A quick look at impredicativity

Type inference for parametric polymorphism is wildly successful, but has always suffered from an embarrassing flaw: polymorphic types are themselves not first class. We present Quick Look, a practical, implemented, and deployable design for impredicative type inference. To demonstrate our claims, we have modified GHC, a production-quality Haskell compiler, to support impredicativity.  The changes required are modest, localised, and are fully compatible with GHC’s myriad other type system extensions.

Here is a video of Simon giving a talk on the ideas in the paper (opens in new tab)