This is a simple lazy form evaluation package for Common Lisp.

A lazy form (or thunk) is simply a function wrapping a body of code that will only every be executed once, and the result remembered for future use.

To create a lazy form use the lazy macro:

(lazy &body form)  ;=> THUNK

Let's try it:

CL-USER > (lazy (  1 2))
#<THUNK UNREALIZED>

Let's resolve it with the lazy-value function:

CL-USER > (lazy-value *)
3

Try it again, but with something that has side-effects:

CL-USER > (lazy (print 'hi) (  1 2))
#<THUNK UNREALIZED>

Resolve it and get the value:

CL-USER > (lazy-value *)
HI
3

Get the value again, and notice that the thunk isn't executed a second time:

CL-USER > (lazy-value **)
3

That's it!