why is :else not (else) in clojure?

Question

In Clojure you can write:

(cond (= 1 2) 1 
      :else 5)

In Scheme the equivalent would be:

(cond ((= 1 2) 1)
       (else 5))

The :else 5 syntax is not as consistent as the (else 5). What is the reason the else syntax is implemented in this seemingly inconsistent way in Clojure?

Answer 1

:else is a actually a bit of a clever trick here:

• cond expects condition/value pairs - and :else is just a value that is "truthy" in Clojure so it guarantees that the condition is satisfied. (anything except null or false counts as "truth"). You could equally use ":donkey" as a guaranteed true condition value if you liked.
• However, :else also conveys meaning to a human reader (i.e. this condition is the one to be executed if none of the other conditions match

So really it's just a convention that works in cond expressions and is meaningful to human readers.

Answer 2

I think (else 5) is less consistent. (cond ...) arguments are stated as condition - value pairs. :else value is consistent because :else is just a convention - it works because :else is just an expression that's always true. There's no special rules for :else at all.