Where example

; k for keyword, f for function
(defmacro where [[k f arg]]
  `(fn [a#] ; Expand to a function
     ; that applies k to the map, then applies f to that result, and the passed arg
     (~f (~k a#) ~arg)))

(filter (where [:id < 2])
  [{:id 0} {:id 1} {:id 2} {:id 3} {:id 4} {:id 5}])

; Returns ({:id 0} {:id 1})

-----

(where [:id < 2]) 

Expands into

(fn [a] (< (:id a) 2))

(defmacro select ; define the macro that acts as a select command
[vara _ coll _ wherearg _ orderarg] `(filter ;call filter command to select only those entries that match the criteria in where arg. Unquote filter block so it is not executed here
(fn ; The first argument to filter is this internally created function
[[word func arg]] ;which takes three arguments, an identifier(:id), a function(>) and a value(2)
(~func (~word ~coll) ~arg) ) ;call the passed in function(func) on each id in coll(persons)
~wherearg) ; the wherearg parameter (:id < 2) is passed into this internally created function
~coll) ; the second argument to filter is coll(persons)