Most programming environments operate using an iterative “generate and test” style of programming.  First, you write the program, then you run the program.  If there are problems with the program you re-write the program and then re-run the program etc..
Impromptu is different.  Impromptu is a dynamic, interactive programming environment where programs are modified and extended while they are running. In fact Impromptu is your program and it is already running.  Let me give you a simple example.  Place your cursor inside the expression below and press the eval button which is located at the top-right of the editor window (copy the (now) expression into your Impromptu editor window and place the cursor inside the expression - the expression will highlight once your cursor is placed correctly).
The message bar at the bottom of the editor window will have displayed the result of evaluating the expression and should look something like this (your number will be different):
Now try pressing the eval button again.  Different result!  (now) is a function that returns the time in audio samples since Impromptu was started.  The important thing to keep in mind for the present is that Impromptu is a program that is already running.  Although Impromptu looks like an ugly text editor, it is actually a powerful notational tool for artists - an artistic environment designed to be extended and manipulated in real-time.  When we evaluate expressions we are actually extending and modifying the behaviour of Impromptu itself.
The eval button evaluates the expression that our cursor is currently located inside (including any sub-expressions that the current expression contains).  The shortcut key for eval is cmd-e.  Try pressing cmd-e with the cursor placed at different locations inside this larger (nested) expression.  You should see the current expression highlight as your cursor moves through each different expression.
(+ 5 (+ 3 (+ 3 4)) (- 10 3))
cmd-e (eval) only evaluates the expression that is highlighted (i.e., the current expression as defined by our cursor position).  There is another way of evaluating an expression in Impromptu that is used much more frequently.  This is the command cntl-space (there is no UI button for cntl-space).  cntl-space always evaluates to the outer expression no matter which sub-expression the cursor is inside.  Try pressing cntl-space with the cursor at different locations inside the expression (+ 5 (+ 3 (+ 3 4)) (- 10 3)).  Notice that the result is 22 regardless of which expression we are in when we evaluate.  cntl-space always evaluates the outermost expression (and thereby all sub-expressions).  For convenience, cntl-space also evaluates the expression immediately following or immediately proceeding the cursor position (i.e., within one space either side of the expression).  This allows you to evaluate an expression that you have just finished typing (i.e., when the cursor is just past the end of the expression).  For the rest of this tutorial assume that when I ask you to evaluate an expression, I mean for you to use cntl-space.
Evaluating expressions is one of our primary mechanisms for interacting with Impromptu and is what differentiates Impromptu from most other creative computer tools.
Evaluating expressions in Impromptu is vitally important but would be limited without a way to extend Impromptu's already running program.  We can extend Impromptu dynamically by defining new values and procedures using define.  Let's make a start by defining a piano instrument.  (Evaluate the next line of code in your impromptu editor - once only!)
(define piano (au:make-node "aumu" "dls " "appl"))
Don't worry to much about how this all works (there are more tutorials to follow).  For now it is just important to understand that you have defined a new instrument (audio unit) and named it “piano”.  
The next two lines modify Impromptu's internal signal flow by connecting the piano to an audio output so that we can play the piano (remember that Impromptu is always running - therefore the audio signal chain is always running).  This is what we mean by dynamic - we are changing Impromptu's internal structure at runtime (in this instance Impromptu's signal flow).
Again, don't worry about the details at this stage, just evaluate each outer expression (pressing cntl-space) in turn. First,
(au:connect-node piano 0 *au:output-node* 0)
And then:
Now we can use the piano to play a note.  Impromptu already defines a play-note function that will allow us to do this.  You should hear a sound when you evaluate the following expression.
(play-note (now) piano 60 80 *second*)
We only wanted one piano, but we're happy to have lots of notes, so you can re-evaluate the play-note function as many times as you like.  Each note will sound for one second.  Because nothing permanent is being defined here we can re-evaluate this code as often as we like. Each time the function executes with no residual effect.
Let's make a small modification to make repeated evaluations more obvious.  We’ll randomise the pitch each time we evaluate play-note.
(play-note (now) piano (random 36 96) 80 *second*)
Notice that Impromptu is responsive, in fact you should be able to evaluate play-note in time with a song playing on the radio.  Impromptu is a live performance instrument so it is designed to be responsive.  This is what we mean by interactive - we can evaluate code and view/hear results from directly inside the editor - hyper-coding.
We can also create functions in Impromptu.  Evaluate the following expression to define a function that, when run, will play a chord using our piano instrument.
(define chord
   (lambda ()
      (play-note (now) piano 60 80 *second*)
      (play-note (now) piano 64 80 *second*)
      (play-note (now) piano 67 80 *second*)))
Once chord is defined you can then call chord as many times as you like by evaluating the following expression.
Congratulations, you have successfully extended the Impromptu program with your own functionality.
You can look up help on any Impromptu function from inside Impromptu by evaluating the help function.  Help on play-note will be printed to the log view.
(help play-note)
Many help references also include example code which you can access by adding #t to help.
(help play-note #t)
Impromptu also displays contextual function signature help.  Whenever you complete typing a functions name it’s signature  (i.e. what arguments it requires) will be displayed in the message bar at the bottom of the editor window.
There are hundreds of Impromptu functions to explore.  To get a general overview of the size of the library you can browse through the function index in Impromptu's Help system (menu help option) or online on the Impromptu website .
One final thing about cmd-e and cntl-space.  If a text region is selected,  both of these forms of evaluation will use the selected region for evaluation.  A quick way to evaluate a whole text buffer is to select using cmd-a and the evaluate using cntl-space.
Impromptu is dynamic, if you re-evaluate a whole buffer you may not get the results you expect!  In particular, remember that if you have multiple text files loaded any evaluations you make all effect the one instance of Impromptu (for example, there is only one audio chain so if you try to evaluate two examples with different audio chain configurations you will almost certainly not get what you expect).  If in doubt, a good idea (particularly when getting started) is to restart Impromptu each time you want to run a new example or start a new project.  (actually most of the examples provided with impromptu are designed to allow you to evaluate the whole buffer - but in general this is not advised).
Have Fun!
Impromptu 101
Impromptu IS your program