects), and has decidable type inference. As such, it is a promising first step towards a practical multi-stage programming language.