Range Over Channels in Lisp

(defpackage :range-over-channels
  (:use :cl))

(in-package :range-over-channels)

(defun main ()
  ;; We'll iterate over 2 values in the `queue` channel.
  (let ((queue (make-channel :buffer-size 2)))
    (send queue "one")
    (send queue "two")
    (close-channel queue)

    ;; This `loop` iterates over each element as it's
    ;; received from `queue`. Because we closed the
    ;; channel above, the iteration terminates after
    ;; receiving the 2 elements.
    (loop for elem = (recv queue)
          while elem
          do (format t "~A~%" elem))))

In a previous example, we saw how iteration works over basic data structures. We can also use similar syntax to iterate over values received from a channel.

This example demonstrates how to use channels and iterate over their contents in Common Lisp. However, it’s important to note that Common Lisp doesn’t have built-in support for channels or concurrent programming constructs like Go does. The code above uses hypothetical functions make-channel, send, recv, and close-channel which would need to be implemented separately, possibly using a library that provides concurrency primitives.

Here’s how the program works:

1. We create a buffered channel queue with a capacity of 2.
2. We send two strings, “one” and “two”, to the channel.
3. We close the channel to signal that no more values will be sent.
4. We use a loop construct to iterate over the channel’s contents. The recv function (which would need to be implemented) retrieves values from the channel. The loop continues as long as recv returns a non-nil value, which it would do until the channel is empty and closed.
5. Each received value is printed to the console.

To run this program, you would need to have the necessary channel implementation and then load and execute the main function:

CL-USER> (range-over-channels:main)
one
two
NIL

This example also shows that it’s possible to close a non-empty channel but still have the remaining values be received. In the Lisp implementation, this behavior would need to be explicitly implemented in the channel functions.