Thinking in Erlang

David N Murray <>
Wed Apr 28 04:29:43 CEST 2010


Hi,

I'm one of those people that learned lisp and moved on, but am a better
programmer for what I learned from lisp.  One of the things I picked up
was function returning functions and the lisp version of closures.  In my
Erlang reading, I tend to think this way when I read about anonymous
functions and closures.

I have a function:

(defun make-fqueue (n)
  (let ((queue '()))
    (lambda (e)
      (push e queue)
      (if (> (length queue) n)
        (setf queue (subseq queue 0 n)))
      queue)))

The above function returns a function that maintains a fixed-length queue,
pushing each element it receives onto the queue and popping the head off
when the queue reaches a certain size and always returning the queue.
Multiple calls to make-fqueue return multiple queue "objects."

What I'm realling doing above is encapsulating a mutable list and carrying
it around inside the function, hidden from the user.  If I try and
implement something like this in Erlang, and don't want to be passing
the list around to whomever may use it, I'm doing it with a gen_server,
correct?

TIA,
Dave



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