Writing loops with tail-recursive function calls is the equivalent of writing them with goto’s. Given that loop packages for Lisp-family languages have been around for over 20 years, it is striking that none have had much success in the Scheme world. I suggest the reason is that Scheme forces us to be precise about the scoping of the various variables introduced by our loop forms, something previous attempts to design ambitious loop forms have not managed to do. I present the design of a loop package for Scheme with a welldefined and natural scoping rule, based on a notion of control dominance that generalizes the standard lexical-scope rule of the λcalculus. The new construct is powerful, clear, modular and extensible. The loop language is defined in terms of an underlying language for expressing control-flow graphs. This language itself has interesting properties as an intermediate representation.

In this paper we present a macro system for the Fortress programming language. Fortress is a new programming language designed for scientific and high-performance computing. Features include: implicit parallelism, transactions, and concrete syntax that emulates mathematical notation. Fortress is intended to grow over time to accommodate the changing needs of its users. Our goal is to design and implement a macro system that allows for such growth. The main challenges are (1) to support extensions to a core syntax rich enough to emulate mathematical notation, (2) to support combinations of extensions from separately compiled macros, and (3) to allow new syntax that is indistinguishable from core language constructs. To emulate mathematical notation, Fortress syntax is specified as a parsing expression grammar (PEG), supporting unlimited lookahead. Macro definitions must be checked for well-formedness before they are expanded and macro uses must be well encapsulated (hygienic, composable, respecting referential transparency). Use sites must be parsed along with the rest of the program and expanded directly into abstract syntax trees. Syntax errors at use sites of a macro must refer to the unexpanded program at use sites, never to definition sites. Moreover, to allow for many common and important uses of macros, mutually recursive definitions should be supported. Our design meets these challenges. The result is a flexible system that allows us not only to support new language extensions, but also to move many constructs of the core language into libraries. New grammar productions are tightly integrated with the Fortress parser, and use sites expand into core abstract syntax trees. Our implementation is integrated into the open-source Fortress reference interpreter.To our knowledge, ours is the first implementation of a modular hygienic macro system based on parsing expression grammars.