Clojure macro demo for Anish

macros.cljs

;; Preliminaries ------------------------------------------------------------
;; Clojure has namespaces. vars can be created in namespaces and referenced
;; from other namespaces.

(ns glam.db)  ;; Switch into namespace glam.db
(var x 1)     ;; define var glam.db/x
x             ;; `x` evals to 1
;;=> 1

(ns glam.other) ;; switch to other namespace
x             
;;ERROR: x is not defined in this namespace or imported
glam.db/x       ;; can access other namespaces' vars this way
;;=> 1

;; can also import from other namespaces when entering a namespace
(ns glam.other  
  (require [glam.db :refer [x]]))
x
;;=> 1

;; The problem ------------------------------------------------------------
(ns glam.db
  (:require [cljs.spec.alpha :as s])
  (:require-macros [cljs.spec.alpha :as s]))
  
;; I often needed to write something like this. (::foo is syntax for "take 
;; a keyword :foo and put the current namespace in front of it, so in this
;; case, ::foo => :glam.db/foo)
(def foo ::foo)
(s/def ::foo string?)

;; Writing this over and over is verbose. One approach would be to write
;; a function that does this:
(ns glam.db.common-functions
(defn def-with-spec
  [key pred]
  (def (-> key name symbol) key)
  (s/def key pred))

;; But this doesn't work for reasons that have to do with how `def` is 
;; implemented.

;; We need to write a macro instead:
(defmacro def-with-spec
  [key pred]
  `(do
     (def ~(-> key name symbol) ~key)
     (s/def ~key ~pred)))

;; Skipping over what all those symbols mean, this has the end result that 
;; an invocation of it will expand to what we want:
(def-with-spec ::foo string?)
;;macroexpand=>
(do
  (def foo ::foo)
  (s/def ::foo string?))