How to write a Clojure macro that mimics Ruby's %w operator

Question

I'm trying to write my first ever macro in Clojure. I want to mimic Ruby's %w{} operator, which works like this:

irb(main):001:0> %w{one two three}
=> ["one", "two", "three"]

I want to write a function similar in Clojure that returns a vector of words. Here is how it would look:

user=> (%w one two three)
=> ["one" "two" "three"]

I know this is something that cannot be defined as an ordinary function because the symbols would be evaluated before applying and we would see something like this:

user=> (%w one two three)
CompilerException java.lang.RuntimeException: Unable to resolve symbol: one in this context, compiling:(NO_SOURCE_PATH:1:1)

Here is my attempt at a macro:

(defmacro %w [& words]
  (map str (vec words)))

But it doesn't work.

user=> (%w one two three)
ClassCastException java.lang.String cannot be cast to clojure.lang.IFn  user/eval801 (NO_SOURCE_FILE:1)

Why is this happening?

ANSWERS

So the problem was that the macro actually returned the correct output, but then the repl tried to evaluate it and "one" is not a valid function.

Thanks to the answers below, here are two correct macros that solve this problem:

(defmacro %w-vec [& words]
  "returns a vector of word strings"
  (mapv str (vec words)))

(defmacro %w-list [& words]
  "returns a list of word strings"
  (cons 'list (map str words)))

freakhill freakhill · Accepted Answer · 2014-12-24T12:40:57

It does not work because after macro-expansion clojure tries to evaluate ("one" "two" "three"), inducing your error message

user=> (%w one two three)
ClassCastException java.lang.String ("one") cannot be cast to clojure.lang.IFn (interface for callable stuff)  user/eval801 (NO_SOURCE_FILE:1)

now you could do that

(defmacro %w [& words]
  (mapv str (vec words)))

generating a vector

or that

(defmacro %w [& words]
  (cons 'list (mapv str (vec words))))

generating (list "one" "two" "three")

or with syntax quote

(defmacro %w [& words]
  `(list ~@(map str (vec words))))

How to write a Clojure macro that mimics Ruby's %w operator

2 Answers