I'm a Clojure beginner and I want to understand the -> macro
This code works:
(-> '(1 2 3) reverse)
But this doesn't even compile and I don't know how to deal with the error message:
user=> (-> '(1 2 3) (fn [x] (reverse x)))
CompilerException java.lang.IllegalArgumentException: Parameter declaration quote should be a vector, compiling:(NO_SOURCE_PATH:1:1)
How can I fix this?
I use macroexpand and friends a lot when unexpected things start happening. If you use these here it becomes really obvious what's going on.
user=> (macroexpand-1 '(-> '(1 2 3) (fn [x] (reverse x))))
(fn (quote (1 2 3)) [x] (reverse x))
I think seeing this it's pretty obvious that the (quote (1 2 3) should not be the first arg to fn.
We can also see that the ugly double-parens approach fixes it:
user=> (macroexpand-1 '(-> '(1 2 3) ((fn [x] (reverse x)))))
((fn [x] (reverse x)) (quote (1 2 3)))
Side note that tripped me up: you have to use macroexpand-1 to get a result here. If you use macroexpand or clojure.walk/macroexpand-all it'll blow up (with your exception) after the first expansion, since fn itself is a macro, and is being called with bad syntax after the first expansion.
