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Fun with Type Functions

Oleg Kiselyov, Ken Shan, and Simon Peyton Jones have a draft paper

which will appear in the proceedings of Tony Hoare's 75th birthday celebration.

Abstract. Tony Hoare has always been a leader in writing down and proving properties of programs. To prove properties of programs automatically, the most widely used technology today is by far the ubiquitous type checker. Alas, static type systems inevitably exclude some good programs and allow some bad ones. This dilemma motivates us to describe some fun we've been having with Haskell, by making the type system more expressive without losing the benefits of automatic proof and compact expression.

Haskell's type system extends Hindley-Milner with two distinctive features: polymorphism over type constructors and overloading using type classes. These features have been integral to Haskell since its beginning, and they are widely used and appreciated. More recently, Haskell has been enriched with type families, or associated types, which allows functions on types to be expressed as straightforwardly as functions on values. This facility makes it easier for programmers to effectively extend the compiler by writing functional programs that execute during type-checking.

This paper gives a programmer's tour of type families as they are supported in GHC today.

This Wiki page is a discussion page for the paper. If you are kind enough to read this paper, you may like to jot down any thoughts it triggers off, and see what others have written. This talk-page lets you do just that.

Deadline is sometime in June 2009.

You can identify your entries by preceding them with four tildes. Doing so adds your name, and the date. Thus:

Simonpj 08:42, 19 April 2007 (UTC) Note from Simon

If you say who you are in this way, we'll be able to acknowledge your help in a revised version of the paper.

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