## 2015-04-08

### The language of types

It's been a while since I posted here. That can be attributed partially to the increased workload I had after starting university, and partially to the fact that most of my ideas had reached either a conclusion or a dead end by the end of summer. But now it's revision time, and I've been able to think about the things I wrote here. Since last post, I've learnt a lot about type theory via Idris. In this post, I apply some of that knowledge.

My aim is to present a type-theoretic interpretation of spoken language, taking advantage of the Curry-Howard correspondence. On the one hand, type theory lends itself very well to the declarative claims of spoken language. It's possible to define predicates as types, with the claim the predicate is making visible to the outside world. Also, it's well documented that logical connectives are easily defined as type families. However, the somehow closed nature of types, in comparison to sets, makes it difficult to work out what types there should be, and how they should be used.

My main idea is to encode verbs as type families and nouns as values. For now, we'll consider all nouns to be of a single, simple type. Notionally, every noun we could want to talk about is specified as its own value constructor, which takes no arguments. Constructors of most real-world verb types are specified similarly, but construct values only of a specific type. For example, taking Lojban's {ciska} (“write”), we make the type family:\begin{align} \mathbf{data}\ &\mathrm{Ciska} : \mathrm{Noun}\to\mathrm{Noun}\to\mathrm{Noun}\to\mathrm{Noun}\to\mathrm{Type}\ \mathbf{where}\\ &\mathrm{MkCiska0} : \mathrm{Ciska}\ \mathit{me}\ \mathit{this\_post}\ \mathit{my\_blog}\ \mathit{keyboard}\\ &\mathrm{MkCiska1} : \mathrm{Ciska}\ \mathit{me}\ \mathit{a\_tweet}\ \mathit{Twitter}\ \mathit{keyboard}\\ &\dots \end{align}Notice that these simple value constructors essentially correspond to events. It has happened that I have written a tweet on Twitter using a keyboard, so the event of that happening constitutes a proof. In this way, we can consider Lojban's {nu} as a wrapper over the contained bridi (type), fixing the type parameters. Of course, we already know that {ka} is used for creating functions resulting in types, and many type constructors take this kind of argument. I'm not sure yet about {du'u} and {si'o}, and {ni} is probably something different again.

Since we now have a reïfied view of statements, it makes sense to quantify them. This is in fact what we are doing with dependently typed Πs and Σs. In implication statements, for example, we are saying that for all proofs of the hypothesis, there exists a proof of the conclusion using that proof. “every time A, B” is logically equivalent to “A ⇒ B”, and is proven using a function of type $A\to B$. We can similarly use dependent pairs for existential quantification, just as in dependently typed programming.

Quantifiers like “almost all” and “almost always when”, naturally, are more difficult to handle, and types don't particularly help us with that. They're probably best dealt with theoretically by having a function of type $A\to\mathrm{Maybe}\ B$ mapping this over a list of inhabitants of A, then interpreting the output list (in some context-sensitive way). How useful this notion is, I don't know.

Anyway, the main point of the latter part of this post was that type families, as opposed to my previous idea of referent sets, are the things that should be quantified over. This becomes apparent when we have polyadic verbs. In my new language, it used to be the case that I'd express “a dog catches a ball” as something like “gerku fasu'o fasu'o kavbu fesu'o fasu'o bolci” (with all those “su'o” usually being implicit). This would literally be read as “a dog is a catching thing; a caught thing is a ball”. However, the problem is that the dog and the ball aren't obviously involved in the same event, and the symmetry of the sentence structure means that we can't make “kavbu” be the main verb. Now, I propose to say “gerku su'o fafa kavbu su'o fefa bolci su'o”, literally “a dog is the catcher in at least one catching event in which a ball is caught”.