Another piece of the stone
A few days ago, I thought that I had understood pi calculus in terms of category theory, and I did, in a way.


To make lambda calculus into a category, we mod out by the rewrite rules and consider equivalence classes of terms instead of actual terms. A model of this category (i.e. a functor from the category to Set) picks out a set of values for each datatype and a function for each program. Given a value from the input set, we get a single value from the output set.
Similarly, a model of the pi calculus assigns to each process a set of states and to each reduction rule a function that updates the state. A morphism in this way of thinking is a certain number of reductions to perform. The reductions are deterministic in the sense that we can model “A or B” as Given an object’s state at a certain point, we can tell what set of states it can be in after the system reduces n messages.
However, what we really want is to know the set of states it can be in after all the messages have been processed: what is its normal form? This is far more like the rewrite rules in lambda calculus. It suggests that we should be treating the reduction rules like 2morphisms instead of 1morphisms. There’s one important difference from lambda calculus, however: the 2morphisms of pi calculus are not confluent! It matters very much in which order they are evaluated. Thus processes can’t map to anything but trivial functions.
It looks like a better fit for models of the pi calculus is Rel, the category of sets and relations. A morphism in Rel can relate a single input state to multiple output states. This suggests we have a single object * in the pi calculus that gets mapped to a set of possible states for the system, while each process gets mapped to a relation that encodes all its possible reductions.
I’m rather embarrassed that it took me so long to notice this, since my advisor has been talking about replacing Set by Rel for years.
category  lambda calculus  pi calculus 
objects  types  only one type *, a system of processes 
a morphism  an equivalence class of terms  a structural congruence class of processes 
dinatural transformation from the constant functor (mapping to the terminal set and its identity) to a functor generated by hom, products, projections, and exponentials (if they’re there)  combinator: template for programs mapping between isomorphic types (usually)  since there’s only one type, this is trivial 
Model of the category in Rel  (usually taken in Set, a subcategory of Rel) a set of values for each data type and a function for each morphism between them  * maps to a set S of states for the system, and each process gets mapped to a relation that relates each element of S to its possible reductions in S 
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