HoTT stands for Homotopy Type Theory. Sounds scary and maybe it is. I’m slowly learning about it in various ways, mostly by watching Bob Harper’s lectures at CMU. The subject area of HoTT is “Proof Theory”. I’ve been known to say that Mathematics is about thinking clearly about problems. Proof Theory is mathematicians trying to think clearly about how to do Mathematics. While the details aren’t sorted out, there seems to be good reason to believe that HoTT is a major advance in this subject, and hence in Mathematics itself.

It has been known for many decades that there is a correspondence between logic and programming that is usually expressed as “Propositions are equivalent to Types, and proofs are the same as programs”. In HoTT this becomes true in a practical way. It looks like all or most of Mathematics can be formalized as files that can be checked by a proof assistant. Note that Mathematics is a human activity, and is about human understanding. So Mathematics can not normally be in a final form in these files since the implications for understanding need to be communicated in a human way.

In the world of Proof Theory we don’t have the law of the excluded middle. For a given proposition we might have a proof or a refutation, but until we do the status of the proposition is unknown. In Wombat there is a WeakBoolean type which also includes Unknown as well as True and False, and this has various applications which can be seen as being related to those in Proof Theory.

The core of HoTT is thinking clearly about equality. The very first bit about that (in Harper’s lectures 5 and 6) talks about the difference between Intentional equality, which follows by direct application of the definition of a Type, and Extentional equality which requires a more substantial proof. This has a close analogy in Wombat where primitive types are always Distinguishable (i.e. have a Boolean equality) just based on the representation as a string of bits. All types are then SemiDistinguishable (i.e. have a WeakBoolean equality) based, ultimately, on that Equality: I.e. (roughly) if 2 values of a Type are equal as bits then they are automatically equal, but finer concepts of equality (up to Distinguishable which excludes Unknown) require actual code (a “program” corresponding to a “proof”).

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