15-851 Computation and Deduction
Lecture 15: Parametric Polymorphism

So far we have modeled a modest amount of polymorphism by type-checking a let name constructs after substitution. In this lecture we avoid this technique in favor of adding an explicit quantifier forall a. t to the language of types.

The resulting system has some remarkable properties: it allows more programs to be type-checked, generally improving the expressive power of the type systems. But it also has an important downside: type-checking is now undecidable. We prove here only the most important property, namely that type preservation still holds.

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Frank Pfenning