Variables in Types

September 25, 2017

These are some recitation notes for an ad-hoc recitation I gave for the class 15-312 Principles of Programming Languages. It was probably my favorite 312 recitation content-wise, because it’s the first recitation where we’ve covered enough stuff to where we can really start connecting the dots.


We’ve seen a number of examples in class of types which use variables. Having variables in our type systems lends a great deal of power to languages using these type systems. We’re going to look at how variables are used in generic programming, inductive & coinductive types, and polymorphic types.

Variables in Types

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