Compiling Environment Classifiers to Contextual Modal Types

Abstract: Type systems for metaprogramming take one of two approaches to safeguarding the scopes of variables in generated code. They either use Environment Classifiers (EC) to identify scopes by name (Taha and Nielsen, 2003), or they use Contextual Modal Type Theory (CMTT) to match scopes by structure (Nanevski, Pfenning, and Pientka, 2008). In this paper, we connect these two approaches by compiling a calculus with polymorphic environment classifiers, subtyping, and mutable references to a CMTT-based calculus with polymorphic contexts, mutable references, and first-class substitutions. The key idea is to map each environment classifier to a context. We prove that the compilation preserves types and that compilation preserves the behavior of metaprograms. Both proofs are mechanized in Agda. The compilation shows that CMTT is at least as expressive as environment classifiers. We conjecture that the converse compilation is difficult, because environment classifiers impose a stricter regime on the metaprogram.