Iowa Type Theory Commute

The problem of word equations is a rather storied one, including frustrated connections to Hilbert's Tenth problem.  Word equations relate expressions consisting of concatenations of variables and constant symbols.  An example is a X = X a, where X is a variable and a is a constant.  A solution maps variables to strings of constant symbols making the two sides identical.  In this episode, I discuss the problem a little, and what I learned so far about how it is solved.

In this episode, I give further arguments in favor of coercive subtyping from a software-engineering perspective.  I also explain the critical concept of coherence.

I explain the story from last episode.

The Curry-Howard isomorphism for the law of excluded middle, as a radio drama.  I first saw a version of this story performed by Phil Wadler and Frank Pfenning (wearing fake horns!) at RTA in Nara, Japan in 2005.  This is my take on it.  In a subsequent episode, I will explain how the story illustrates the computational interpretation of the law of excluded middle.

I discuss a nice paper I quite enjoyed reading, called The Calculated Typer, by Garby, Bahr, and Hutton.  The authors take a very nice general look at the specification of a type checker, for a very simple expression language.  They then manually derive the actual code for the type checker by effectively trying to prove that this as yet unknown code satisfies its spec.  (This is what is meant by calculating the type checker.)