Re: how to emulate lisp's labels functionality?

2009-02-21 Thread Christopher Taylor 


On 15.02.2009, at 06:26, Chouser wrote:


 On Sat, Feb 14, 2009 at 11:32 PM, GS gsincl...@gmail.com wrote:

 On Feb 14, 12:21 pm, Chouser chou...@gmail.com wrote:

 (defn count-instances [obj lsts]
  (let [instances-in (fn thisfn [lst]
   (if (seq lst)
 (+ (if (= (first lst) obj) 1 0)
(thisfn (rest lst)))
 0))]
(map instances-in lsts)))

 Notwithstanding your more idiomatic implementation (snipped),  
 wouldn't
 recur be better than thisfn?

 (defn count-instances [obj lsts]
  (let [instances-in (fn [lst]
   (if (seq lst)
 (+ (if (= (first lst) obj) 1 0)
(recur (rest lst)))
 0))]
(map instances-in lsts)))

 Yes, you're absolutely right.

however, the recur isn't in tail position, so it doesn't work:

user= (defn count-instances [obj lsts]
  (let [instances-in (fn [lst]
   (if (seq lst)
 (+ (if (= (first lst) obj) 1 0)
(recur (rest lst)))
 0))]
(map instances-in lsts)))
java.lang.UnsupportedOperationException: Can only recur from tail  
position (NO_SOURCE_FILE:12)

all the best,
   --Chris


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how to emulate lisp's labels functionality?

2009-02-13 Thread wubbie 

Hi, do we have labels equiv. in clojure?
The code below is from OnLisp. Trying to convert to clj file,
but have minor difficulties.


(defun count-instances (obj lsts)
  (labels ((instances-in (lst)
(if (consp lst)
  (+ (if (eq (car lst) obj) 1 0) (instances-in (cdr lst)))
  0)))
 (mapcar #’instances-in lsts)))

 (count-instances ’a ’((a b c) (d a r p a) (d a r) (a a)))
(1 2 1 2)

thanks
sun

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Re: how to emulate lisp's labels functionality?

2009-02-13 Thread Phil Hagelberg 

wubbie sunj...@gmail.com writes:

 Hi, do we have labels equiv. in clojure?
 The code below is from OnLisp. Trying to convert to clj file,
 but have minor difficulties.

You can use let since variables and functions are kept in the same
namespace.

 (defun count-instances (obj lsts)
   (labels ((instances-in (lst)
 (if (consp lst)
   (+ (if (eq (car lst) obj) 1 0) (instances-in (cdr lst)))
   0)))
  (mapcar #’instances-in lsts)))

...becomes...

 (defn count-instances [obj lsts]
   (let [instances-in (fn [lst]
(if (cons? lst)
  (+ (if (= (first lst) obj) 1 0) 
 (instances-in (rest lst)))
  0))]
   (map instances-in lsts)))

This is a pretty direct translation; you could make it more idiomatic in
other ways; I'm just trying to compare let to labels.

-Phil

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Re: how to emulate lisp's labels functionality?

2009-02-13 Thread Chouser 

On Fri, Feb 13, 2009 at 8:01 PM, Phil Hagelberg p...@hagelb.org wrote:

 wubbie sunj...@gmail.com writes:
 Hi, do we have labels equiv. in clojure?
 The code below is from OnLisp. Trying to convert to clj file,
 but have minor difficulties.

 You can use let since variables and functions are kept in the same
 namespace.

This is true, but this...

 (defn count-instances [obj lsts]
   (let [instances-in (fn [lst]
(if (cons? lst)
  (+ (if (= (first lst) obj) 1 0)
 (instances-in (rest lst)))
  0))]
   (map instances-in lsts)))

...doesn't work.  You can't refer to the 'let' local from within the
fn definition.  You can however give the fn a name, like thisfn:

(defn count-instances [obj lsts]
  (let [instances-in (fn thisfn [lst]
   (if (seq lst)
 (+ (if (= (first lst) obj) 1 0)
(thisfn (rest lst)))
 0))]
(map instances-in lsts)))

That works, but is not very Clojurey.  Just for the record, I might do
it more like:

(defn count-instances [obj lsts]
  (map #(count (filter #{obj} %)) lsts))

--Chouser

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