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			     I N R I A - Paris
                 2 rue Simone Iff (ou: 41 rue du Charolais)
                          Salle Lions 1, bâtiment C

                           JEUDI 21 février, 10h30

                                 -----------
                                 Jeremy Siek
                                 -----------

                              Indiana University

                       ===============================
                       Toward Efficient Gradual Typing
                       ===============================

Gradual typing combines static and dynamic typing in the same
program. Siek et al. (2015) describe five criteria for gradually
typed languages, including type soundness and the gradual guarantee.
A significant number of languages have been developed in academia
and industry that support some of these criteria (TypeScript, Typed
Racket, Safe TypeScript, Transient Reticulated Python, Thorn, etc.),
but relatively few support all the criteria (Nom, Gradualtalk,
Guarded Reticulated Python). Of those that do, only Nom does so
efficiently. The Nom experiment shows that one can achieve efficient
gradual typing in languages with only nominal types, but many
languages have structural types: function types, tuples, record and
object types, generics, etc.

In this talk I describe a compiler, named Grift, that addresses the
difficult challenge of efficient gradual typing for structural
types. The input language includes a selection of difficult
features: first-class functions, mutable arrays, and recursive
types. I show that a close-to-the-metal implementation of run-time
casts inspired by Henglein's coercions eliminates all of the
catastrophic slowdowns without introducing significant average-case
overhead. As a result, Grift exhibits lower overheads than those of
Typed Racket.