Clojure Programming/Examples/Norvig Spelling Corrector

Peter Norvig wrote an excellent example of a spelling corrector in Python. You can see How to Write a Spelling Corrector for a more detailed explanation.

This code, translated into Clojure by language creator Rich Hickey, demonstrates the Python-like brevity of Clojure:

(defn words [text] (re-seq #"[a-z]+" (.toLowerCase text)))

(defn train [features]
  (reduce (fn [model f] (assoc model f (inc (get model f 0)))) {} features))

(def *nwords* (train (words (slurp "big.txt"))))

(defn edits1 [word]
  (let [alphabet "abcdefghijklmnopqrstuvwxyz", n (count word)]
    (distinct (concat
      (for [i (range n)] (str (subs word 0 i) (subs word (inc i))))
      (for [i (range (dec n))]
        (str (subs word 0 i) (nth word (inc i)) (nth word i) (subs word (+ 2 i))))
      (for [i (range n) c alphabet] (str (subs word 0 i) c (subs word (inc i))))
      (for [i (range (inc n)) c alphabet] (str (subs word 0 i) c (subs word i)))))))

(defn known [words nwords] (not-empty (set (for [w words :when (nwords w)]  w))))

(defn known-edits2 [word nwords]
  (not-empty (set (for [e1 (edits1 word) e2 (edits1 e1) :when (nwords e2)]  e2))))

(defn correct [word nwords]
  (let [candidates (or (known [word] nwords) (known (edits1 word) nwords) 
                       (known-edits2 word nwords) [word])]
    (apply max-key #(get nwords % 1) candidates)))

To use:

(correct "misstake" *nwords*)
(correct "speling" *nwords*)

Note: training with big.txt may require the jvm to be started with a heap bigger than its default. You can increase the heap size either by starting java with the "-server" option or setting the maximum heap size explicitly: "java -Xmx128m ..."