On Lisp in Clojure chapter 11 (11.3 - 11.6)

I am continuing to translate the examples from On Lisp by Paul Graham into Clojure. The examples and links to the rest of the series can be found on github.

Section 11.3 Conditional Evaluation

Graham makes such a great point in this section that I wanted to start with it: "When faced with a choice between a clean idiom and an efficient one, we go between the horns of the dilemma by transforming the former into the latter."

I decided to write Graham's if3 macro using true, false and nil for 3 value truthiness. Normally in an if statement nil and false are evaluated the same. The nif macro, is very similar. In the Clojure form, the test is evaluated in a let binding, so that it only has to be evaluated once.

(defmacro if3 [test t-case nil-case f-case]
  `(let [expr# ~test]
     (cond
      (nil? expr#) ~nil-case
      (false? expr#) ~f-case
      :default ~t-case)))

(defmacro nif  [expr pos zero neg]  `(let [expr# ~expr]
    (cond
     (pos? expr#) ~pos
     (zero? expr#) ~zero
     :default ~neg)))

Graham presents us with an `in` macro which tests for membership using a series of tests of equality joined in an or expression.

(defmacro in? [needle & haystack]  ( let [target# needle]
    `(or ~@(map (fn [x#] `(= ~x# ~target#)) haystack))))

(macroexpand-1
 '(in? 1 1 2 3))

;;(clojure.core/or (clojure.core/= 1 1) (clojure.core/= 2 1) (clojure.core/= 3 1))


;; Just to make sure it is working the way we hope
(in? 1 1 (do (println "called") 2) 3)

The Clojure function `some` uses `or` recursively to find the first match.

Clojure's lazy sequences provide another way to get the same functionality. The member? function below returns the first match in the sequence. The argument list is different in this implementation because i wanted the caller to pass a collection, rather than several elements that I wrap in to a collection, because this allows me to work with an infinite collection, such as all positive integers.

;; lazy function
(defn member? [needle haystack]
  (take 1 (filter (partial = needle) haystack)))

(member? 2 (iterate inc 1) )

in-f is almost the same as in?, except that it allows us to pass the function to use for the comparison.

(defmacro in-if [func needle & haystack]
  (let [target# needle]
    `(or ~@(map (fn [x#] `(~func ~x# ~target#)) haystack))))

Graham creates a >case macro that applies a case statement with expressions as keys instead of constants. Each key will be evaluated until a match is found. Once a match is found, no more keys will be evaluated. Clojure's cond statement already behaves like that.

(cond
 (do (println "First cond") false) (println "one")
 (do (println "Second cond") true) (println "two")
 (do (println "Third cond") true) (println "three")
 :default (println "Nothing matched"))

Section 11.4 iteration

Clojure's partition function makes breaking the source parameter into chunks pretty easy. Of course, partition makes the macro easier, it also makes for a short inline invocation. Here is do-tuples-o, followed by an example call to the macro, and an example of writing the map expression directly.

(defmacro do-tuples-o [parms source & body]
  (let [src# source]
    `(map (fn [~parms] ~@body)
          (partition (count (quote ~parms)) 1 ~src#))))

(do-tuples-o [x y] [1 2 3 4 5] (+ x y))

(map
     (fn [[x y]] (+ x y))
     (partition 2 1 [1 2 3 4 5]))

If we use partition in conjunction with cycle, we can create our parameter list that wraps around. The only change we have to make for do-tuples-c is to change partition to partition-round. I also changed my sample call, to show it can work with a function of any arity.

(defn partition-round [size step source]
  (partition size step
             (take (- (+ size (count source)) step)
                   (cycle source))))
(defmacro do-tuples-c [parms source & body]
  (let [src# source]
    `(map (fn [~parms] ~@body)
          (partition-round (count ( quote ~parms)) 1 ~src#))))

(do-tuples-c [x y z] [1 2 3 4 5] (+ x y z))

(map
     (fn [[x y z]] (+ x y z))
     (partition-round 3 1 [1 2 3 4 5]))

Section 11.5 Iteration with Multiple Values

In this section, Graham shows us a macro that executes a do loop that increments several variables in parallel and allows for multiple return values. He then shows us an example of a game that might use this sort of construct to track moving objects.

Multiple return values in a list based language really seems like a non-issue to me.

The sample game that Graham describes looks interesting. I hope to do a Clojure implementation of it soon, and then I will have a better context to evaluate the need for a multi-varibale do.

Section 11.6 Need for Macros

In earlier sections, Graham described using macros for conditional evaluation and iteration. Here he shows us how the some of the same things can be done with functions.

(defn fnif [test then else]
  (if test (then) (else)))

(fnif true (fn [] (do (println "true") (+ 1 2))) (fn []  (do (println "false")  (- 2 1))))

(defn forever [fn]
  (if true
    (do
      (fn)
      (recur fn))
    'done))

#_(forever (fn [] (println "this is dumb")))

We have to wrap the code we want executed within an anonymous function which we invoke when we want the code evaluated. Graham points out that in these situations, the macro solution is much cleaner, if not strictly necessary. He also says simple binding situations can be handled with map, but that more complicated situations are only possible with macros.

On Lisp in Clojure chapter 11 (section 11.2)

I am continuing to translate the examples from On Lisp by Paul Graham into Clojure. The examples and links to the rest of the series can be found on github.

I am only covering one section in this post, but this one section includes file I/O, exception handling and locking. This is a post about the examples from On Lisp, and so isn't a tutorial on any of these topics. Hopefully, it provides a gentle introduction to each topic.

Section 11.2 The with- macro

The with-open-file macro Graham describes is just with-open in Clojure. It can be used with any resource that implements a close method.

(with-open [writer (clojure.java.io/writer "output-file" :append true)]
  (.write writer "99"))

Clojure has a pair of functions for doing stream I/O. slurp and spit, for reading and writing, both use with-open to manage their streams.

(spit "output.txt" "test" :append true )
(slurp "output.txt")

Graham's unwind-protect becomes a try-catch-finally block, which works just like you would expect.

(try
  (do (println "What error?")
      (throw (Exception. "This Error."))
      (println "This won't run"))
  (catch Exception e (.getMessage e))
  (finally (println "this runs regardless")))

Graham's with-db example combines mutations, locks and exception handling. In his first example, he rebinds *db* to a new value, locks it, uses the new value, releases the lock and resets the value. In Clojure, you can create dynamic variables, but changes to their values only appear in the current thread. For Clojure datatypes locks are unnecessary.

(def ^:dynamic *db* "some connection")

(binding [ *db* "other connection"]
         (println *db*))

Because the value assigned to a var with binding is only visible on the current thread, this will not work with code that you want to execute on a different thread. If we are using mutable Java objects across different threads, locking can come into play. Clojure has a locking macro which accepts the object to lock and the code to be executed.

Strings in Java are immutable, so I am going to use a StringBuilder. Graham's let form becomes something like this:

(def db2 (StringBuilder. "connection"))

(let [old-val (.toString db2)]
  (.replace db2 0 (.length db2) "new connection")
  (locking db2
    (println (.toString db2)))
  (.replace db2 0 (.length db2) old-val))

Clearly Graham's call to with-db is preferable to writing the let form over and over. And, as he points out, it is easy enough to add a try-finally block to make a safer implementation.

(defmacro with-db [db & body]
  `(let [temp# (.toString db2)]
    (try
      (.replace db2 0 (.length db2) ~db)
      (locking db2
        ~@body)
      (finally
       (.replace db2 0 (.length db2) temp#)))))

(with-db "new connection"
   (println (.toString db2)))

Graham also gives an example that uses both a macro and a function, which has most of the work being done in the function inside of a context created by the macro. I am not going to claim to understand how Common Lisp manages memory, or how that is different from Clojure. Instead, I will simply acknowledge his point, that perhaps a macro can be used to create a context, and the rest of the work can be done in a function.