Closure-Free Functional Programming in a Two-Level Type Theory

Journal article, 2024

Many abstraction tools in functional programming rely heavily on general-purpose compiler optimization to achieve adequate performance. For example, monadic binding is a higher-order function which yields runtime closures in the absence of sufficient compile-time inlining and beta-reductions, thereby significantly degrading performance. In current systems such as the Glasgow Haskell Compiler, there is no strong guarantee that general-purpose optimization can eliminate abstraction overheads, and users only have indirect and fragile control over code generation through inlining directives and compiler options. We propose a two-stage language to simultaneously get strong guarantees about code generation and strong abstraction features. The object language is a simply-typed first-order language which can be compiled without runtime closures. The compile-time language is a dependent type theory. The two are integrated in a two-level type theory. We demonstrate two applications of the system. First, we develop monads and monad transformers. Here, abstraction overheads are eliminated by staging and we can reuse almost all definitions from the existing Haskell ecosystem. Second, we develop pull-based stream fusion. Here we make essential use of dependent types to give a concise definition of a concatMap operation with guaranteed fusion. We provide an Agda implementation and a typed Template Haskell implementation of these developments.