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Question

I feel offended by re-frame's reg-* API. How is it functional to side effect globally?

Background

A re-frame app is defined collectively by its handlers. As an app boots, calls to registration functions like reg-event-db and reg-sub collectively "build up" an app, infusing it with behaviour and capability.

Currently, this "building up" process involves progressive mutation of a global registrar (map) held internally within re-frame. Each registration adds a new entry to this registrar.

How should we analyse this from a functional point of view?

Answer

There's three ways to view this:

  1. Egads! Say it isn't true. Mutation of a global? Summon the functional lynch mob!

  2. In theory, top level side effects will introduce some pain points, but re-frame's design represents a conscious decision to trade off functional purity for simplicity of everyday developer experience. So, yes, re-frame represents a point in the possible design space, with associated pros and cons. But the cons tend to be theoretical and the pros are real.

  3. Actually, there's no purity problem! As a Clojure program starts, each defn (becomes a def which) happily interns a symbol and function in a map-ish structure representing a namespace. The lynch mob stays home for that. The pitchforks remain in their rack. re-frame handler registration is the same pattern - an id and handler function are interned within a map-ish structure (a registrar), once, on program load. So, if you feel uncomfortable with what re-frame does, you should also feel uncomfortable about using defn. Also, it would be useful to understand how you are creating a virtual machine when you program re-frame

While Point 3 is valid, and perhaps interesting to consider, the real discussion should be around points 1 and 2: is it a good idea for re-frame to tradeoff purity for simplicity? You can't really judge this properly until you have used it and experienced the simplicity, and/or found pain points. For most people it never comes up as a problem.

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