John Carmack on mutable variables

After a 2 year Clojure stint I find it very hard to explain the clarity that comes with immutability for programmers used to trigger effects with a mutation.

I think it may be one of those things you have to see in order to understand.

Carmack is talking about variable reassignment here, which Clojure will happily let you mutate.

For example:

  (let [result {:a 1}
        result (assoc result :b 2)]
    ...)

He mentions that C and C++ allow const variables, but Clojure doesn't support that.

clj-kondo has a :shadowed-var rule, but it will only find cases where you shadow a top-level var (not the case in my example).

It's more nuanced, because the shadowing is block-local, so when the lexical scope exits the prior bindings are restored.

I think in practice this is the ideal middle ground of convenience (putting version numbers at the end of variables being annoying), but retaining mostly sane semantics and reuse of prior intermediate results.