The design and implementation of typed scheme

Abstract

When scripts in untyped languages grow into large programs, maintaining them becomes difficult. A lack of types in typical scripting languages means that programmers must (re)discover critical pieces of design information every time they wish to change a program. This analysis step both slows down the maintenance process and may even introduce mistakes due to the violation of undiscovered invariants.This paper presents Typed Scheme, an explicitly typed extension of an untyped scripting language. Its type system is based on the novel notion of occurrence typing, which we formalize and mechanically prove sound. The implementation of Typed Scheme additionally borrows elements from a range of approaches, including recursive types, true unions and subtyping, plus polymorphism combined with a modicum of local inference. Initial experiments with the implementation suggest that Typed Scheme naturally accommodates the programming style of the underlying scripting language, at least for the first few thousand lines of ported code. Categories and Subject Descriptors Type Refactoring: From Scripts to ProgramsRecently, under the heading of "scripting languages", a variety of new languages have become popular, and even pervasive, in weband systems-related fields. Due to their popularity, programmers often create scripts that then grow into large applications.Most scripting languages are untyped and have a flexible semantics that makes programs concise. Many programmers find these attributes appealing and use scripting languages for these reasons. Programmers are also beginning to notice, however, that untyped scripts are difficult to maintain over the long run. The lack of types means a loss of design information that programmers must recover every time they wish to change existing code. Both the Perl community (Tang 2007) and the JavaScript community (ECMA International 2007) are implicitly acknowledging this problem with the Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. POPL'08, January 7-12, 2008 In the meantime, industry faces the problem of porting existing application systems from untyped scripting languages to the typed world. Based on our own experience, we have proposed a theoretical model for this conversion process and have shown that partial conversions can benefit from type-safety properties to the desired extent (Tobin-Hochstadt and Felleisen 2006). The key assumption behind our work is the existence of an explicitly typed version of the scripting language, with the same semantics as the original language, so that values can freely flow back and forth between typed and untyped modules. In other words, we imagine that programmers can simply add t...

Preemptive Type Checking in Dynamically Typed Languages

“…This work however does not define a formal type system or describe how omitted type annotations are treated. Felleisen and Tobin-Hochstadt [21] propose the notion of occurrence typing for implementing a statically typed version of Scheme. A translation of the simple example in Fig.…”

Quotients revisited for Isabelle/HOL

“…To achieve this, it builds types representing α-equivalent terms. Earlier versions of Nominal Isabelle have been used successfully in formalisations of an equivalence checking algorithm for LF [12], Typed Scheme [11], several calculi for concurrency [2] and a strong normalisation result for cut-elimination in classical logic [13].…”

Practical Variable-Arity Polymorphism

“…For the past two years, Tobin-Hochstadt and Felleisen [1,2] have been developing Typed Scheme, an explicitly and statically typed sister language of PLT Scheme [3]. In many cases, Typed Scheme accommodates existing Scheme programming idioms as much as possible.…”

“…The goal of our Typed Scheme [2] project is to design a typed sister language for an untyped scripting language in which programmers can transfer programs to the typed world one module at a time. Like PLT Scheme, Typed Scheme is a modular programming language; unlike plain Scheme programs, Typed Scheme programs have explicit type annotations for function and structure definitions that are statically checked.…”