Finally Tagless, Partially Evaluated

Abstract

We have built the first family of tagless interpretations for a higher-order typed object language in a typed metalanguage (Haskell or ML) that require no dependent types, generalized algebraic data types, or postprocessing to eliminate tags. The statically type-preserving interpretations include an evaluator, a compiler (or staged evaluator), a partial evaluator, and call-by-name and call-by-value CPS transformers. Our main idea is to encode HOAS de Bruijn or higher-order abstract syntax using cogen functions rather than data constructors. In other words, we represent object terms not in an initial algebra but using the coalgebraic structure of the λ-calculus. Our representation also simulates inductive maps from types to types, which are required for typed partial evaluation and CPS transformations. Our encoding of an object term abstracts over the various ways to interpret it, yet statically assures that the interpreters never get stuck. To achieve self hyp interpretation and show Jones hyp optimality, we relate this exemplar of higher-rank and higher-kind polymorphism (provided by ML functors and Haskell 98 constructor classes) to plugging a term into a context of let hyp polymorphic bindings.