I find it most useful to define referential transparency in terms of substitution (which is the definition Wikipedia starts with). Namely: if I substitute a fun

I find it most useful to define referential transparency in terms of substitution (which is the definition Wikipedia starts with). Namely: if I substitute a function call with its result is there an observable different in program behaviour? If there is, the program is not referentially transparent. This is clearly the case with `println`. (println "Hi there") is clearly not the same program as nil as the former causes output to the console, while the latter does not -- an observable difference. Using Scheme (because I don't know Clojure that well) (define (foo x) (let ([y 0]) (set! y 10) (+ x 1))) is a referentially transparent program. Although it contains a side-effect (the assignment to y) this effect is not observable outside `foo`, and thus any call to `foo` can be substituted with its result.