"Syntax error macroexpanding" when applying the quote form to the binding form of let macro:
(def x '[y 1])
(let x y)
How to make it work if the binding form is predefined outside let expression?
@EDIT
By chance I tried locally on this problem that asks to fill the same bind form in each of the three test cases. I failed below hence came up with this question.
(def __ '[x 7 y 3 z 1])
(println
(= 10 (let __ (+ x y)))
(= 4 (let __ (+ y z)))
(= 1 (let __ z)))
You can't predefine the let binding outside the let form. The 4clojure problem isn't asking you to do that although I can see why you would try to do what you showed (defining __ to be a literal vector).
What 4clojure is doing behind the scenes here is taking your literal input and macroexpanding forms so that your input is placed where the __ would be. It's doing something like this:
user=> (let [__ '[x 7 y 3 z 1]] `(let ~__ (+ ~'x ~'y)))
(clojure.core/let [x 7 y 3 z 1] (clojure.core/+ x y))
and then evaluating the result:
user=> (clojure.core/let [x 7 y 3 z 1] (clojure.core/+ x y))
10
4clojure is expecting that literal vector, without the quote.