Notes on pattern languages

main.ts

@@ -12,6 +12,28 @@ import {
 import { Dictionary, IdentityMap, is, Record } from 'preserves';
 import { Bag, ChangeDescription } from './bag';
 
+// Q. Why keep "Observe"? Why not do the clever trick of asserting the
+// observer, and having the dataspace read the implicit pattern it's
+// interested in off its attenuator?
+//
+// A. (1) Because we want to have the possibility of more than one
+// variety of pattern language. For example, here we have a simple
+// label match, but we'll quickly want something about as rich as the
+// pattern language in attenuators. And later, we could easily want
+// something with, perhaps, as much power as RELAX-NG or similar. (2)
+// Because we want to have onlookers have some hope of seeing whether
+// a pattern of interest to them is being observed, and if we used
+// attenuators to match, we'd have to expose visibility into
+// attenuators into the pattern language. See next question.
+//
+// Q. What kinds of constraints on the pattern language are there?
+//
+// A. (1) It should be fast to evaluate; ideally, JITtable? (2) It
+// should allow patterns on patterns, to some degree. We occasionally
+// want to observe observers. As such, it'd be good to choose a
+// language that enforced some kind of normal forms for its patterns,
+// so observer-observers didn't have to deal with spurious variation.
+//
 const Observe = Record.makeConstructor('Observe', ['label', 'observer']);
 
 class Dataspace implements Partial<Entity> {