Quick Practical Clojure macro question

2011-10-24 Thread Alan O'Donnell 
Hi everyone,

I'm working my way through Practical Clojure's macro chapter, and I'd like 
to check my understanding about one of the examples.

The book discusses writing a macro to randomly evaluate a form out of a list 
of forms--essentially a cond that randomly selects which branch to evaluate.

The book's version looks like this:

(defmacro rand-expr-multi [ exprs] 

  `(let [ct# ~(count exprs)]

(case (rand-int ct#) 

  ~@(interleave (range (count exprs)) exprs

My version looks like this:

(defmacro rand-expr-mulit [ exprs]

  (let [n (rand-int (count exprs))]

(nth exprs n)))

Is there any difference between the two? I'm a little shaky on macro 
expansion-time vs run-time, hygiene, etc.

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SICP sqrt function generates unexpected NullPointerException

2011-10-14 Thread Alan O'Donnell 
Hi everyone,

I've encountered an unexpected NullPointerException while translating
some early SICP code into Clojure 1.3.0. In particular, I'm
implementing the iterative sqrt procedure from section 1.1.7.

Here's my code:

(defn square [x] (* x x))

(defn abs [x]
  (cond
( x 0) (- x)
:else x))

(defn average [x y]
  (/ (+ x y) 2))

(defn sqrt
  ([x] (sqrt 1.0 x))
  ([guess x]
(letfn [(good-enough? [guess]
  ( (abs (- (square guess) x)) 0.001))
(improve [guess]
  (average guess (/ x guess)))]
  (if (good-enough? guess)
guess
(recur (improve guess) x)

Rather mysteriously, this works correctly for inputs less than roughly
(square 2718.259...); anything larger throws a NullPointerException
clojure.lang.Numbers.lt (Numbers.java:3693).

Any ideas?

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Re: SICP sqrt function generates unexpected NullPointerException

2011-10-14 Thread Alan O'Donnell 
Armando, I get the same behavior as you with Clojure 1.2.1. But if I lein 
dep Clojure 1.3.0, I'm back to NullPointerExceptions.

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