Simplified JSON parser written in Clojure. Created for blog article. http://mpieciukiewicz.blogspot.com/2013/08/writing-simplified-json-parser-in.html

json_parser.clj

(ns pl.marpiec.json-parser)

(defn is-string-opening? [json] (= \" (first json)))

(defn is-string-ending? [json] (= \" (first json)))

(defn is-object-opening? [json] (= \{ (first json)))

(defn is-object-ending? [json] (= \} (first json)))

(defn is-array-opening? [json] (= \[ (first json)))

(defn is-array-ending? [json] (= \] (first json)))

(defn is-boolean-opening? [json] (or (= \t (first json)) (= \f (first json))))

(defn is-null-opening? [json] (= \n (first json)))

(defn is-digit? [json]
  (and
    (>= (int (first json)) (int \0))
    (<= (int (first json)) (int \9))))

(defn non-digit? [json]
  (not (is-digit? json)))

(defn char-to-number [character]
  (- (int character) (int \0)))

(defn parse-number [json number]
  (if (non-digit? json)
    [number json]
    (recur (rest json) (+ (char-to-number (first json)) (* number 10)))))

(defn parse-string [json string]
  (if (is-string-ending? json)
    [string (rest json)] ; rest to skip closing "
    (recur (rest json) (str string (first json)))))

(defn parse-boolean [json]
  (if (= \t (first json))
     [true (drop 4 json)] ; drop 4 characters of "true"
     [false (drop 5 json)])) ; drop 5 characters of "false"

(defn parse-null [json]
    [nil (drop 4 json)]) ; drop 4 characters of "null"


(declare parse-object parse-array)

(defn parse-value [json]
  (cond
    (is-digit? json) (parse-number json 0)
    (is-boolean-opening? json) (parse-boolean json)
    (is-null-opening? json) (parse-null json)
    (is-string-opening? json) (parse-string (rest json) "")
    (is-object-opening? json) (parse-object (rest json) {})
    (is-array-opening? json) (parse-array (rest json) [])
    :else (println "Incorrect character?")))

(defn parse-property-name [json name]
  (if (= \: (first json))
    [name json]
    (recur (rest json) ; rest to skip ':' delimiter
           (str name (first json)))))

(defn drop-comma-if-first [json]
  (if (= \, (first json))
    (rest json)
    json))

(defn parse-array [json array]
  (if (is-array-ending? json)
    [array (rest json)] ; rest to skip closing ']'
    (let
      [value-result (parse-value (drop-comma-if-first json))
       value (nth value-result 0)
       rest-json (nth value-result 1)]
      (recur rest-json (conj array value)))))

(defn parse-object [json object]
  (if (is-object-ending? json)
    [object (rest json)]; rest to skip closing '}'
    (let
      [property-name-result (parse-property-name (drop-comma-if-first json) "")
       property-name (nth property-name-result 0)
       value-json (rest (nth property-name-result 1)) ;rest to skip \:
       value-result (parse-value value-json)
       value (nth value-result 0)
       rest-json (nth value-result 1)]
      (recur rest-json (assoc object property-name value)))))

(defn parse-json [json]
  (nth (parse-value json) 0))

(defn test-parser []
  ;(assert (== (parse-number "124,xxx" 0) 124))
  (parse-json "{value:1050, name:\"My account\",active:true,owner:{name=\"Marcin Pieciukiewicz\",id:1017,active:false}, operationsIds:[12,14,17,23], keywords:[\"vacation savings\",\"retirement savings\"],additionalInfo:null}"))