Abstract

note: SS03 not GS15

Haskell provides static, ad-hoc polymorphism using its “type class” mechanism. Type classes allow polymorphic types to be constrained to a set, or class, of types over which some functions are defined.

The popular Glasgow Haskell Compiler has extended the Haskell 98 standard type system with several powerful features, leading to an expressive language of type terms. In contrast, constraints over types have received much less attention, creating an imbalance in the expressivity of the type system.

We rectify this imbalance, transferring familiar type-level constructs, synonyms and families, to the language of constraints, providing a symmetrical set of features at the type-level and constraint-level. In this talk, the new features, constraint synonyms and constraint families, will be introduced along with examples of their increased expressivity for improving the utility of polymorphic EDS Ls in Haskell, amongst other examples.

This talk will start with a brief tour of Haskell’s current menagerie of type system features, followed by a look at our new features.

This is joint work with Tom Schrijvers, and is a practice talk for FLOPS 2010 .