Wishful Coding

Didn't you ever wish your computer understood you?

Clojure micro pattern matcher

Today I had some ideas for my game related to cellular automata that involved pattern matching. I googled, and found Matchure.

It might be a great pattern matcher, but the syntax and the amount of macro trickery makes me feel uncomfortable.

In an attempt to write one that relies only on functions, I made this.

Update: This is now a cute project-in-a-gist that can be used as a checkout dependency in Leiningen and Cake.

(ns seqex
  (:import [clojure.lang
            Fn
            IPersistentVector
            IPersistentMap]))

(defn cps [funct cont]
  (fn [[f & r]]
    (when-let [f (funct f)]
      (when-let [r (cont r)]
        (if (true? r)
          (if (true? f) [] [f])
          (if (true? f) r (cons f r)))))))
;    (and (funct f) (cont r))))

(def _ (constantly true))

(def end nil?)

(defmulti matcher type)

(defn match [pattern items]
  ((reduce #(cps (matcher %2) %1)
           (rseq pattern))
     items))

(defmethod matcher :default [l]
  (partial = l))

(defmethod matcher Fn [f] f)

(defmethod matcher IPersistentVector [pattern]
  (partial match pattern))

(defmethod matcher IPersistentMap [pattern]
  (fn [m]
    (every? identity
      (for [[k v] pattern]
        (when-let [mv (get m k)]
          ((matcher v) mv))))))

(defmethod matcher java.util.regex.Pattern [pattern]
  (partial re-find pattern)) ; returns match

(defn store [pattern]
  (fn [s]
    (when ((matcher pattern) s)
      s)))

(defmacro cond-let
  [& clauses]
    (when clauses
      (list `if-let (first clauses)
            (if (next clauses)
                (second clauses)
                (throw (IllegalArgumentException.
                         "cond-let requires an even number of forms")))
            (cons `cond-let (next (next clauses))))))
(match
 [1
  [3 _ end]
  {:a even?, :b #(= 2 (count %))}
  end]
 [1 [3 5] {:a 4, :b [1 2], :d :a}])
; [] (is truthy)

(match [1 2 3 _] [1 2 3 4 5 6 7 8])
; []

(let [[x y] (match
              [1 #"\w.+?\w" 3 (store _) end]
              [1 "foo bar"  3        4])]
  (dotimes [_ y] (println x)))
; foo
; foo
; foo
; foo
; nil
(defproject seqex "1.0.0-SNAPSHOT"
  :description "a tiny pattern matcher"
  :dependencies [[org.clojure/clojure "1.2.1"]]
  :source-path ""
  :aot [seqex]
  :omit-source true)

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