In Clojure, is there a way to make a var constant such that it can be used in case statements?
e.g.
(def a 1)
(def b 2)
(let [x 1]
(case x
a :1
b :2
:none))
=> :none
I understand I can use something like cond or condp to get around this, but it would be nice if I could define something that does not require further evaluation so I could use case.
Related and answer stolen from it:
As the docstring tells you: No you cannot do this. You can use Chas Emericks macro and do this however:
(defmacro case+
"Same as case, but evaluates dispatch values, needed for referring to
class and def'ed constants as well as java.util.Enum instances."
[value & clauses]
(let [clauses (partition 2 2 nil clauses)
default (when (-> clauses last count (== 1))
(last clauses))
clauses (if default (drop-last clauses) clauses)
eval-dispatch (fn [d]
(if (list? d)
(map eval d)
(eval d)))]
`(case ~value
~@(concat (->> clauses
(map #(-> % first eval-dispatch (list (second %))))
(mapcat identity))
default))))
Thus:
(def ^:const a 1)
(def ^:const b 2)
(let [x 1]
(case+ x
a :1
b :2
:none))
=> :1
An alternative (which is nice since it's more powerful) is to use core.match's functionality. Though you can only match against local bindings:
(let [x 2
a a
b b]
(match x
a :1
b :2
:none))
=> :2