variant.clj

(ns variant
  (:require [clojure.core.typed :as t]))

(t/defalias V
  (t/Rec [V]
    (t/U '[':lambda t/Sym V]
         '[':if V V V]
         '[':val t/Any])))

(t/ann command [V -> t/Str])
(defmulti command first)

(defmethod command :lambda
  [[_ s body]]
  (command body))

(defmethod command :val
  [[_ val]]
  (str val))

(defmethod command :if
  [[_ test then else]]
  (str (map command [test then else])))

And for convenience,

(defmacro defvariant [name variant-binder body]
  (let [[tag & binders] variant-binder]
    `(defmethod ~name ~tag [[_ ~@binders]] ~body)

(defvariant command [:print val]
  (println val))

and if you like, it's small but not insignificant

(defmacro defmulti-variant [name & args]
  `(defmulti ~name first ~@args))

Maybe it could use a better name though...