An expression is a piece of Clojure code that can be evaluated and return a result
This expression calls the
+ function with the arguments
1 2 3 4 5. As this code works, we get a result.
(+ 1 2 3 4 5)
-e option an expression can be passed to the Clojure CLI tools and a value returned
clojure -e (+ 1 2 3 4 5)
If the expressing used returns a value of
nil, a legal value in Clojure, then no result is printed out.
When to use this?
clojure -e is a quick way to see what an expression does without having to set anything up (although starting a REPL is very little extra effort).
-e option is useful for running simple scripts written in Clojure, especially on servers and remote environments.
The lack of return value for nil is very useful when using Clojure CLI tools to evaluate Clojure code within another script.