Clojure precondition with local variable

Question

Is there any way to store the result of a computation performed in a precondition so that it can be used in the actual function body.

This is awful (costly-computation runs twice):

(defn bar [x]                                                                           
  {:pre [(> (costly-computation x) 1337)]}                                                
  (costly-computation x))

I would like to do something along these lines. But this doesn't work.

(defn smartbar [x]
  (let [res (costly-computation x)]                                                       
    {:pre [(> res 1337)]}
    res))

Answer 1

You have multiple ways to tackle this. For example you can compute (costly-computation x) outside of (bar) function and pass it as a parameter to (bar) function.

Or you can compute (costly-computation x) inside (bar) function and validate there, no {:pre} condition.

Or write a Macro to do exactly what you want!

Your smartbar isn't going to work because that isn't how defn macro is created to do.
Try this inside the REPL:

(source defn)