Question

I was reading the source for ChanL the other day. It contains an example use of channels, to implement futures. The DEFUNs were declared inside a LET, like so:

(let ((some-var some-value))
  (defun foo () ... (reference some-var) ... )
  (defun bar () ...))

What purpose does this serve? Is it just to provide some common value that several functions can share, and keep the encapsulation clean?

Answer 1

You already answered your question: to provide shared bindings for a group of functions and keep encapsulation clean.

Simple example from http://letoverlambda.com/textmode.cl/guest/chap2.html:

(let ((direction 'down))
  (defun toggle-direction ()
    (setq direction
          (if (eq direction 'up)
              'down
              'up))))
(toggle-direction)   => UP
(toggle-direction)   => DOWN
(toggle-direction)   => UP
(toggle-direction)   => DOWN

You can also add a function inside this closure which behavior depends on direction.