ezyang's blog

This year at ICFP, we had some blockbuster attendance to the Haskell Implementor’s Workshop (at times, it was standing room only). I had the pleasure of presenting the work I had done over the summer on Backpack.

You can grab the slides or view the presentation itself (thank you ICFP organizers for being incredibly on-the-ball with videos this year!) The talk intersects a little bit with my blog post A taste of Cabalized Backpack, but there are more pictures, and I also emphasize (perhaps a little too much) the long term direction we are headed in.

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One of the major open problems for building a module system in Haskell is the treatment of type classes, which I have discussed previously on this blog. I’ve noted how the current mode of use in type classes in Haskell assume “global uniqueness”, which is inherently anti-modular; breaking this assumption risks violating the encapsulation of many existing data types.

As if we have a choice.

In fact, our hand is forced by the presence of open type families in Haskell, which are feature many similar properties to type classes, but with the added property that global uniqueness is required for type safety. We don’t have a choice (unless we want type classes with associated types to behave differently from type classes): we have to figure out how to reconcile the inherent non-modularity of type families with the Backpack module system.

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