Manuel Chakravarty

Title: Type Families in Haskell

One of the main contributions of Haskell was the elegant combination
of ad hoc polymorphism, in the form of type classes, with the
parametric polymorphism of the Hindley-Milner type system.  It
essentially re-interprets function overloading as type-indexed
families of functions.  In Haskell 98 this concept of type-indexed
families is however restricted to functions, or to be precise to
values.  In recent years, the Haskell community has aimed at extending
this capability to type-indexed families of types.  Most of the
approaches have centered around the use of type classes with
functional dependencies and various forms of generic programming.

In this talk, I describe a more direct approach adding type families,
also known as associated types, as an explicit feature to Haskell.
The extension is fully backwards compatible and integrates with other
cutting-edge type extensions, such as generalised abstract data types
(GADTs).  We already have implemented data type families fully into
the Glasgow Haskell Compiler, the most widely used Haskell system, and
are currently implementing synonym families.  The aim is to include
type families in the next stable release - version 6.8 - of GHC.