One of the primary differences between Impromptu and most other general purpose programming environments is an emphasis on precisely scheduled code execution.
Impromptu incorporates a scheduling engine directly into the programming environment allowing tasks to be scheduled for execution at precise times into the future. Unlike using threading constructs such as sleep(), wait() etc. that do not provide any strong guarantees about temporal accuracy the scheduling engine is guaranteed to execute its tasks at the requested time (the number of audio samples since Impromptu was started). This temporal guarantee is significant for time critical domains such as music.
This example defines a piano instrument and plays three notes in sequence.
; make sure that everything is disconnected
; piano -> output
(define piano (au:make-node "aumu" "dls " "appl"))
(au:connect-node piano 0 *au:output-node* 0)
; schedule three nodes to play in succession
(define play-seq
  (lambda ()
    (play-note (now) piano 60 80 10000)
    (play-note (+ (now) 22050) piano 64 80 10000)
    (play-note (+ (now) 44100) piano 67 80 10000)))
; play
Impromptu uses a schedule and forget, or asynchronous, style of programming, and a design pattern called temporal recursion. This is different from languages such as ChucK that use a synchronous approach. What's the difference? Synchronous timing works by holding up code until some specified time in the future. This is basically the same concept as using sleep() although strongly timed languages like ChucK guarantee the length of the sleep. Impromptu works by scheduling tasks to be executed at some time in the future. Once a task has been scheduled thread execution moves immediately onto the next expression. A pseudo code example may help to illustrate.
//synchronous timing
time = (now) + 44100
//asynchronous timing
play-note(now + 44100)
Strongly timed code holds up thread execution until the global time is equal to the value 'time'. Multitasking is achieved by running multiple concurrent threads of execution. The asynchronous code example schedules tasks into the future and immediately continues execution.  Multitasking in Impromptu is achieved with very little effort by evaluating multiple simultaneous temporal recursions.
Scheduling events is a fairly common programming technique and there wouldn't be much else to say if Impromptu wasn't a dynamic language. Impromptu allows us to create and schedule code for future execution as well as data events such as notes and graphics objects. The ability to schedule blocks of code for precise future execution is a very useful technique for time based programming.
There is a common design pattern in Impromptu programming called temporal recursion. By writing a function that schedules itself as its final action a temporally recursive callback loop can be established. Here is an example demonstrating a foo function that will play a note and then schedule itself to be called back in one second. This loop will continue indefinitely.
(define foo
   (lambda ()
      (play-note (now) piano 60 80 *second*)
      (callback (+ (now) *second*) 'foo)))
You can create as many temporal recursions as you like.  Try evaluating foo multiple times.  Notice that you get multitasking “for free” (i.e you don’t need to do anything special to run two event streams). You can even create temporal recursions inside temporal recursions. A temporal recursion need not run at a constant rate. By adjusting the time increment on each cycle the callback rate (control rate) can be constantly adjusted. Here is an extension to the previous example that will randomize the note length (NOTE: that each callback is now scheduled at (now) + the duration of the note).
Another important factor to keep in mind is that you can revaluate a function while it is temporally recursing - changing its functionality on the fly (provided that the signature of the method does not change - i.e. same arguments and same name).  Try evaluating the code below while the old version of foo is running.
; re-define foo
(define foo
   (lambda ()
      (let ((note-length (random '(0.25 0.5 1.0 2.0))))
         (play-note (now) piano (random 60 80) 80 (* *second* note-length))
         (callback (+ (now) (* note-length *second*)) 'foo))))
One off anonymous functions can also be scheduled for future evaluation. The code example below shows a one off anonymous function scheduled for evaluation in one minute from now.
(callback (+ (now) *minute*)
          (lambda () (play-note (now) piano 60 80 *second*)))
There are a couple of gotcha's to keep in mind when doing schedule and forget programming. The first thing to keep in mind is that (now) is an airy fairy thing. In the example below the two notes may be scheduled to play on the same sample - but then again, they may not - (now) may have moved forward in time between the two calls (even if evaluated at the same time).
(play-note (now) piano 60 80 *second*)
(play-note (now) piano 72 80 *second*)
Often this lack of precision is fine (i.e. too small a change to be noticeable) but where absolute accuracy is required a time variable should be used.
(let ((time (now)))
   (play-note time piano 60 80 *second*)
   (play-note time piano 72 80 *second*))
This inaccuracy becomes more of an issue when amplified over time such as using (now) inside a recursive callback loop. We can avoid the problem by precisely incrementing a time value between each recursive callback (note that any arguments required by the function being called back must also be passed to callback).
;This is bad
(define loop
  (lambda ()
    (play-note (now) piano 60 80 *second*)
    (callback (+ (now) *second*) 'loop)))
;This is good (precise time arg is now incremented each recursion)
(define loop
  (lambda (time)
    (play-note time piano 60 80 *second* )
    (callback (+ time *second*) 'loop (+ time *second*))))
(loop (now))
The second major gotcha in recursive callback loops is that (now) is now. Code requires some time to execute. If you are executing a call to evaluate a note (now) by the time the code is evaluated it will already be late. You should always try to schedule your code execution ahead of the scheduled time of your tasks.
;This is best (callback happens 4100 samples earlier than new time)
(define loop
  (lambda (time)
    (play-note time piano 60 80 1.0)
    (callback (+ time 40000) 'loop (+ time 44100))))
(loop (now))
In the this-is-good version of loop the time sent as an argument to loop is exactly the same time as the scheduled callback time. The problem with this is that the next note needs to be scheduled at exactly the same time that the function is called. The note will always be late. The this-is-best version schedules the callback just ahead of the time that we want to schedule the note. This gives us 4100 samples to execute the code to schedule the note before the note is required to sound.
Temporal recursion is a fundamental pattern in Impromptu and something that you will use constantly.  Take a look at some of the basic Music and Animation tutorials to get more of an idea about using temporal recursion in more practical settings.
Have Fun!
Temporal recursion with callback