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


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 the 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?


There are 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 an interesting perspective to consider, the real discussion should probably 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 (devcards!). Many people experience few problems and live happily ever after. For others, the conceptual distaste is insurmountable and nagging. Like it or hate it, please realise it was a deliberate and conscious design decision, not some oversight.