Practical variable-arity polymorphism

Abstract

Just as some functions have uniform behavior over distinct types, other functions have uniform behavior over distinct arities. These variable-arity functions are widely used in scripting languages such as Scheme and Python. Statically typed languages also accommodate modest forms of variable-arity functions, but even ML and Haskell, languages with highly expressive type systems, cannot type check the wide variety of variable-arity functions found in untyped functional languages. Consequently, their standard libraries contain numerous copies of the same function definition with slightly different names. As part of the Typed Scheme project-an on-going effort to create anexplicitly typed sister language for PLT Scheme-we have designed and implemented an expressive type system for variable-arity functions. Our practical validation in the context of our extensive code base confirms the usefulness of the enriched type system.