Keyword literal as function apply(:<<) result

Question

Fails:

(let [x [:k1 :k2 :k3]]
  (m/match [x]
           [(:k1 :<< first)] :first))



Invalid list syntax :<< in (:k1 :<< first). Valid syntax:
   [[:default :guard] [:or :default] [:default :only] [:default :seq]
   [:default :when] [:default :as] [:default :<<] [:default
   :clojure.core.match/vector]]


Work:

(let [k1 :k1
      x [:k1 :k2 :k3]]
  (m/match [x]
           [(k1 :<< first)] :first))


Is it a bug or a feature?
How to match keywords correctly?

Answer 1

Comment made by: glchapman

This is an unfortunate corner case. A list pattern is expected to have a literal keyword in position 1 or 2; the keyword is used in dispatching the emit-pattern-for-syntax multimethod. In your first case, you have literal keywords in both positions; if you look at the dispatch function for emit-pattern-for-syntax, you'll see that in this case, the first keyword wins. Since no pattern syntax is defined for :k1, you get the error. I believe quoting the :k1 literal ought to work in this case:

`
(let [x [:k1 :k2 :k3]]
(m/match [x]

       [(':k1 :<< first)] :first))```

Answer 2

Reference: https://clojure.atlassian.net/browse/MATCH-125 (reported by alex+import)